Chemical Reactions and Equationssndpublicschool.org/wp-content/uploads/2020/04/Ch_1... · 2020. 4. 16. · Chemical Reactions and Equations 3 Change in temperature Some chemical reactions

TOPIC 1 : CHEMICAL REACTIONS AND CHEMICAL EQUATIONSEverything in nature undergoes changes or transformations that involves changes in either their physical properties or their chemical properties. The change in which the original substances do not lose their nature and identity but accompanied with change in physical properties are called physical changes e.g., evaporation of water, dissolution of salt in water, etc. The changes which are accompanied by changes in composition and chemical properties of the original substance thereby forming new substances are called chemical changes e.g., burning of wood, cooking of food, ripening of fruit, etc. The process that involves a chemical change is called a chemical reaction.

Chemical ReactionsThe change of one or more substances into other substances having different composition and different properties is called a chemical reaction. A chemical reaction simply is a rearrangement of atoms. The substances which take part in a chemical reaction are called reactants. The new substances produced as a result of a chemical reaction are called products.In a chemical reaction, breaking of chemical bonds takes place in the reactant molecules and making of new chemical bonds take place to form the products.

(Reactants)A + B

(Products)C + D

Examples of some chemical reactionsBurning of magnesium in air : On heating, magnesium combines with oxygen present in air to form white powder of magnesium oxide.

TOPICS

Chemical Reactions and

Chemical Equations

Types of Chemical Reactions and Effects of Oxidation Reactions in Everyday Life

Chemical Reactions and Equations

Everything around us changes with time. Some changes can be noticed immediately, while some others go unnoticed. A few examples of processes occurring in everyday life are : Turning brown of freshly cut apple in air, cooking of food, Digestion of food in our body, respiration, and burning of candle wax. In all the above processes, the nature and identity of the initial substances changes. We may also, say that during such changes substances are undergoing permanent changes and new substances are formed. When different substances react and new substances having different nature and identity are formed, we say a chemical reaction has occurred. In the present unit we will discuss what is actually meant by a chemical reaction and how these reactions take place.

1CHAPTER

2 100PERCENTScience Class-10

2Mg(s) + O2(g) → 2MgO(s) Magnesium Oxygen Magnesium oxide (white powder)

Reaction between zinc and dilute hydrochloric acid : Zinc reacts with dilute hydrochloric acid to produce zinc chloride and hydrogen gas.

Zinc chloride Hydrogen gas

ZnCl2 + H2 ↑Zn + 2HClZinc Hydrochloric acid

Characteristics of chemical reactionsWhen a chemical reaction takes place, some changes are observed. The easily observable changes that take place in a chemical reaction are called the characteristics of the chemical reaction. Some important characteristic of chemical reactions are discussed below :

Change in physical stateSome chemical reactions are accompanied by change in physical state of reactants.Examples :(i) When a mixture of carbon monoxide and hydrogen is reacted at high pressure liquid methyl alcohol is formed. CO(g) + 2H2(g) CH3OH(l )

340 atm

(ii) Ammonia reacts with hydrogen chloride gas forming solid ammonium chloride. NH3(g) + HCl(g) NH4Cl(s)

Room temperature

Change in colourSome chemical reactions are accompanied by change in colour.Examples :(i) When hydrogen sulphide gas is passed through copper sulphate solution, black copper sulphide is formed.

CuSO4(aq) + H2S(g) CuS(s) + H2SO4(aq)

(Black)(Blue)

(ii) When red lead oxide is heated, yellow lead monoxide is formed.

2Pb3O4(s)

(Red) (Yellow)6PbO(s) + O2(g)↑

Heat

Evolution of a gas

Reaction between Zn metaland dilute sulphuric acid

Dilute H2SO4

Zn

Evolution ofH2 gas

Some chemical reactions take place with the evolution of a gas.Examples :(i) Reaction between a metal such as zinc, magnesium or iron and dilute sulphuric acid produces hydrogen gas.

Zn(s) + H2SO4(dil ) ZnSO4(aq) + H2(g)↑(ii) Reaction between iron sulphide and dilute sulphuric acid produces hydrogen sulphide gas.

FeS(s) + H2SO4(dil ) FeSO4(aq) + H2S(g)↑

To study the reaction between zinc and dilute hydrochloric acid " Take a conical flask. Fit it with a cork and a glass tube passing through the cork. " Put a few zinc granules in the flask. Add dilute hydrochloric acid or sulphuric acid to this. " Zinc reacts with dilute hydrochloric acid to form hydrogen gas which appears as bubbles. " Heat is produced during the reaction causing warming of the flask.

LAB TIMELet's Do & Learn

3Chemical Reactions and Equations

Change in temperatureSome chemical reactions involve a change in temperature, i.e., rise or fall of temperature.Exothermic reactions : Reactions which are accompanied by evolution of heat are known as exothermic reactions e.g., when small amount of water is added to quick lime, a large amount of heat is evolved and reaction mixture becomes hot.

CaO(s) + H2O(l ) Ca(OH)2(s) + HeatQuick lime Slaked lime

Endothermic reactions : Reactions which are accompanied by absorption of heat are known as endothermic reactions e.g., burning of limestone requires lots of heat which causes fall of temperature.

CaCO3(s) + Heat CaO(s) + CO2(g)Limestone Quick lime

Formation of a precipitateSome chemical reactions are characterised by the formation of a precipitate. A precipitate is an insoluble solid substance produced on mixing two solutions.Examples :(i) When potassium iodide solution is added to lead nitrate solution, yellow precipitate of lead iodide is formed.

(Yellowprecipitate)

Pb(NO3)2(aq) + 2KI(aq) PbI2(s)↓ + 2KNO3(aq)

To study the reaction between lead nitrate and potassium iodide

I–

K++

Pb2+

NO3–

Pb(NO3)2(aq) → PbI2(s)↓ + 2KNO3(aq)2KI(aq) +

" Take lead nitrate solution in one beaker and potassium iodide solution in another beaker. Mix the two solutions.

" A yellow solid (called precipitate) appears. This yellow solid is a new chemical compound, namely, lead iodide. Another substance formed is potassium nitrate which we cannot see as it remains dissolved in the solution.


(ii) When silver nitrate solution is mixed with sodium chloride solution, white precipitate of silver chloride is formed.

(Whiteprecipitate)

AgNO3(aq) + NaCl(aq) AgCl(s) ↓ + NaNO3(aq)

" There are some chemical reactions which can show more than one

High Energy Food

characteristics, e.g., the chemical reaction between zinc granules and dilute hydrochloric acid (or dilute sulphuric acid) shows two characteristics : evolution of a gas (hydrogen gas) and change in temperature (rise in temperature). Similarly the chemical reaction between potassium iodide solution and lead nitrate solution shows two characteristics : change in colour (from colourless to yellow) and formation of a precipitate (lead iodide precipitate).

" We get the energy for our living from the food that we eat. The food such as rice, potatoes and bread contain carbohydrates. During digestion, they are broken down into simpler substances such as glucose. The air we breathe in during respiration, oxidizes glucose in the cells of our body. This reaction is exothermic and provides us the required energy. C6H12O6(aq) + 6O2(g) 6CO2(g) + 6H2O(l) + Energy Glucose


ILLUSTRATIONS (i) Name two solutions which on mixing form a yellow precipitate and what is it due to?

(ii) Name two solutions which on mixing form a white precipitate and what is it due to?Ans. (i) Lead nitrate and potassium iodide solutions on mixing form a yellow precipitate due to formation of lead iodide.

Pb(NO3)2 + 2KI → PbI2 ↓ + 2KNO3 (Yellow ppt.) (ii) Barium chloride and sodium sulphate solutions on mixing form a white precipitate due to formation of barium sulphate. BaCl2 + Na2SO4 → BaSO4↓ + 2NaCl (White ppt.)

(i) Give an example of a chemical reaction which is accompanied by evolution of a gas as well as rise in temperature.

(ii) Classify the following reactions as exothermic and endothermic : (a) CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) (b) N2(g) + O2(g) → 2NO(g)

Ans. (i) When zinc reacts with dilute sulphuric acid, a gas (i.e., hydrogen) is evolved and also a rise in temperature takes place.

Zn(s) + H2SO4(dil) → ZnSO4(aq) + H2(g) ↑(ii) (a) This is an exothermic reaction as heat is evolved during the reaction.(b) This is an endothermic reaction as heat is absorbed during the reaction.

YOURSELF1. What are the characteristics of a chemical reaction?

2. Why is respiration considered an exothermic process and photosynthesis considered an endothermic process?

3. What happens chemically when quick lime is added to water filled in a bucket?

For YOURSELF Solutions visit http://bit.ly/mtg-100-sc-tryyourself

Chemical EquationsThe method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it is known as a chemical equation.Chemical equation is a way to represent the chemical reaction in concise and informative way.

– The reactants are written on the left hand side with plus sign (+) between them.

– The products are written on the right hand side with plus sign (+) between them.

– An arrow (→) separates the reactants from the products. The direction of the arrow head points towards the products, i.e., direction of occurrence of reaction.

There are two ways of representing a chemical equation :In terms of words (called word equation) :

Magnesium + oxygen → Magnesium oxideIn terms of symbols and formulae (called chemical equation) :Mg + O2 →MgOBalanced and unbalanced chemical equationsA balanced chemical equation is one which contains an equal number of atoms of each element on both the sides of the equation.An unbalanced chemical equation is one in which the number of atoms of the elements on the two sides of the equation is not the same. An unbalanced chemical equation is also called skeletal chemical equation.KI + CI2 KCl + I2 (Unbalanced chemical equation)2KI + CI2 2KCl + I2 (Balanced chemical equation)


H O

C3H8 5O2 4H2O

C

+ → 3CO2 +

Need for balancing a skeletal chemical equationAccording to the law of conservation of mass, “Mass can neither be created nor destroyed in a chemical reaction”. To obey this law the total mass of elements present in reactants must be equal to the total mass of elements present in the products. In other words, the number of atoms of each element remain the same, before and after a chemical reaction. Hence, we need to balance skeletal chemical equation.

Balancing of chemical equationsThe number of atoms of each element should be equal on both sides of the equation. The equation can be balanced by hit and trial method.

