Honors Chemistry, Chapter 8 Page 1 Chapter 8 – Chemical Equations and Reactions

Honors Chemistry, Chapter 8Page 1

Chapter 8 – Chemical Equations and Reactions


Chemical Equations

• A chemical equation represents, with symbols and formulas, the identities and relative amounts of the reactants and products in a chemical reaction.– Reactants – Starting substances on the left

side of the equation– Products – Final substances on the right side

of the equation

• Obeys the Law of Conservation of Mass


Indications of a Chemical Reaction

1. Evolution of heat and light. (Examples: Decomposition of ammonium dichromate, burning of methane.)

2. Production of a gas. (Example: Carbon dioxide released when baking soda and vinegar react.)


Indications of a Chemical Reaction

3. Formation of a precipitate. (Example: Reaction of ammonium sulfide and cadmium nitrate Yellow ppt.)

4. Color Change. (Example: When iron and water react the iron changes from metallic color to red/brown.)


Characteristics of Chemical Equations

1. The equation must represent the known facts (all reactants and products must be identified by chemical analysis.)

2. The equation must contain the correct formulas for the reactants and products.

3. The law of conservation of mass must be satisfied. (Coefficients added as necessary. Coefficient: a small whole number in front of a formula in a chemical equation.)


Elements That Normally Appear as Diatomic or Polyatomic Molecules

Element Symbol Molecular Formula

Physical State

Hydrogen H H2 Gas

Nitrogen N N2 Gas

Oxygen O O2 Gas

Fluorine F F2 Gas

Chlorine Cl Cl2 Gas


Elements That Normally Appear as Diatomic or Polyatomic Molecules

Element Symbol Molecular Formula

Physical State

Bromine Br Br2 Liquid

Iodine I I2 Solid

Sulfur S S8 Solid

Phosphorous P P4 Solid


Word Equations

• Word equation: An equation in which the reactants and products of a chemical reaction are represented by words.Methane + oxygen carbon dioxide + Water

• Arrow ( ) is read as: “react to yield”, “yield”, “produce”, or “form”


Formula Equations

• Formula equation: An equation which represents the reactants and products of a chemical reaction by their symbols or formulas.CH4(g) + O2(g) CO2 (g) + H2O(g) (not balanced)

CH4(g) + O2(g) CO2 (g) + 2H2O(g) (partially balanced)

CH4(g) + 2O2(g) CO2 (g) + 2H2O(g) (balanced)


Symbols Used in Chemical Equations

Symbol Explanation

Yields; indicates result of a reaction

Indicates a reversible reaction

(s) Reactant or product in the solid state

Precipitate

(l) Reactant or product in the liquid state

(aq) Reactant or product in aqueous solution

(g) Reactant or product in the gaseous state


Symbols Used in Chemical Equations

Symbol Explanation

Product in the gaseous state

Reactants are heated ( or heat)

2 atm Reaction at 2 atmospheres pressure

pressure Reaction at elevated pressure

0oC Reaction at 0oC

MnO2 Reaction in the presence of a catalyst


Reaction Symbol Definitions

• Catalyst: A catalyst is a substance that changes the rate of a chemical reaction but can be recovered unchanged.

• Reversable Reaction: A chemical reaction in which the products re-form the original reactants.


Practice Writing Formulas

Barium Acetate Ammonium Sulfate

Aluminum Carbonate Iron (III) Chromate

Nickel (II) Hydroxide Copper (II) Nitrate

Rubidium Chloride Lithium Permanganate

Strontium Peroxide Calcium Phosphate

Mercury (I) Sulfate Magnesium Bromide

Zinc Thiosulfate Tin (IV) Chlorate


Example Reactions

3Fe(s) + 4 H2O(g) Fe3O4 + 4 H2(g)

2HgO(s) 2Hg(l) + O2(g)

C2H4(g) + H2(g) C2H6(g)

Pressure

Pt


Significance of a Chemical Equation

1. The coefficients of a chemical reaction indicate relative, not absolute amounts of reactants and products.

H2(g) + Cl2(g) 2HCl(g)

In other words:1 molecule H2 : 1 molecule Cl2 : 2 molecules HCl

Or

1 mole H2 : 1 mole Cl2 : 2 mole HCl



2. The relative masses of the reactants and products of a chemical reaction can be determined from the reaction’s coefficients.