Hit and trial method There is no definite rule for balancing the equation by this method. The following steps are followed for balancing :– Write the skeletal equation for the reaction using symbols and chemical formulae and count the number of atoms of each

element on both sides of the equation.– Select the biggest formula and balance the elements of this compound by hit and trial and then balance other elements one

by one.– If the selection of the biggest formula is inconvenient then balance the atoms of that element which occurs at minimum

number of places on both sides of the equation.– Check the correctness of the chemical equation by counting the number of atoms of each element on both sides of the

equation.– Do not change the formulae of the substances while balancing the equation. To be precise, do not change the subscripts of

the formulae but only place a whole number coefficient before the formula to increase the number of atoms e.g., for four O atoms, write 4H2O and not H2O4 or (H2O)4.

– Let us understand the balancing of chemical equations with the help of following example :Magnesium metal reacts with hydrochloric acid to form magnesium chloride and hydrogen gas.The word equation for the reaction is written asMagnesium + Hydrochloric acid Magnesium chloride + HydrogenThe symbol equation for the same reaction is written as Mg + HCl MgCl2 + H2Step I : Let us count the number of atoms of all the elements in the reactants and products on both sides of the equation.

Element No. of atoms of reactants (L.H.S) No. of atoms of products (R.H.S)

MgClH

111

122

A simple look at the equation reveals that the number of Mg atoms are equal (1) on both sides. At the same time, the number of Cl and H atoms are not equal.Step II : It is always convenient to start with the compound that contains the maximum number of atoms, whether a reactant or a product. Then, use appropriate coefficients to balance its atoms. In this case, the compound is MgCl2. Here, Mg atoms are already balanced on both sides. To balance the atoms of Cl, put coefficient 2 before HCl on the reactant side. The equation may now be written as : Mg + 2HCl MgCl2 + H2


Step III : Now, check the number of atoms of different elements on both sides of the equation. They are equal. This means that the equation is balanced.

A balanced chemical equation conveys the following informations – Name of the reactants and products– Qualitative information– No. of molecules or atoms of reactants and products taking part in the reaction– No. of moles of each substance involved in the reaction– Mass of each substance involved in the reaction– Mass-mass, mass-volume, volume - volume relationship between the reactants and products

Making a chemical equation more informativeTo make a chemical equation more informative the following facts needs to be mentioned :

The physical states of the substances– Gaseous state is represented by the symbol ‘g ’ or by the symbol (↑).– Liquid state is represented by the symbol ’l ’.– Solid state is represented by the symbol ’s ’.– Aqueous solution (substance dissolved in water) is represented by the symbol ‘aq’.– The precipitate, if formed, during the reaction is indicated by the symbol (↓). e.g., 2Na(s) + 2H2O(l ) 2NaOH(aq ) + H2(g) or H2↑ AgNO3(aq) + NaCl(aq) AgCl(s) ↓ or AgCl(ppt.) + NaNO3(aq )

Concentration of the acidIf acid is present as one of the reactants, it may be diluted or concentrated.– The symbol ‘dil.’ is used for dilute acid.– The symbol ‘conc.’ is used for concentrated acid. e.g., Zn(s) + H2SO4(dil.) ZnSO4(aq) + H2(g) Cu(s) + 4HNO3(conc.) Cu(NO3)2(aq) + 2NO2(g) + 2H2O(l)

Reaction conditions :The specific conditions such as temperature, pressure, catalyst etc. for the reaction are indicated above or below the arrow in the equation.

N2(g) + 3H2(g) 450°C, 200-900 atm

Fe + Mo 2NH3(g)

– A catalyst is a substance which increases the rate of a reaction. However, a catalyst is not used up or consumed during a reaction. The property of a catalyst to alter the rate of a chemical reaction is called catalysis.

– If a reaction takes place on heating, the sign D(delta) is put on the arrow. e.g., 2KClO3(s)

D 2KCl(s) + 3O2(g)– If a reaction takes place in presence of sunlight, the word ‘sunlight’ is written on the arrow. e.g.,

H2(g) + Cl2(g) Sunlight

2HCl(g) Such reactions are called photochemical reactions.

Evolution/absorption of energyEvolution or absorption of heat can be indicated by writing (+ Heat) or (– Heat) respectively on the right hand side of a chemical equation.e.g., C(s) + O2(g) CO2(g) + Heat (393 kJ / mole) (Exothermic reaction) C(s) + 2S(g) CS2(g) – Heat (92 kJ / mole) (Endothermic reaction)or C(s) + 2S(g) + Heat CS2(g)

Information not conveyed by chemical equation(i) A chemical equation does not tell us about the feasibility of a chemical reaction. It means that from the equation, we cannot

make out whether a particular reaction will actually take place or not.


(ii) We cannot get any clue or idea about the rate (speed) of the reaction from the chemical equation i.e., it does not tell us that a particular reaction will be slow, moderate or fast.

ILLUSTRATIONS Aqueous solutions of sulphuric acid and sodium hydroxide react to form aqueous sodium

sulphate and water. Write the balanced equation.

Ans. 2NaOH(aq) + H2SO4(aq) Na2SO4(aq) + 2H2O(l)

When hydrogen is passed over copper oxide, copper and steam are formed. Write a balanced equation for this reaction and state which of the chemicals are (i) elements (ii) compounds (iii) reactants and (iv) products.Ans. CuO(s) + H2(g) Cu(s) + H2O(g)(i) Elements Cu, H2 (ii) Compounds CuO, H2O(iii) Reactants CuO, H2 (iv) Products Cu, H2O

YOURSELF4. State the difference between balanced and unbalanced equations.5. Balance the following chemical equation : MnO2 + HCl → MnCl2 + Cl2 + H2O6. Represent the following statement into chemical equation and then balance it : An aqueous solution of ferrous sulphate reacts with an aqueous solution of sodium hydroxide

to form a precipitate of ferrous hydroxide and sodium sulphate remains in solution.


NCERT FOCUS1. Why should a magnesium ribbon be cleaned before burning in air?

2. Write the balanced equations for the following chemical reactions.(a) Hydrogen + Chlorine → Hydrogen chloride(b) Barium chloride + Aluminium sulphate →

Barium sulphate + Aluminium chloride(c) Sodium + Water → Sodium hydroxide

+ Hydrogen

3. Write a balanced chemical equation with state symbols for the following reactions.(a) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.(b) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

4. A solution of the substance ‘X’ is used for white washing.(i) Name the substance ‘X’ and write its formula.(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

5. What is a balanced chemical equation? Why should chemical equations be balanced?

6. Translate the following statements into chemical equations and then balance them.(a) Hydrogen gas combines with nitrogen to form ammonia.(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.


7. Balance the following chemical equations.(a) HNO3 + Ca(OH)2 →Ca(NO3)2 + H2O(b) NaOH + H2SO4 → Na2SO4 + H2O(c) NaCl + AgNO3 → AgCl + NaNO3(d) BaCl2 + H2SO4 → BaSO4 + HCl

8. Write the balanced chemical equations for the following reactions.(a) Calcium hydroxide + Carbon dioxide →

Calcium carbonate + Water

(b) Zinc + Silver nitrate → Zinc nitrate + Silver(c) Aluminium + Copper chloride →

Aluminium chloride + Copper (d) Barium chloride + Potassium sulphate →

Barium sulphate + Potassium chloride

9. What does one mean by exothermic and endothermic reactions? Give examples.

10. Why is respiration considered as an exothermic reaction ? Explain.

CBSE FOCUSObjective Type QuestionsMCQs and VSA Type Questions (1 Mark)1. In which of the following chemical equations, the abbreviations represent the correct states of the reactants and products involved at reaction temperature?(a) 2H2(l) + O2(l) → 2H2O(g)(b) 2H2(g) + O2(l) → 2H2O(l)(c) 2H2(g) + O2(g) → 2H2O(l)(d) 2H2(g) + O2(g) → 2H2O(g) (NCERT Exemplar)2. Assertion : Respiration is an exothermic process.Reason : The glucose combines with oxygen in the cells of our body and provides energy.(a) If both assertion and reason are true and reason is the correct explanation of assertion.(b) If both assertion and reason are true but reason is not the correct explanation of assertion.(c) If assertion is true but reason is false.(d) If assertion is false but reason is true.3. Which one of the following processes involve chemical reaction?(a) Storing of oxygen gas under pressure in a gas cylinder(b) Liquefaction of air(c) Keeping petrol in a china dish in the open(d) Heating copper wire in presence of air at high temperature (NCERT Exemplar)4. When lead nitrate reacts with potassium iodide, yellow precipitate of _________ is formed.

(a) PbI2 (b) KNO3(c) Pb(NO3)2 (d) PbIO35. Consider the following reaction:pMg3N2 + qH2O →rMg(OH)2 + sNH3When the equation is balanced, the coefficients p, q, r, s respectively are(a) 1, 3, 3, 2 (b) 1, 6, 3, 2(c) 1, 2, 3, 2 (d) 2, 3, 6, 26. What is wrong with the following chemical equation :Mg + O → MgO Write it after correcting it.7. Write chemical equation for the following reaction: When solid mercury (II) oxide is heated, liquid mercury and oxygen gas are produced.8. What do you mean by ‘skeletal equation’?9. Give a reaction which is associated with a change in colour and in the physical state.10. Which among the following changes are exothermic or endothermic in nature?(i) Decomposition of ferrous sulphate(ii) Dilution of sulphuric acid(iii) Dissolution of sodium hydroxide in water(iv) Dissolution of ammonium chloride in water

Short Answer Type QuestionsSA Type Questions (2/3 Marks)11. Balance the following chemical equations and state whether they are exothermic or endothermic.(i) Na + H2O NaOH + H2(ii) FeSO4 Fe2O3 + SO2 + SO3

For Solutions visit http://bit.ly/mtg-100-sc-ncertfocus


12. Change the following reactions into balanced chemical equations :(i) Manganese dioxide is heated with aluminium powder.(ii) Iron is treated with steam.13. Why is the burning of a candle a physical change whereas burning of the candle wax a chemical change ?14. (a) State the law that is followed for balancing a chemical equation.(b) Balance the following chemical equation : CaC2 + H2O Ca(OH)2 + C2 H215. Balance the following chemical equations :