1 mol H2 x 2.02 g H2/mol H2 = 2.02 g H2

1 mol Cl2 x 70.90 g Cl2/mol Cl2 = 70.90 g Cl2

2 mol HCl x 36.46 g HCl/mol HCl = 72.92 g HCl



3. The reverse for a chemical reaction has the same relative amounts of substances as the forward reaction.


Balancing Chemical Equations

1. Identify the names of the reactants and the products and write the word equation.

Water hydrogen + oxygen

2. Write a formula equation by substituting correct formulas for the names of the reactants and the products.H2O(l) H2(g) + O2(g) (not balanced)


Balancing Chemical Equations3. Balance the equation according to the

law of conservation of massa. Balance different types of atoms one at a time.b. First balance the atoms of elements that are

combined and that appear only once on each side of the equation.

c. Balance polyatomic ions that appear on both sides of the equation as single units.

d. Balance H atoms and O atoms after all other elements are balanced.

2H2O(l) H2(g) + O2(g) (Partially balanced)

2H2O(l) 2H2(g) + O2(g) (balanced)


Balancing Chemical Equations

4. Count atoms to be sure the equation is balanced.2H2O(l) 2H2(g) + O2(g) (balanced)

(4H + 2O) = (4H) + (2O)


When Balancing . . .

• Always write the correct formulas for both the reactants and the products

• Never try to balance an equation by changing subscripts(!)


Balancing Exercise #1

Using the four steps, write a balanced chemical equation for the reaction between zinc and aqueous hydrochloric acid which produces zinc chloride and hydrogen gas.


Exercise #1 - Solution

1. zinc + hydrochloric acid zinc chloride + Hydrogen

2. Zn(s) + HCl(aq) ZnCl2(aq) + H2(g)

3. Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)

4. (1 Zn)+(2H + 2Cl) = (1Zn+2Cl) + (2H)



Using the four steps, write a balanced chemical equation for the reaction between aluminum carbide (Al4C3) and water which produces methane gas (CH4) and aluminum hydroxide.



1. aluminum carbide + water methane + aluminum hydroxide

2. Al4C3(s) + H2O(l) CH4(g) + Al(OH)3(s)

3. Al4C3(s)+12H2O(l)3CH4(g)+4Al(OH)3(s)

4. (4Al+3C)+(24H+12O)

= (3C+12H) + (4Al+12O+12H)



Using the four steps, write a balanced chemical equation for the reaction between calcium carbide (CaC2) and water which produces ethyne gas (HCCH) (also known as acetylene) and calcium hydroxide.



1. calcium carbide + water ethyne + calcium hydroxide

2. CaC2(s)+H2O(l) HCCH(g)+Ca(OH)2(s)

3. CaC2(s)+2H2O(l) HCCH(g)+Ca(OH)2(s)

4. (1Ca+2C)+(4H+2O)

= (2H + 2C) + (1Ca+2O+2H)


Chapter 8, Section 1 Review

1. List four observations that indicate that a chemical reaction has taken place.

2. List three requirements for a correctly written chemical equation.

3. Write a word equation and a formula equation for a given chemical reaction.

4. Balance a formula equation by inspection.


Broad Types of Chemical Reactions

• Synthesis: A + X AX

• Decomposition: AX A + X

• Single-Replacement: A + BX AX + B or Y + BX BY + X

• Double-Replacement: AX + BY AY + BX

• Combustion: 2A + O2 2AO

(coefficients and subscripts may vary)


Synthesis Reactions

• In a synthesis reaction, also known as a composition reaction, two or more substances combine to form a new compound.

• A + X AX• Three Types

– Elements + Oxygen or Sulfur– Metals with Halogens– Reactions with Oxides


Reactions of Elements with Oxygen or Sulfur

• Mg(s) + O2(g) ? 2Mg(s) + O2(g) 2MgO(s)

• Rb(s) + S8(s) ? 16Rb(s)+S8(s) 8Rb2S(s)

• Fe(s) + O2(g) ?2Fe(s) + O2 2FeO4Fe(s) + 3O2 2Fe2O3


Reactions of Elements with Oxygen or Sulfur

• C(s) + O2(g) ? C(s) + O2(g) CO2(g)2C(s) + O2(g) 2CO(g)

• S8(s) + O2 ? S8(s) + 8O2 8 SO2(g)

• H2(g) + O2(g) ? 2H2(g) + O2(g) 2H2O(g)


Reactions of Metals with Halogens

• Na(s) + Cl2(g) ? 2Na(s) + Cl2(g) 2NaCl(s)

• K(s) + I2(s) ? 2K(s) + I2(s) 2KI(s)

• Mg(s) + F2(g) ? Mg(s) + F2(g) MgF2(s)

• Sr(s) + Br2(l) ? Sr(s) + Br2(l) SrBr2(s)


Reactions of Metals with Halogens

• Na(s) + F2(g) ? 2Na(s) + F2(g) 2NaF(s)

• Co(s) + F2(g) ? 2Co(s) + 3F2(g) 2CoF3(s)

• U(s) + F2(g) ? (Hint: Uranium (VI))

U(s) + 3F2(g) UF6(g)


Synthesis Reactions with Oxides

• CaO(s) + H2O(l) ? CaO(s)

+H2O(l)Ca(OH)2(s)

• SO2(g) + H2O(l) ? SO2(g)+H2O(l)

H2SO3(aq)

• H2SO3(aq) + O2(g) ? 2H2SO3(aq) + O2(g) 2H2SO4(aq)

• CaO(s) + SO2(g) ? CaO(s)+ SO2(g)

CaSO3(s)


Decomposition Reactions• In a decomposition reaction, a single

compound undergoes a reaction that produces two or more simpler substances.