(i) Al(OH)3 D

Al2O3 + H2O(ii) Pb(NO3)2 + Fe2(SO4)3

Fe(NO3)3 + PbSO4↓16. Represent each of the following word equations with a balanced chemical equation.(a) Disilane gas (Si2H6) undergoes combustion to form solid silicon dioxide and water.(b) Solid aluminium hydride is formed by a combination reaction of its two elements.(c) When solid calcium bisulphite Ca(HSO3)2 is heated, it decomposes to solid calcium oxide, sulphur dioxide gas, and water.17. What are the products formed when magnesium ribbon is burnt in air ? If the product

is dissolved in water, what will be the nature of the solution and what is it due to ?18. Explain the significance of photosynthesis. Write the balanced chemical equation involved in the process. (Board Term-I 2017)

Long Answer Type QuestionsLA Type Questions (5 Marks)19. Express the following reactions in the form of chemical equations and then balance these equations :(i) Phosphorus burns in oxygen to form phosphorus pentoxide.(ii) Silver is precipitated out when a copper strip is dipped in silver nitrate solution. The solution turns blue due to the formation of copper(II) nitrate.(iii) Magnesium ribbon is burnt in a jar containing nitrogen gas when a white powder of magnesium nitride is obtained.(iv) Marble (calcium carbonate) dissolved in hydrochloric acid to give calcium chloride, water and carbon dioxide.(v) Sodium hydroxide reacts with sulphuric acid to form sodium sulphate and water.20. What information is conveyed by the following chemical equation?CaCO3 + 2HCl →CaCl2 + H2O + CO2

1. (d) : 2H2(g) + O2(g) → 2H2O(g)Hydrogen gas reacts with oxygen gas to give water vapours.

2. (a)

3. (d) : When copper wire is heated in presence of air at high temperature copper oxide is formed hence, it is a chemical reaction. Other changes are just physical changes.

4. (a) : Pb(NO3)2 + 2KI → PbI2↓ + 2KNO3 Yellow ppt.

5. (b) : Mg3N2 + 6H2O → 3Mg(OH)2 + 2NH36. Oxygen should be in molecular form (O2).2Mg + O2 → 2MgO7. 2HgO(s) 2Hg(l) + O2(g)8. An equation in which the number of atoms of one or more elements is not equal on the two sides of the equation is called a skeletal (or unbalanced) equation.

9.

colourless

Pb(NO3)2(aq)colourless yellow precipitate

+ +PbI2(s)↓ 2KNO3(aq)2KI(aq)

10. (i) Endothermic(ii) Exothermic(iii) Exothermic(iv) Endothermic

11. (i) 2Na + 2H2O 2NaOH + H2 (Exothermic)(ii) 2FeSO4

Heat Fe2O3 + SO2 + SO3 (Endothermic)

12. (i) 3MnO2 + 4AI D 3Mn + 2AI2O3

(ii) 2Fe + 3H2O D Fe2O3 + 3H2

13. Burning of candle is a physical change because the wax of the candle melts to liquid state which again solidifies to solid state but chemically it remains wax. Burning of candle wax is a chemical change because wax burns to form compounds like CO2, water etc.

14. (a) Law of conservation of mass is followed for balancing a chemical equation which states that mass can neither be created nor destroyed in a chemical reaction. That is, the total mass of the elements present in the products of a chemical

ANSWERS


reaction has to be equal to the total mass of the elements present in the reactants in a balanced equation.

(b) CaC2 + 2H2O →Ca(OH)2 + C2H2

15. (i) 2AI(OH)3 D

AI2O3 + 3H2O

(ii) 3Pb(NO3)2 + Fe2(SO4)3 →2Fe(NO3)3 + 3PbSO416. (i) 2Si2H6(g) + 7O2(g) → 4SiO2(s) + 6H2O(l)(ii) 2Al(s) + 3H2(g) → 2AlH3(s)(iii) Ca(HSO3)2(s)

D CaO(s) + 2SO2(g) + H2O(l)

17. Magnesium oxide and magnesium nitride are formed.2Mg + O2 → 2MgO3Mg + N2 → Mg3N2On dissolving in water, magnesium hydroxide is formed.MgO + H2O → Mg(OH)2Mg3N2 + 6H2O →3Mg(OH)2 + 2NH3Solution will be basic due to formation of Mg(OH)2.

18. Photosynthesis means synthesis with the help of light. It is the process that gives life to all living beings.Photosynthesis is a process by which plants utilise carbon dioxide and water in the presence of sunlight to produce glucose and oxygen.

6CO2 + 12H2O C6H12O6 + 6O2 + 6H2OSunlight

ChlorophyllGlucose

19. (i) P4(s) + 5O2(g) → 2P2O5(s)(ii) Cu(s) + 2AgNO3(g) → Cu(NO3)2(aq) + 2Ag↓(iii) 3Mg(s) + N2(aq) → Mg3N2(s)(iv) CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)(v) 2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l)20. CaCO3 + 2 HCI → CaCI2 + H2O + CO2 (40 +12 + 3 ×16) 2(1 + 35.5) (40+2×35.5) (2 ×1 + 16) (12+ 2×16) = 100 = 73 = 111 = 18 = 44

This equation gives the following formation :(i) One molecule of calcium carbonate reacts with two molecules of hydrochloric acid to give one molecule of calcium chloride, one molecule of water and one molecule of carbon dioxide.(ii) One mole of CaCO3 reacts with two moles of HCI acid to give one mole of CaCl2, one mole of H2O and one mole of CO2.(iii) 100 g of CaCO3 react with 73 g of HCI acid to yield 111 g of CaCl2, 18 g of H2O and 44 g of CO2.(iv) 100 g of CaCO3 react with 73g of HCI acid to produce 111 g of CaCl2, 18 g of H2O and 22.4 litres of CO2 at STP.

TOPIC 2 : TYPES OF CHEMICAL REACTIONS AND EFFECTS OF OXIDATION REACTIONS IN EVERYDAY LIFE

Types of Chemical ReactionsA chemical reaction, in general, involves the rearrangement of atoms in the reactants to form products. According to the law of conservation of mass, the total mass of the atoms of the reactants is equal to the total mass of the atoms of the products formed. Thus, neither the atoms disappear from the reactants nor new atoms appear in the products. However, the rearrangement of atoms of the reactants can take place in a number of ways in chemical reactions leading to different types of reactions.

Combination reactionsA reaction in which two or more reactants combine to form a single product is called combination reaction. In combination reactions following three cases are observed :Combination of two elements : Two elements combine to form a new compound. Some examples are :– Combination of hydrogen and chlorine :

Mg

+ →

MgO2Mg(s) + O2(g) → 2MgO(s)

O2

H2(g) + Cl2(g) Sunlight

2HCl(g )

– Formation of iron sulphide :

Fe(s) + S(s) Heat FeS(s)

– Burning of coal : C(s) + O2(g) CO2(g)– Combustion of hydrogen : 2H2(g) + O2(g) 2H2O(l )

Combination of an element and a compound : An element may also combine with a compound to form a new compound some examples are :


– Combination between nitric oxide and oxygen : 2NO(g) + O2(g) 2NO2(g) Nitric oxide Nitrogen dioxide (brown gas)

– Formation of sulphur trioxide :

2SO2(g) + O2(g) 2SO3(g) Sulphur dioxide Sulphur trioxide

Combination of two compounds : Two compounds may combine to form a new compound. Some examples are :– Combination between ammonia and hydrogen chloride gas : NH3(g) + HCl(g) NH4Cl(s) Ammonia Hydrochloric acid Ammonium chloride

– Formation of slaked lime : CaO(s) + H2O(l ) Ca(OH)2(aq) + Heat Calcium oxide Water Calcium hydroxide (quick lime) (slaked lime)

Slaked lime or calcium hydroxide formed by adding water to quicklime (CaO) is in LimestoneCaCO3

Water added

HeatedCarbonated

Slaked limeCa(OH)2

QuicklimeCaO

the form of a white suspension. It is filtered and the solution containing soluble calcium hydroxide is applied on the wall with a brush. Carbon dioxide present in air will come into its contact and as a result, a thin layer of calcium carbonate (white) will be deposited on the wall. This is known as white wash. Ca(OH)2(aq) + CO2(g) CaCO3(s) + H2O(l)

Decomposition reactionsA reaction in which a single reactant breaks down to form two or more products is known as decomposition reaction.Decomposition reaction is just the opposite of combination reaction. The decomposition reaction takes place only when the energy in the form of heat, electricity or light is supplied. Thus, there are three types of decomposition reactions, classified on the basis of energy supplied to carry out the decomposition.Thermal decomposition reactions : When a decomposition reaction is carried out by heating, it is called thermal decomposition reaction.Examples :

2Pb(NO3)2(s) Heat 2PbO(s) + 4NO2(g) + O2(g)

Lead nitrate Lead oxide Nitrogen dioxide (White) (Yellow) (Brown fumes)

CaCO3(s) Heat CaO(s) + CO2(g)

Calcium carbonate Calcium oxide (limestone) (quick lime)

Electrolytic decomposition reactions : When a substance is decomposed by passing electric current, the process is called electrolysis (electro = electric ; lysis = break down).– When electric current is passed through water (mixed with few drops of acid), it is decomposed into hydrogen and

oxygen gas.

9 volt Battery

2H2O(l ) Electric current

2H2(g) + O2(g)– When electric current is passed through molten sodium chloride, it decomposes to give

sodium metal and chlorine gas.

2NaCl(l ) Electric current

2Na(s) + Cl2(g )Photodecomposition reactions : When a chemical compound decomposes on absorbing light energy, then the reaction is called photodecomposition reaction or photolysis.Example : – Silver bromide decomposes in presence of sunlight to give silver metal and bromine gas.


2AgBr(s) Sunlight

2Ag(s) + Br2(g) Silver Silver Bromine bromide (grey) (brown)

The decomposition reactions of silver halides are used in black and white photography.

To demonstrate photodecomposition of silver chloride " Take about 2 g of silver chloride (white in colour) in a clean watch-glass. Place the

watch-glass in bright sunlight for few minutes.

" Silver chloride slowly changes to deep grey colour. It is because, silver chloride on absorbing light energy undergoes photodecomposition and hence, forms silver metal and chlorine.

2AgCl(s) Sunlight → 2Ag(s) + Cl2(g)

Silver chloride Silver Chlorine (grey)

The chlorine gas escapes in air and grey silver metal is left in the watch-glass.


Displacement reactionsThe chemical reaction in which one element takes the position or place of another element present in a compound is called displacement reaction. In these reactions an atom or group of atoms in a molecule is replaced by another atom or group of atoms. These reactions are very common in inorganic chemistry.These reactions occur mostly in solution form and a more active element displaces or removes another less active element from a compound.Examples : – Zinc displaces copper from copper sulphate solution :

Zn CuSO ZnSOZinc Copper sulphate Zinc sulphate

( ) ( ) ( )s aq+ →4 4 aq ++ CuCopper

( )s

– Magnesium displaces copper from copper sulphate solution :

Mg + CuSO MgSO( )Magnesium

4( )Copper sulphate

4( )Magnesi

s aq aq→uum sulphate

( )Copper

+ Cu s

In displacement reactions, only one element displaces the another element in the compound. Hence, they are also called simple displacement reactions.

K

Na

Ca

Mg

Al

Zn

Fe

Pb

H

Cu

Hg

Ag

Pd

Pt

Au

Most reactive

Least reactive

Metals less reactive

than hydrogen

Metals more reactive than

hydrogen

Reactivity Series

Decr

easin

g ch

emic

al r

eact

ivity

Reactivity series of metals or activity series The arrangement of metals in the decreasing order of their reactivities is called reactivity series or activity series of metals.In the activity series, the most reactive metal is placed at the top whereas the least reactive metal is placed at the bottom. The metal which is higher in the reactivity series is more reactive because it can readily lose electrons and change into positive ion.A more reactive metal can displace any less reactive metal from its aqueous salt solution.Displacement of hydrogen from acids by active metals : When metallic zinc reacts with dilute sulphuric acid, it gives hydrogen gas and zinc sulphate.

Zn(s) + H2SO4(aq) ZnSO4(aq) + H2(g)Zinc Dil. sulphuric

acid

Zinc sulphate Hydrogen gas


Displacement of hydrogen gas from water by active metals : Active metals like K, Na, Ca displace hydrogen from cold water while Mg, Al, Zn, Fe from boiling water or steam. 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g) Sodium Cold water Sodium Hydrogen hydroxide

Displacement of a less active non-metal by a more active non-metal : When chlorine water is added to potassium bromide solution, bromine is liberated. 2KBr(aq) + Cl2(g) 2KCl(aq) + Br2(g) Potassium Chlorine Potassium Bromine bromide chloride

" Most of the common displacement reactions occur in aqueous solutions. However, some displacement reactions also occur between solid substances. The displacement reactions of metals with metal oxides are examples of such type. In these a more reactive metal displaces a less reactive metal from its oxide.

CuO(s) + Mg(s) MgO(s) + Cu(s) Copper oxide Magnesium Magnesium oxide Copper (Black) (Silvery white) (White powder) (Reddish brown)

" All displacement reactions are exothermic.

" Displacement reactions are used in the extraction of silver and gold.

To study the reaction between copper and silver nitrate

" Take silver nitrate solution in a beaker. The solution of silver nitrate is clear and colourless. Place a copper coil in this solution. Keep the beaker undisturbed for some time.

" After sometime the solution becomes blue due to the formation of Cu(NO3)2 and a shiny coating of silver gets deposited on the wire.

" Copper is more reactive than silver. Hence, it displaces silver from silver nitrate.


Double displacement reactionsThe reactions in which two compounds react to form two different compounds by mutual exchange of ions are called double displacement reactions.Double displacement reactions are generally ionic reactions taking place in aqueous solution in which ions participate and there is an exchange of ions. Examples :

– Exchange of ions between silver nitrate and sodium chloride solution

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq) Silver nitrate Sodium chloride Silver chloride Sodium nitrate (White ppt.)

– Barium chloride solution reacts with dilute sulphuric acid with the exchange of partner ions to give a precipitate of barium sulphate.

BaCl2(aq) + H2SO4(dil) BaSO4(s) ↓ + 2HCl Barium chloride Sulphuric acid Barium sulphate Hydrochloric acid (white precipitate)

Precipitation reactionsWhen the aqueous solution of two ionic compounds react by exchanging their ions/radicals, to form two or more new compounds, of which one product is an insoluble salt that forms precipitate, the double displacement reaction is said to be precipitation reaction.


Examples :

Reaction between lead nitrate and potassium iodide solution

– When lead nitrate solution is mixed with potassium iodide solution a yellow precipitate is formed. This reaction is a precipitation reaction and can be expressed as follows :

Pb(NO3)2(aq) + 2KI(aq) PbI2(↓) + 2KNO3(aq) Lead nitrate solution Potassium iodide Lead iodide Potassium nitrate (colourless) solution (colourless) (yellow ppt.) solution

– Aqueous solution of sodium sulphate reacts with barium chloride solution to give white precipitate of barium sulphate.

Na2SO4(aq) + BaCl2(aq) 2NaCl(aq) + BaSO4↓ Sodium sulphate Barium chloride Sodium chloride Barium sulphate (White ppt.)

Neutralisation reactions

Neutralisation

BaseAcid

Salt + Water

When an aqueous solution of an acid reacts with a base (alkali) by exchanging their ions/radicals to form salt and water as the only products, the reaction which takes place is called neutralisation reaction. e.g., H2SO4(aq) + 2NaOH(aq) Na2SO4(aq) + 2H2O(l) Dil. sulphuric acid Sodium hydroxide Sodium sulphate Water

Oxidation and Reduction ReactionsOxidationAny process which involves – addition of oxygen (or non-metallic element)– or removal of hydrogen (or metallic element)– or loss of electron(s) from an atom or an ion is called oxidation reaction.The substance which can bring about oxidation of other substances is called oxidising agent.

Reduction Any process which involves– addition of hydrogen (or metallic element)– or removal of oxygen (or non-metallic element)– or gain of electrons(s) by atom an or ion is called reduction reaction.The substance which can bring about reduction of other substances is called a reducing agent.

According to the electronic concept, oxidation and reduction are defined as follows :

" The reduction in which a chemical species loses one or more electrons is called oxidation reaction. Examples : Cu Cu2+ + 2e– (loss of two electrons by Cu) Fe2+ Fe3+ + e– (loss of one electron by Fe2+)

" The reaction in which a chemical species accepts/gains one or more electrons is called reduction reaction. Examples : Fe3+ + e– Fe2+

ferric ion ferrous ion

Cl2 + 2e– 2Cl–

chlorine chloride ion

" The chemical species which can remove electrons from some other species is called an oxidising agent. In a redox reaction, the oxidising agent gets reduced.

Examples : Sn4+ + 2e– Sn2+ (Sn4+ is an oxidising agent) tin(IV) ion tin(II) ion

F2 + 2e– 2F– (F2

is an oxidising agent)

fluorine fluoride ion


" The chemical species which can lose electrons to some other species is called a reducing agent. A reducing agent itself gets oxidised in a redox reaction.

Examples : Fe2+ Fe3+ + e– (Fe2+ is a reducing agent) iron(II) ion iron(III) ion 2I–

I2 + 2e– (I– ion is a reducing agent)

iodide ion iodine

Redox reactionsChemical reactions where oxidation and reduction both take place simultaneously are called redox reactions. In the word, ‘REDOX’ ‘RED’ stands for reduction and ‘OX’ stands for oxidation.Examples :

Cu2O(s) + H2(g)Cu2O – Oxidising agent

H2 – Reducing agent

;Cu(s) + H2Oheat

Oxidation

Reduction

Fe2O3 – Oxidising agent

CO – Reducing agent;Fe2O3(s) + 3CO(g) 2Fe + 3CO2(g)

Oxidation

Reduction

Cu2+ – Oxidising agent

Zn – Reducing agent;Zn(s) + Cu

2+(ag) Zn

2+(aq) + Cu(s)

Oxidation

Reduction

Redox reactions play a vital role in our daily life. Generation of electricity in batteries, production of heat energy by burning chemical substances, extraction of metals, manufacture of a number of useful products are common examples of redox reactions.

ILLUSTRATIONS What are non-redox reactions? Explain with the help of examples.

Ans. The reaction in which neither oxidation nor reduction takes place are called non-redox reactions. e.g.(i) HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l ) (ii) BaCl2(aq) + Na2SO4(aq) BaSO4(s) + 2NaCl(aq)

Can a displacement reaction be a redox reaction? Explain with the help of an example.Ans. Yes, a displacement reaction can be a redox reaction. Consider a displacement reaction,e.g. Cu + 2AgNO3 Cu(NO3)2 + 2AgHere, Cu displaces Ag from AgNO3 solution. Cu is oxidised to Cu(NO3)2 and AgNO3 is reduced to Ag. Therefore, it is a redox reaction.

YOURSELF7. Oxidation can never occur without reduction and vice-versa comment.8. What is the difference between displacement and double displacement reactions. Give

examples for these reactions.9. In the following chemical reaction, represent oxidation and reduction reactions. Also

mention oxidising and reducing agents. H2S + Cl2 2HCl + S10. What is meant by combination reactions and decomposition reactions? Give one example

in each.



Effects of Oxidation Reactions in Everyday LifeWe all are aware of the fact that oxygen is the most essential element for sustaining life. It is involved in a variety of reactions which have wide range of effects on our daily life. Most of them are quite useful while a few may be harmful in nature. Some of these effects are briefly discussed here :

Corrosion Corrosion is the process in which metals are eaten up gradually by the action

Corrosion on an iron object

of air, moisture or a chemical such as an acid on their surface.Corrosion needs oxygen (or air) and moisture to take place and is accelerated by the presence of electrolytes in water.Rusting of iron metal is the most common example of corrosion.Rust is a chemical substance brown in colour and is formed by the chemical action of moist air (containing O2 and H2O) on iron. It is basically an oxidation reaction and formula of rust is Fe2O3.x H2O. Corrosion is very slow process in nature and once started keeps on occurring.

232 2 2 2 3 2

Fe O H O Fe O H OHydrated ferric oxide

(rust)

+ + →x x.