• AX A + X• Five Types

– Decomposition of Binary Compounds– Decomposition of Metal Carbonates– Decomposition of Metal Hydroxides– Decomposition of Metal Chlorates– Decomposition of Acids


Decomposition of Binary Compounds

• H2O(l) electricity ? 2H2O(l) electricity 2H2(g) + O2(g)

(electrolysis)

• HgO(s) ? 2 HgO(s) 2 Hg(l) + O2(g)

(thermolysis)


• Decompostion of Metal Carbonates

CaCO3(s) ?CaCO3 CaO(s) +

CO2(g)

• Decomposition of Metal Hydroxides

Ca(OH)2(s) ? Ca(OH)2(s) CaO(s)

+H2O(g)


• Decompostion of Metal Chlorates

KClO3(s) ?2KClO3(s) 2KCl(s)+3O2(g)

• Decomposition of Acids

H2CO3(aq) ? H2CO3(aq) CO2(g)+H2O(l)

H2SO4(aq) ? H2SO4(aq) SO3(g)

+H2O(l)

(Sulfurous Acid ?)


Single-Replacement Reactions• In a single-replacement reaction, also

known as a displacement reaction, one element replaces a similar element in a compound.

• A + BX AX + B or• Y + BX BY + X• Four Types

– Replacement of a Metal by Another Metal– Replacement of H2 in Water by a Metal– Replacement of H2 in an Acid by a Metal– Replacement of Halogens


•Replacement of a Metal with another Metal

Al(s) + Pb(NO3)2(aq) ? 2Al(s)+3Pb(NO3)2(aq)3Pb(s)+2Al(NO3)3

•Replacement of H2 in Water by a Metal

Na(s) + H2O(l) ?2Na(s)+ 2H2O(l) 2NaOH(aq) +

H2(g)

Fe(s) + H2O(g) ? 3Fe(s)+ 4H2O(g) Fe3O4(s)+ 4H2(g)

(Oxidation State of Fe?)


•Replacement of H2 in an Acid by a Metal

Mg(s) + HCl(aq) ?

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

•Replacement of Halogens

Cl2(g) + 2KBr(aq) 2KCl(aq) + Br2(l)

F2(g) + 2NaCl(aq) 2NaF(aq) + Cl2(g)

Br2(l) + KCl(aq) no reaction


Double-Replacement Reactions• In double-replacement reactions, the ions

of two compounds exchange places to make two new compounds.

• AX + BY AY + BX• Three Types

– Formation of a Precipitate– Formation of a Gas– Formation of Water


•Formation of a Precipitate

KI(aq) + Pb(NO3)2(aq) ?

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

•Formation of a Gas

FeS(s) + 2HCl(aq) H2S(g) + FeCl2(aq)

•Formation of Water (acid-base reactions)

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)


Combustion Reactions• In a combustion reaction, a substance

combines with oxygen, releasing a large amount of energy in the form of light and heat.

2H2(g) + O2(g) 2H2O(g)

C3H8(g) + 5O2 3CO2(g) + 4H2O(g)



1. Define and give general equations for:a) Synthesis reactions

b) Decomposition reactions

c) Single-replacement reactions

d) Double-replacement reactions

e) Combustion reactions

2. Classify a reaction into one of the five categories above.


Chapter 8, Section 2 Review continued

3. List three types of synthesis reactions and five types of decomposition reactions.

4. List four types of single replacement reactions and three types of double replacement reactions.

5. Predict the products of simple reactions given the reactants.


Activity Series

• An Activity series is a list of elements organized according to the ease with which the elements undergo certain chemical reactions.

• Each element in the series will replace all elements below it in the series in a single-replacement reaction.


Activity Series of MetalsRx with water

Rx with steam

Rx with acids

Rx with oxygen

Unreactive

Li

Rb

K

Ba

Sr

Ca

Na

Mg

Al

Mn

Zn

Cr

Fe

Cd

Co

Ni

Sn

Pb

H2

Sb

Bi

Cu

Hg

Ag

Pt

Au


Activity Series of Halogens

Fluorine

Chlorine

Bromine

Iodine



1. Explain the significance of an activity series.

2. Use an activity series to predict whether a given reaction will occur and what the products will be.

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Honors Chemistry, Chapter 8 Page 1 Chapter 8 – Chemical Equations and Reactions