Similarly, copper objects lose their lustre after sometime. In the presence of moist air, a thin film of green basic copper carbonate is formed on its surface and therefore, it corrodes.

2Cu(s) + CO2(g) + O2(g) + H2O(l) CuCO3·Cu(OH)2Air

Watervapour

Basic coppercarbonate (Green)

Likewise, silver metal gets tarnished in an atmosphere containing traces of H2S due to the formation of black silver sulphide.2Ag(s) + H2S(g) Ag2S(g) + H2(g)↑

Hydrogensulphide(From air)

Harmful effect of corrosion Both corrosion and rusting are very harmful and cause damage to the buildings, railway tracks automobiles and other objects/materials where metals are used. We quite often hear that an old building has collapsed of its own causing loss of both lives and property. This is on account of the rusting of iron which is used in making the structures particularly the roof.

There are some cases where corrosion plays a useful role. We all know that aluminium is placed high in the activity series and is expected to be quite reactive. It combines with oxygen present in air to form its oxide called aluminium oxide (Al2O3) and this is a case of corrosion. The metal oxide formed slowly gets deposited on the surface of aluminium and forms a protective coating on the surface. This coating makes the metal passive to the attack by water, air, acids and alkalies, etc. hence protects the metal underneath from further damage. Aluminium foils are commonly used for packing food preparations, cigarettes, etc.

Methods used to prevent rustingRusting can be prevented if iron objects are not allowed to come in contact with damp air by using a barrier. This method is called barrier protection. The barrier protection can be achieved by any of the following methods :– The surface is coated with paint or some chemicals, e.g., biphenol.– The surface is protected by applying a thin film of oil or grease.– The metal is electroplated with metals like tin, nickel, zinc, chromium, aluminium, etc.Sacrificial protection : In this, iron is protected from rusting by covering it with a layer of more active metal (e.g., Mg, Zn) than iron. The active metal like Mg, Zn, etc., gets corroded itself but saves the object. This is called sacrificial protection. Zinc is commonly used for covering the iron surfaces. The process of coating iron with zinc is called galvanisation.Using anti-rust solutions : To retard the corrosion of iron, certain anti-rust solutions like solutions of alkaline phosphates and alkaline chromates are generally used.


RancidityThe aerial oxidation of fats and oils present in food materials resulting in unpleasant smell and taste is called rancidity. Rancidity spoil the food materials prepared in fats and oils which have been kept for a considerable time and make them unfit for eating.

Methods to check rancidity

Fresh Oil Rancid Oil

Following measures can be adopted to prevent or slow down rancidity : – By vacuum packing : Food materials are often packed in air tight

containers. Oxygen has no access to them and oxidation resulting in rancidity is prevented. These days, preference is given to vacuum packing.

– By refrigeration of food stuffs : Refrigeration of food also slows down rancidity because the temperature inside refrigerator is very low and direct contact with air or oxygen is avoided.

– By packaging fat and oil containing foods in nitrogen gas : In bags containing potato chips and other similar stuff, the air is quite often replaced by nitrogen. This checks their oxidation as well as rancidity.

– By storing food away from light : It is always advisable to place food materials and cooked food in places away from direct sunlight. This will slow down the process of rancidity.

– By adding antioxidants : Two commonly used antioxidants are BHA (Butylated hydroxyanisole) and BHT (Butylated hydroxytoluene). Addition of these antioxidants to food materials increases their storage life.

ILLUSTRATIONS (a) What is rust chemically?

(b) Why iron corrodes but aluminium does not?Ans. (a) Rust is hydrated ferric oxide (Fe2O3 ⋅ x H2O).(b) Iron reacts with oxygen and moisture of the air to form rust which peels off easily and fresh surface of iron is again exposed to air. Aluminium reacts with oxygen to form impervious and hard layer of aluminium oxide which does not allow the underneath surface to come in contact with air.

To preserve food items, we keep them in a refrigerator. why? Ans. Keeping in refrigerator lowers the temperature. As a result, oxidation of the food is slowed down. Hence, the food can be preserved for longer time.

YOURSELF11. What are the necessary conditions for rusting of an iron article?12. What do you mean by rancidity?13. Suggest two methods to prevent rusting.14. Why is copper vessel covered with a green coating in rainy season?


NCERT FOCUS1. Identify the substances that are oxidised and the substances that are reduced in the following reactions.(i) 4Na(s) + O2(g) → 2Na2O(s)(ii) CuO(s) + H2(g) → Cu(s) + H2O(l)

2. Write the balanced chemical equations for the following and identify the type of reaction in each case.(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)


(b) Zinc carbonate(s) → Zinc oxide(s)+ Carbon dioxide(g)

(c) Hydrogen(g) + Chlorine(g) →Hydrogen chloride(g)

(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)

3. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.4. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.5. What is the difference between displacement and double displacement reactions? Write equations for these reactions.6. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.7. What do you mean by a precipitation reaction? Explain by giving examples.8. Explain the following in terms of gain or loss of oxygen with two examples each.(a) Oxidation (b) Reduction9. A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.10. Why do we apply paint on iron articles?

11. Oil and fat containing food items are flushed with nitrogen. Why?12. Explain the following terms with one example each.(a) Corrosion (b) Rancidity13. Identify the oxidising agent (oxidant) in the following reactions.(a) Pb3O4 + 8HCl → 3PbCl2 + Cl2 + 4H2O(b) 2Mg + O2 → 2MgO(c) CuSO4 + Zn → Cu + ZnSO4(d) V2O5 + 5Ca → 2V + 5CaO(e) 3Fe + 4H2O → Fe3O4 + 4H2(f) CuO + H2 → Cu + H2O14. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.(a) Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773 K to form ammonia gas.(b) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water.(c) Ethanol is warmed with ethanoic acid to form ethyl acetate in the presence of concentrated H2SO4.(d) Ethene is burnt in the presence of oxygen to form carbon dioxide, water and releases heat and light.

15. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

CBSE FOCUSObjective Type Questions MCQs and VSA Type Questions (1 Mark)1. The following reaction is an example of a 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g)(i) displacement reaction(ii) combination reaction(iii) redox reaction(iv) neutralisation reaction(a) (i) and (iv) (b) (ii) and (iii)(c) (i) and (iii) (d) (iii) and (iv) (NCERT Exemplar)2. The given diagram represents a __________ reaction.

MercuryOxygen gas

Mercury(II)oxide

(a) photodecomposition(b) electrolysis(c) displacement(d) thermal decomposition

3. Which of the following statements is correct?Rusting of iron is a chemical change because

For Solutions visit http://bit.ly/mtg-100-sc-ncertfocus


(a) a new substance with new properties is produced(b) chemical composition of reactant is changed(c) change is permanent and cannot be reversed easily(d) all of these.4. Assertion : Following reaction of iron is a redox reaction.4Fe + 3O2 → 4Fe

3+ + 6O2–.Reason : The metallic iron is oxidised to Fe3+ and O2 is reduced to oxide ion.(a) If both assertion and reason are true and reason is the correct explanation of assertion.(b) If both assertion and reason are true but reason is not the correct explanation of assertion.(c) If assertion is true but reason is false.(d) If assertion is false but reason is true.

5. Which of the following informations about the reaction of quick lime with water is not true?(a) Quick lime reacts with water vigorously.(b) During the reaction the test tube becomes hot.(c) Quick lime reacts with water to form slaked lime.(d) During the reaction dazzling white light is produced.6. Why does zinc react with dilute sulphuric acid to give hydrogen gas but copper does not ? 7. Define the term oxidising agent.8. What is the difference between the following two type of reactions? AgNO3 + HCl → AgCl + HNO3 Mg + 2HCl → MgCl2 + H29. Double displacement reactions are also known as precipitation reactions. Why?10. A silver spoon is kept immersed in an aqueous copper sulphate solution. What change will take place?

Short Answer Type QuestionsSA Type Questions (2/3 Marks)11. Write combination reactions that occur when the calcium reacts with the following :(a) Hydrogen (b) Bromine(c) Nitrogen12. Below are given two chemical reactions :(i) 2KBr(aq) + Cl2(aq) → 2KCl(aq) + Br2(aq)(ii) Fe(s) + S(s) → FeS(s)Which is combination reaction and which is displacement reaction? Give explanation also.

13. Select the oxidising agent and the reducing agent from the following reaction : H2S(s) + I2 → 2HI + S Hydrogen sulphide Iodine Hydrogen iodide Sulphur

14. 2 g of ferrous sulphate crystals are heated in a dry boiling tube.(a) List any two observations.(b) Name the type of chemical reaction taking place.(c) Write balanced chemical equation for the reaction and name the products formed. (AI 2019, Board Term I 2017)15. Decomposition reactions require energy either in the form of heat or light or electricity for breaking down the reactants. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light and electricity. (2018)16. What is reduction reaction? Identify the substances that are oxidised and the substances that are reduced in the following reactions :(i) Fe2O3 + 2Al →Al2O3 + 2Fe(ii) 2PbO + C →2Pb + CO217. (a) What is ‘corrosion’ ? Why aluminium sheets do not corrode easily ?(b) The marble statues often slowly get corroded when kept in open for a long time. Assign a suitable explanation.18. (a) Can a displacement reaction be a redox reaction? Explain with the help of an example. (b) Write the type of chemical reaction in the following :(i) Reaction between an acid and a base(ii) Rusting of iron. (Board Term I, 2017)

Long Answer Type QuestionsLA Type Questions (5 Marks)19. You are provided with two containers made up of copper and aluminium. You are also provided with solutions of dilute HCl, dilute HNO3, ZnCl2 and H2O. In which of the above containers these solutions can be kept?20. (a) Aluminium is a reactive metal but sill used for packing food articles. Why?(b) Can rusting of iron take place in distilled water?(c) Give one example each of the reaction involving combination of(i) an element with another element (ii) an element with a compound(iii) a compound with another compound.


1. (c) : It is a displacement reaction since H of NH3 is displaced by O. It is a redox reaction since NH3 is oxidised to NO and O2 is reduced to H2O.

2. (d) : It is a thermal decomposition reaction. When mercury(II) oxide is heated strongly, it decomposes to give the elements mercury and oxygen.

3. (d) : Iron reacts with oxygen in presence of water to give hydrated iron oxide which is known as rust. The change is an irreversible change.

4. (a) : Fe is oxidised to Fe3+ and acts as reducing agent.

5. (d) : Quick lime (calcium oxide) reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

6. This is because zinc is more reactive than hydrogen whereas copper is less reactive than hydrogen in the activity series.

7. The substance which either gives oxygen or gains hydrogen in a reaction, is known as an oxidising agent.

8. The first reaction is a double displacement reaction whereas second reaction is a single displacement reaction.

9. Double displacement reactions are also known as precipitation reactions because they are accompanied by the formation of a precipitate.

10. No change will take place and the silver spoon will remain unaffected because no chemical reaction takes place between silver and copper sulphate solution as silver is placed below copper in the activity series.

11. (a) Ca(s) + H2(g) → CaH2(s)(b) Ca(s) + Br2(l) → CaBr2(s)(c) 3Ca(s) + N2(g) → Ca3N2(s)12. (i) In the first reaction, potassium bromide solution reacts with chlorine solution to form potassium chloride solution and bromine. So, in this reaction, chlorine is displacing bromine from potassium bromide to form potassium chloride and bromine. Thus, it is a displacement reaction.(ii) In the second reaction, iron combines with sulphur to form iron(II) sulphide. So, it is a combination reaction.

13. (i) H2S is changing into S. This is the removal of hydrogen form H2S. By definition, the removal of hydrogen is known as oxidation, therefore, hydrogen sulphide is being oxidised to sulphur. Iodine is removing the hydrogen from H2S, so iodine is the oxidising agent.

(ii) I2 is changing into HI. This is the addition of hydrogen to iodine. By definition, addition of hydrogen is known as reduction, therefore, iodine is being reduced to hydrogen iodide. Hydrogen sulphide is supplying the hydrogen required for reduction, so hydrogen sulphide is the reducing agent.

14. (a) Ferrous sulphate crystals (FeSO4.7H2O) lose water when heated and anhydrous FeSO4 is formed. It then further decomposes to ferric oxide (Fe2O3), sulphur dioxide (SO2) and sulphur trioxide (SO3).(i) Colour of the crystals changes from green (FeSO4⋅7H2O) to reddish brown (Fe2O3).(ii) Smell of burning sulphur.(b) This is a decomposition reaction.(c) 2FeSO4(s) Fe2O3(s) + SO2(g) + SO3(g)

Ferrous sulphate Ferric oxide Sulphurdioxide

Sulphurtrioxide

15. Decomposition reaction involving absorption of heat :ZnCO3(s) D ZnO(s) + CO2(g)

Zinc carbonate Zinc oxide Carbon dioxide

Decomposition reaction involving absorption of light : 2H2O2(l)

Light 2H2O(l) + O2(g) Hydrogen peroxide Water Oxygen

Decomposition reaction involving absorption of electrical energy :

2Al2O3(l) Electric current 4Al(s) + 3O2(g)

Alumina Aluminium Oxygen

16. Reduction reaction involves addition of hydrogen or removal of oxygen from a substance in a reaction.(i) Fe2O3 is reduced and AI is oxidised.(ii) PbO is reduced and C is oxidised.

17. (a) The process of slowly eating up of the metals due to attack of atmospheric gases such as oxygen, carbon dioxide, hydrogen sulphide, water vapour etc. on the surface of the metals so as to convert the metal into oxide, carbonate, sulphide etc. is known as corrosion. Aluminium sheets do not corrode easily because in the presence of air, the surface of aluminium is covered with a protective layer of aluminium oxide.(b) We all know that vapours of poisonous gas sulphur trioxide (SO3) are being constantly released into the atmosphere as a result of the combustion of sulphur which is taking place. These are dissolved by rain water and sulphuric acid results. The rain water containing the acid is often known as acid rain. The acid present slowly reacts with the compound CaCO3 present in marble statue and the latter gets corroded.

ANSWERS


SO3 + H2O → H2SO4CaCO3 + H2SO4 → CaSO4 + CO2↑ + H2O(Marble)

18. (a) Consider the following displacement reaction :Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s)Here, Zn has changed into ZnSO4 (i.e., Zn

2+ ions) by loss of electrons. Hence, Zn has been oxidized. CuSO4 (i.e., Cu

2+) has changed into Cu by gain of electrons. Hence, CuSO4 has been reduced. Thus, the above reaction is a displacement reaction as well as a redox reaction.(b) (i) Neutralisation reaction (ii) Oxidation - reduction reaction

19. (i) When solutions are kept in copper container :(a) Dilute HCl - Copper does not react with dilute HCl Hence, HCl can be kept in it. (b) Dilute HNO3 - Dilute HNO3 being a strong oxidising agent, reacts with copper. Hence, dilute HNO3 cannot be stored in it.(c) ZnCl2 - Copper is less reactive than zinc therefore, no displacement reaction occurs hence, ZnCl2 can be stored in it.(d) H2O - Water does not react with copper hence, water can be stored in it.(ii) When solutions are kept in aluminium container :(a) Dilute HCl - Aluminium reacts with dilute HCl. So, HCl cannot be kept in it. 2Al + 6HCl 2AlCl3 + 3H2

(b) Dilute HNO3 - Aluminium reacts with dilute HNO3 to form a protective layer of aluminium oxide and there is no further reaction hence, HNO3 can be stored in it.(c) ZnCl2 - Aluminium is more reactive than zinc hence, can displace zinc from the solution. ZnCl2 cannot be stored in aluminium vessel. 2Al + 3ZnCl2 2AlCl3 + 3Zn(d) H2O - Aluminium does not react with cold or hot water, therefore, water can be stored in it.Aluminium is attacked by steam to form aluminium oxide and hydrogen.2Al(s) + 3H2O(g) Al2O3(s) + 3H2(g)20. (a) From the position of the aluminium (Al) metal in the activity series, it seems to be quite reactive. However, it is not so reactive. Actually, when the metal is kept in air or oxygen for sometimes, it is converted into its oxide called aluminium oxide (Al2O3). This gets deposited at the surface of the metal as a thin coating. It is rather passive which means that it is not reactive. Therefore, the metal is used for packing food articles.

(b) No, rusting of iron can not take place in distilled water because it neither contains dissolved oxygen nor carbon dioxide. Both are essential for the rusting of iron.

(c) (i) Fe + S D

FeS

(ii) 2CO + O2 2CO2(iii) NH3 + HCI NH4CI

22 100PERCENTScience Class-10CONCEPTMAP

h Unbalanced chemical equation or skeletal equation

A chemical equation in which number of atoms of each kind are not equal in the reactants side and products side e.g.,

Na + H2O → NaOH + H2h Balanced chemical equation

A chemical equation which has an equal number of atoms of each element in reactants and products side e.g.,

2Na + 2H2O → 2NaOH + H2

h Balancing of chemical equation Step 1 : Count the number of

atoms on both sides of equation.

Step 2 : Start balancing with the compound which has maximum number of atoms.

Step 3 : Then balance H-atoms in different compounds.

Step 4 : Balance remaining atoms if any.

Step 5 : Check for the correctness of balanced equation.

Word Equationh The simplest way to write a reaction is

in the form of words. Magnesium + Oxygen → Magnesium Oxide

Reactants Product

Change of State

h H2(g) +

12

O2 (g) → H2O(l)

Change of Colour

h 2Pb(NO3)2(s) → 2PbO(s) + 4NO2(g) + O2(g) (Colourless) (Yellow) (Brown fumes)

Evolution of Gas

h Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)↑

heat

Symbol Equation

h 2Mg + O2 → 2MgO

ObservationsEvolution or absorption of energy

Displacement Reactions

h The chemical reactions in which more reactive element takes the place of less reactive element in a compound.

h Single displacement reactions : The chemical reactions in which one element takes the place of another element in a compound.

Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

+

+AB C A BC

→

→

+

+

h Double displacement reactions : Two compounds react by exchange of ions to form two new compounds.

CuSO4(aq) + H2S(g) → CuS(s) + H2SO4(aq)

Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq) (White ppt.)

This is also known as precipitation reactions.

CHEMICAL EQUATIONS Represented by

The process which transforms one or more substances into

new substances is called chemical reaction.

CHEMICAL REACTIONS

AND EQUATIONS

TYPES OF CHEMICAL REACTIONS

Endothermic reactions

h Reactions which occur with the absorption of heat.

N2(g) + O2(g) + heat → 2NO(g)

Exothermic reactions

h Reactions which occur with the evolution of heat

C(s) + O2(g) → CO2(g) + heat

Heat

Heat HeatCooler than Surroundings

Heat

Heat

Heat HeatHotter than Surroundings

Heat


Combination Reactions

h The reactions in which two or more substances combine to form a single substance. CaO(s) + H2O(l ) → Ca(OH)2(aq) Quick lime Slaked lime

+ →

3Mg(s) + N2(g) → Mg3N2(s)

Reactivity Series of Metals

h The arrangement of metals in the decreasing order of their reactivities is called reactivity series of metals.h In general, a more reactive metal (which is placed higher in the reactivity series) can displace the less reactive metal from its salt solution.

Cu + AgNO3 → Cu(NO3)2 + Ag More reactive Less reactive

Oxidation–Reduction Reactions

h Oxidation – Addition of oxygen or removal of hydrogen or loss of electrons.

h Reduction – Removal of oxygen or addition of hydrogen or gain of electrons.

h Ranc id i t y – The change in odour and flavour of oily and fatty foods by oxidation.

h Corrosion – The process of deterioration of a metal as a result of its reaction with air and water present in atmosphere, e.g., Rusting is oxidation of iron.

CuO + H2 → Cu + H2OOxidising

agentReducing

agent Addition of oxyge

n or

Loss of electro

ns

Removal of oxygen or

Gain of electrons

Ways to

prevent

rancidity

Vacuum packaging e.g., is used to store fresh fruits and

vegetables

Storing in a dark place

Using inert gases in

food packaging like N2 gas in chips packet

Using antioxidants e.g., vitamin

A, C & E

Refrigeration

Decomposition Reactions

h The reactions in which a compound breaks up into simpler substances when the energy is supplied in the form of heat, light and electricity.

Photodecompositionh Energy is supplied in

the form of sunlight. 2AgBr(s) → 2Ag(s) + Br2(g)h This reaction is used

in photography.

Sunlight

Thermal decompositionh Energy is supplied in the form of heat.

2FeSO4.7H2O (s) → 2FeSO4(s) → Fe2O3(s) (Green) (Pale green) (Brown) + SO2(g) + SO3(g)

–H2O

Electrical decompositionh Energy is supplied in the form of electricity.

2H2O(l) → 2H2(g) + O2(g)Electrolysis

Potassium Sodium Calcium Magnesium Aluminium Zinc Iron Lead Copper Silver Gold

D E C R E A S I N G R E A C T I V I T Y

Iron

O2 O2Rust

Water dropletsAir

Fe2+

Fe3+

+–

e–

O2 H2

e–

Water

Oxygen gas rises in this test tube.

Hydrogen gas rises in this test tube.

(Reverse reaction is not possible.)


QUESTIONSMISCELLANEOUS1. Grapes hanging on the plant do not ferment but after being plucked from the plant can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change?

Ans. Grapes when attached to the plants are living and their own immune system prevents fermentation, therefore the microbes cannot grow into them. Once plucked, the microbes grow in them under anaerobic conditions and cause fermentation. It is a chemical change.

2. In electrolysis of water, why is the volume of gas collected over one electrode double than that of gas collected over the other electrode?

Ans. In electrolysis of water, hydrogen (H2) gas is liberated at cathode while oxygen (O2) gas is liberated at anode. The overall reaction is

2H2O(l) Electric current

2H2(g) + O2(g)The ratio of H : O in H2O is 2 : 1. Thus, amount of H2 liberated is twice that of O2.

3. You have collected some silver coins. After few days you observe a black coating on silver coins. On rubbing with toothpaste the silver coins regain shine.(a) Which chemical phenomenon is responsible for these coatings ?(b) Write the chemical name and formula of the black coating.(c) Why the silver coins again start shining by rubbing with toothpaste?

Ans. (a) Corrosion is responsible for the coatings.(b) Black coating on silver is silver sulphide, Ag2S. It is formed by the reaction of silver with hydrogen sulphide present in the atmosphere.(c) Toothpaste contains traces of hydrogen peroxide which reacts with Ag2S formed on the surface and restore whiteness.

4. The white solid compound A decomposes quite rapidly on heating in the presence of a black substance X to form a solid compound B and a

gas C. When an aqueous solution of compound B is reacted with silver nitrate solution, then a white precipitate of silver chloride is obtained alongwith potassium nitrate solution. Gas C does not burn itself but helps in burning other things?(i) What is compound A?(ii) What is compound B?(iii) What is gas C?(iv) What do you think is the black substance X? What is its function?

Ans. (i) Potassium chlorate, KCIO3(ii) Potassium chloride, KCI(iii) Oxygen, O2(iv) Manganese dioxide, MnO2; It acts as a catalyst in the decomposition of potassium chlorate to form oxygen gas.

5. During the reaction of some metals with dilute hydrochloric acid, following observations were made, (a) Silver metal does not show any change.(b) The temperature of the reaction mixture rises when aluminium (Al) is added.(c) The reaction of sodium metal is found to be highly explosive.(d) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid.Explain these observations giving suitable reasons.

Ans. (a) Silver does not react with HCI(aq) because it is less reactive than hydrogen.(b) Aluminium reacts with HCI(aq) forming aluminium chloride and hydrogen gas. This is because aluminium is more reactive (or more electropositive) than hydrogen.

2Al(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2(g)↑(c) Sodium metal is highly reactive. It displaces hydrogen from acid solution. The reaction being highly exothermic and violent.(d) Lead displaces hydrogen from acid solutions but slowly. It is because lead chloride formed in the reaction gets deposited on the surface of lead and slows down the reaction.


FOCUSCOMPETITIONMultiple Choice Questions1. Among the following, the endothermic reaction is(a) combination of carbon and oxygen to form carbon monoxide(b) combination of nitrogen and oxygen to form nitrogen monoxide(c) combination of glucose and oxygen to form carbon dioxide and water(d) combination of zinc and hydrochloric acid to form zinc chloride and hydrogen.2. Formation of carbon disulphide from carbon and sulphur takes place by(a) absorption of heat (b) evolution of heat(c) no change in heat content(d) none of the above.3. Study the given diagram carefully.

Identify X, Y and the type of reaction occurring.(a) H2, O2, Decomposition(b) O2, H2, Displacement(c) H2, O2, Displacement(d) O2, H2, Decomposition4. Copper displaces which of the following metal from its salt solution(a) ZnSO4 (b) FeSO4(c) AgNO3 (d) NiSO45. The chemical reaction involved in the corrosion of metal is that of(a) oxidation as well as displacement(b) reduction as well as combination(c) oxidation as well as combination(d) reduction as well as displacement.6. Which of the following is not an example of decomposition reaction?(a) CaCO3(s) → CaO(s) + CO2(g)(b) Ca(OH)2(s) → CaO(s) + H2O(l)(c) CuSO4.5H2O(s) → CuSO4(s) + 5H2O(l)(d) 2KClO3(s) → 2KCl(s) + 3O2(g)

7. In the balanced equation,Cu + xHNO3 → Cu(NO3)2 + yNO2 + 2H2Othe values of x and y are(a) 3 and 5 (b) 8 and 6(c) 4 and 2 (d) 7 and 18. Which of the following statements about the following reactions is correct? ZnO + CO → Zn + CO2(a) ZnO is being oxidized.(b) CO is being reduced.(c) CO2 is being oxidized.(d) ZnO is being reduced.9. In the reaction between lead sulphide and hydrogen peroxide which substance is reduced?

PbS + 4H2O2 → PbSO4 + 4H2O(a) Lead sulphide (b) Hydrogen peroxide(c) Lead sulphate (d) Water10. Which of the following reactions is used in white washing walls?(a) 2Ca + O2 → 2CaO(b) Ca(OH) CaO + H O2

Heat2 →

(c) Ca(OH)2 + CO2 → CaCO3 + H2O(d) CaO + H2O → Ca(OH)2

Match the Columns Match the column I with column II and select the correct answer by choosing an appropriate option.11. Column I Column II (Gas evolved) (Reaction)P. SO2 gas 1. Reaction between zinc

and sulphuric acidQ. H2 gas 2. Reaction between

manganese dioxide and sulphuric acid

R. CO2 gas 3. Reaction between copper and conc. sulphuric acid on heating

S. O2 gas 4. Reaction of sulphuric acid on marble.

T. NH3 gas 5. Reaction between magnesium nitride and water.

(a) P-5, Q-3, R-1, S-2, T-4(b) P-3, Q-1, R-4, S-5, T-2 (c) P-4, Q-5, R-3, S-1, T-2(d) P-3, Q-1, R-4, S-2, T-5


12. Column I Column II (Reaction) (Types of reaction)P. CuO + H2 → 1. Photodecomposition Cu + H2O reactionQ. 2AgBr → 2. Redox reaction 2Ag + Br2R. 2H2O → 3. Thermal 2H2 + O2 decomposition reactionS. ZnCO3 → 4. Electrolytic ZnO + CO2 decomposition reactionT. NaOH + HCl → 5. Neutralization NaCl + H2O reaction(a) P-3, Q-1, R-5, S-4, T-2(b) P-2, Q-4, R-3, S-5, T-1(c) P-2, Q-1, R-4, S-3, T-5(d) P-3, Q-4, R-2, S-1, T-5

13. Column I Column II (Reaction) (Observation)P. KI + Cl2 → 1. Violet solutionQ. KBr + Cl2 → 2. Yellow ppt.R. PbNO3 + KI → 3. Light green

solutionS. CuSO4 + Fe → 4. Light brown solutionT. Zn + HCl → 5. Evolution of gas(a) P-1, Q-4, R-2, S-3, T-5(b) P-2, Q-5, R-4, S-1, T-3(c) P-1, Q-3, R-5, S-2, T-4(d) P-2, Q-1, R-3, S-5, T-4

Passage BasedDirection (Q. No. 14 and 15) : Read the given passage and answer the following questions.Oxidation and reduction always occurs simultaneously. These reactions which involve simultaneous oxidation and reduction are called as redox reactions. Therefore every redox reaction is made up of two half reactions. One half reaction represents oxidation and other half reaction represents the reduction. Oxidation and reduction of an atom, molecule or ion can also be defined in terms of electrons. The substance that gains electrons, is reduced to a lower oxidation state act as an oxidising agent. Similarly, the substance which loses electrons is oxidised to higher oxidation state, is called as a reducing agent.

14. I2 reacts with caustic soda, the products are NaIO3 and NaI. The reaction is an example of(a) oxidation (b) reduction(c) disproportionation (d) neutralization.

15. In which of the following reaction hydrogen peroxide is acting as reducing agent?(a) 2FeCl2 + 2HCl + H2O2 → 2FeCl3 + 2H2O(b) Cl2 + H2O2 → 2HCl + O2(c) 2HI + H2O2 → 2H2O + I2(d) H2SO3 + H2O2 → H2SO4 + H2ODirection (Q. No. 16 to 18) : Read the given passage and answer the following questions.A balanced chemical equation is one which contain an equal number of atoms of each element on both the sides of the equation.For example, balanced chemical equation of burning of magnesium in oxygen to form magnesium oxide is written as

2Mg + O2 → 2MgOA balanced chemical equation must obey the law of conservation of mass. This means that the total mass of the reactants and products participating in a reaction must be the same.

16. Which of the following chemical equations is an unbalanced one?(a) 2NaHCO3 → Na2CO3 + H2O + CO2(b) 2C4H10 + 12O2 → 8CO2 + 10H2O(c) 2Al + 6H2O → 2Al(OH)3 + 3H2(d) 4NH3 + 5O2 → 4NO + 6H2O17. The information not conveyed by a balanced and fully labelled chemical equation is(a) whether the reaction is exothermic or endothermic(b) whether the reaction is feasible or not (c) the number of products and reactants taking part in the reaction(d) whether any product is a solid or a soluble substance.

18. The chemical equations are balanced to satisfy one of the following laws in chemical reactions. This law is known as(a) law of conservation of momentum(b) law of conservation of mass(c) law of conservation of motion(d) law of conservation of magnetism.


Assertion & Reason Direction : In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as :(a) If both assertion and reason are true and reason is the

correct explanation of assertion.(b) If both assertion and reason are true but reason is not the

correct explanation of assertion.(c) If assertion is true but reason is false.(d) If both assertion and reason are false.

19. Assertion : In the reaction,Zn(s) + 2H

+(aq) → Zn

2+(aq) + H2(g), zinc acts as an

oxidising agent and H+ acts as a reducing agent.Reason : An oxidising agent loses electrons while a reducing agent accepts electrons.

20. Assertion : Corrosion of iron is commonly known as rusting.Reason : Corrosion of iron occurs in presence of moist air.

21. Assertion : Food materials are often packed in air tight container.

Reason : Oxidation, resulting in rancidity, is prevented.

22. Assertion : When a mixture of hydrogen and chlorine is placed in sunlight, hydrogen chloride is formed.Reason : It is an example of combustion reaction.

Integer/Numerical Value23. C2H5OH(l) + 3O2(g) →xCO2(g) + yH2O(l) Value of (x + y) is24. Among the following reactions, the number of reaction(s) in which H2S acts as reducing agent is (are)CuSO4 + H2S → CuS + H2SO4Cd(NO3)2 + H2S → CdS + 2HNO32FeCl3 + H2S → 2FeCl2 + 2HCl + S25. Number of double displacement reactions among the following reaction are :(i) CuO + H2 → H2O + Cu(ii) Zn + CuSO4 → ZnSO4 + Cu(iii) BaCl2 + H2SO4 → BaSO4 + 2HCl(iv) NaOH + HCl → NaCl + H2O

1. (b)2. (a) : C + 2S → CS2It is an endothermic reaction, resulting in absorption of heat.

3. (d) : The given diagram shows electrolysis of water which is a type of decomposition reaction. When electric current is passed through acidified water, it decomposes to give hydrogen and oxygen gases.

Since in the above reaction, 2 volumes of H2 gas and 1 volume of O2 gas are produced hence, Y is H2 gas and X is O2 gas.

4. (c) : Cu + 2AgNO3 → Cu(NO3)2 + 2Ag Copper can displace silver from its salt solution since copper is more reactive than silver.

5. (c)6. (c) : In all other reactions except (c) the compound decomposes to give simpler products while in (c) only the water of crystallisation is separated from the compound.

7. (c) : Cu + 4HNO3 → Cu(NO3)2 + 2NO2 + 2H2O8. (d) : ZnO is reduced to Zn and CO is oxidised to CO2.9. (b) : H2O2 is reduced to water by removal of oxygen.

10. (c) : Lime is mixed with water to give slaked lime and applied on the walls. When slaked lime or Ca(OH)2 absorbs CO2 from the atmosphere it forms a shining white substance lime stone which gives shining white colour to the walls.Ca(OH)2 + CO2 → CaCO3 ↓ + H2O.11. (d) 12. (c)13. (a)14. (c) : Oxidation number of iodine is changing from zero (I2) to + 5 (NaIO3) and – 1 (NaI).15. (b) : H2O2 is oxidised to O2, hence acts as reducing agent.16. (b)17. (b) 18. (b)19. (d) : Zn(s) + 2H

+(aq) → Zn

2+(aq) + H2(g)

In this reaction, zinc loses electrons and so it is a reducing agent. While H+ gains electrons and so it is an oxidising agent.20. (b) 21. (a)22. (c) : Hydrogen and chlorine combine to give hydrogen chloride, which is a combination reaction.23. (5) : C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l)24. (1) : 2FeCl3 + H2S → 2FeCI2 + 2HCl + S25. (2) : (iii) and (iv) are double displacement reactions.

ANSWERS


SECTION - A

1. Write balanced equation for the following reaction :Zinc carbonate(s) Zinc oxide(s) + Carbon dioxide(g)2. Give one example of a reaction which is double displacement reaction as well as precipitation reaction.

3. Answer question numbers 3(i) - 3(iv) on the basis of your understanding of the following paragraph and the related studied concepts.

Oxidation and reduction reactions are based on the addition and removal of oxygen or hydrogen. Those reactions which involve simultaneous oxidation and reduction are called redox reactions. Oxidation may be defined as the addition of oxygen to a substance or removal of hydrogen from a substance in a chemical reaction. The substance which either gives oxygen or removes hydrogen in an oxidation reaction is known as an oxidising agent. Reduction may be defined as the addition of hydrogen to a substance or removal of oxygen from a substance. The substance which either gives hydrogen or remove oxygen in a reduction reaction is known as reducing agent.

3(i) Write the reaction in which hydrogen peroxide is acting as a reducing agent.

3(ii) Identify the substance that is oxidised and the substance that is reduced in the following reaction :4Na(s) + O2(g) 2Na2O(s)3(iii) Can a displacement reaction be a redox reaction ?

3(iv) Why combustion reaction is an oxidation reaction ?

4. Question numbers 4(i) - 4(iv) are based on the table given below. Study this table related to the different types of reactions / processes and answer the questions that follow.

S. No.

Name ofProcess

Wordequation

i. Combustion Magnesium + Oxygen heat

Magnesium dioxide

ii. Photosynthesis Carbon dioxide + Water sunlight

Chlorophyll

Glucose + Oxygen + Water

iii. Combination Iron + Sulpher heat Iron sulphide

iv. Photodecomposition Silver bromide light Silver + Bromine

4(i) Define combination reaction.

4(ii) What is photosynthesis ?

Time: 3 hrs. Max. Marks: 80

(i) The question paper comprises three sections – A, B and C. Attempt all the sections.

(ii) All questions are compulsory.

(iii) Internal choice is given in each section.

(iv) All questions in Section A are one-mark questions comprising MCQ, VSA type and assertion-reason type questions. They are to be answered in one word or in one sentence.

(v) All questions in Section B are three-marks, short-answer type questions. These are to be answered in about 50 - 60 words each.

(vi) All questions in Section C are five-marks, long-answer type questions. These are to be answered in about 80 - 90 words each.

(vii) This question paper consists of a total of 30 questions.

EXAM

DRILL

FOR BOARD EXAM


4(iii) When a chemical compound decomposes on absorbing light and energy, then the reaction which takes place is known as(a) photosynthesis(b) photodecomposition(c) combination(d) thermal decomposition.

4(iv) Which of the following reactions is an example of combustion reaction ?(a) C(s) + O2(g) CO2(g)(b) Zn(s) + H2SO4(aq) ZnSO4(aq) + H2(g)(c) Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)(d) 3Mg(s) + N2(g) Mg3N2(s)5. Precipitation occurs in the solution when(a) barium chloride is added to sodium sulphate(b) barium chloride is added to sodium chloride(c) sodium chloride is added to sodium sulphate(d) hydrochloric acid is added to barium sulphate.

6. 10 mL of freshly prepared iron sulphate solution was taken in each of the four test tubes. Strips of copper, iron, zinc and aluminium were introduced, each metal in a different test tube. A black residue was obtained in two of them. The right pair of metals forming precipitates is(a) copper and zinc(b) aluminium and copper(c) iron and aluminium(d) zinc and aluminium.

7. Which of the following is not a chemical reaction?(a) Souring of milk(b) Dissolution of sugar in water(c) Rusting of iron(d) Digestion of food in our body

ORWhen ferrous sulphate crystals are heated, the residue left behind is(a) FeO with black colour(b) Fe2O3 with reddish brown colour(c) Fe3O4 with red colour(d) a mixture of all the above.

8. 4M + O2 → 2M2O, in this reaction M is(a) reduced(b) oxidized

(c) both (a) and (b)

(d) neither (a) nor (b).

9. In order to prevent the spoilage of potato chips, they are packed in plastic bags in an atmosphere of

(a) Cl2 (b) H2(c) N2 (d) O2

ORIn the reaction, PCl3 + Cl2 → PCl5(a) PCl3 is acting as reductant

(b) Cl2 is acting as reductant

(c) both PCl3 and Cl2 are acting as reductant

(d) both PCl3 and Cl2 are acting as oxidant.

10. Calcium oxide reacts vigorously with water.

Beaker

Water

Calcium oxide

Identify the incorrect statements.

(I) It is an endothermic reaction.

(II) Slaked lime is produced.

(III) Quick lime is produced.

(IV) It is an exothermic reaction.

(V) It is a combination reaction.

(a) (I) and (II)

(b) (III) and (IV)

(c) (I) and (III)

(d) (II), (IV) and (V)

11. In the following equation :

Na2CO3 + xHCl 2NaCl + CO2 + H2O, the value of x is (a) 1 (b) 2(c) 3 (d) 4

12. Marble reacts with HCl to give (i) gas whereas zinc reacts with HCl to give (ii) gas. (i) and (ii) are respectively(a) H2, O2 (b) CO2, H2(c) H2, CO2 (d) O2, H2

ORIdentify the type of reaction for each of the following asCombination – (p), Decomposition – (q),


Displacement – (r), Double displacement – (s), Combustion – (t)(A) A compound breaks apart into its elements.(B) A metal and a non-metal react to form an ionic compound.(C) A compound of hydrogen and carbon reacts with oxygen to produce carbon dioxide and water.(D) Silver ion from AgNO3(aq.) forms a precipitate with bromide ion from KBr(aq.).(a) (A) – q, (B) – p, (C) – r, (D) – s(b) (A) – q, (B) – p, (C) – t, (D) – s(c) (A) – s, (B) – r, (C) – q, (D) – p(d) (A) – p, (B) – t, (C) – q, (D) – rFor question numbers 13 and 14, two statements are given- one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below :(a) Both A and R are true and R is correct explanation of the

assertion.(b) Both A and R are true but R is not the correct explanation

of the assertion.(c) A is true but R is false.(d) A is false but R is true.

13. Assertion : The reaction, Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) is an example of displacement reaction.Reason : Displacement reactions occur mostly in solution form.

14. Assertion : The food items containing oil and fat are flushed with nitrogen.Reason : Oil and fat become rancid on oxidation which has the bad taste and smell.

SECTION - B15. (a) Distinguish between endothermic and exothermic reactions.(b) (i) Describe the change you will observe whe

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