Energy From Chemicals

ENERGY FROM CHEMICALS

By the end of the lesson, you should be able to:

□ Describe the meaning of the terms exothermic and endothermic.

□ State the connection between making and breaking bonds and exothermic and endothermic.

□ State the unit of energy.

□ Explain what is meant by ΔH in an equation.

□ Explain in terms of bonds why a reaction is exothermic and endothermic.

□ Sketch graphs of temperature against time for endothermic and exothermic laboratory reactions.

□ Explain overall enthalpy changes in terms of the energy changes associated with the breaking and making of covalent bonds.

□ Describe petroleum as a mixture of hydrocarbons and its separation into useful fractions by fractional distillation.

□ Name six main petroleum fractions and state the uses for each one.

□ Describe hydrogen, derived from water or hydrocarbons, as a potential fuel for use in future,

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EXOTHERMIC AND ENDOTHERMIC CHANGES

When heat is ______________ in a chemical reaction, the reaction is said to be exothermic.

Exothermic reactions have the following features:- the temperature _______________.- The products have _______ energy than the reactants (because

heat energy is lost).

When heat is ________________ in a chemical reaction, the reaction is said to be endothermic.

Endothermic reactions have the following features:- the temperature ___________________.- The products have __________ energy than the reactants

(because heat energy is gained).

Several physical changes are exothermic while most chemical changes are exothermic.

Several physical changes are endothermic, but only a few chemical changes are endothermic.

Below are some examples of physical and chemical changes, classify each of the following changes as exothermic or endothermic.

1. Changes of state During the freezing of water, water changes to ice and heat energy is

given out to the surroundings.

H2O(l) H2O(s) + heat

Similar reaction takes place during condensation.

Question 1Classify freezing and condensation as either exothermic or endothermic.______________________________________________________________

Heat energy is absorbed during melting and boiling.

For example, during the boiling water, water changes to steam and heat is absorbed.

Heat + H2O(l) H2O(g)

Question 2Classify melting and boiling as either exothermic or endothermic.______________________________________________________________

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2. Dissolving of some salts in water.Part a:

Take note of the initial temperature of water in a test tube.

Put in sodium carbonate into the test tube and take note of the final temperature.

Initial Temperature Final Temperature Temperature change(Increase or Decrease)

Question 3Classify dissolving sodium carbonate in water as either exothermic or endothermic.______________________________________________________________

Part b:

Take note of the initial temperature of water in a test tube.

Put in ammonium chloride into the test tube and take note of the final temperature.


Question 4Classify dissolving ammonium chloride in water as either exothermic or endothermic.______________________________________________________________

3. Combustion reactions

Combustion reactions involve the burning of chemicals in air to produce water. For example, the burning of hydrogen in air to form water is exothermic.

2H2(g) + O2(g) 2H2O(l) + heatQuestion 5Classify combustion reactions as either exothermic or endothermic.______________________________________________________________

4. Decomposition of compounds

Many compounds require heat to decompose them. For example, when green copper (II) carbonate is heated, it absorbs heat and decomposes into black copper (II) oxide and carbon dioxide:

Heat + CuCO3(s) CuO(s) + CO2(g)

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Question 6Classify decomposition of compounds as either exothermic or endothermic.______________________________________________________________

5. Neutralisation reactions

Put in hydrochloric acid into a test tube. Take note of the initial temperature of the acid in the test tube.

Put in sodium hydroxide into the test tube and take note of the final temperature.

H+(aq) + OH-(aq) H2O(aq) + heat


Question 7Classify neutralization reactions as either exothermic or endothermic.______________________________________________________________

6. Metal Displacement reactions

For example, when zinc reacts with copper (II) sulphate solution in a test tube, the reaction gives out heat and the test tube becomes warm.

Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s) + heatQuestion 8Classify metal displacement reactions as either exothermic or endothermic.______________________________________________________________

7. Reactions between acids and hydrogencarbonates

For example, when some headache tablets ( a mixture of solid acids such as citric acid and sodium hydrogencarbonate) are added to water, the temperature of the solution decreases as the acid and hydrogencarbonate react. The word equation for this reaction is:

Heat + sodium hydrogen carbonate + citric acid sodium citrate + carbon dioxide + water

Question 9Classify reactions between acids and hydrogencarbonates as either exothermic or endothermic.______________________________________________________________

Please proceed to do exercise from: WORKSHEET 1

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HEAT CHANGES IN A REACTION

The amount of energy involved in a reaction is known as the heat change or enthalphy change of the reaction.

It is measured in _____________ and represented by the symbol ______.

For an exothermic reaction, ΔH is negative. Why?______________________________________________________________

The heat of the reaction can be shown in the chemical equation. An example is the reaction between magnesium and hydrochloric acid.

For endothermic reaction, ΔH is positive. Why?______________________________________________________________

An example of a reaction with a positive ΔH is given below.

Please proceed to do exercise from:

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WORKSHEET 2

ENERGY LEVEL DIAGRAMS

A convenient way to show energy changes in a reaction is by means of energy level diagrams.

In an exothermic reaction, heat energy is given out to the surroundings. This energy comes from the energy stored in the reactants. Thus, the products of the reaction have less energy than the reactants.

Draw the energy level diagram for an exothermic reaction.

In an endothermic reaction, heat energy is taken in.

So what will happen to the energy of the products and the reactants?

Draw the energy level diagram for an endothermic reaction.

BOND BREAKING AND BOND MAKING

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Energy changes in reactions are caused by the making and breaking of chemical bonds.

Bond breaking requires heat energy to be ___________. Therefore bond breaking is _____________.

Bond making ______________ heat energy. Therefore bond making is __________________.

The energy absorbed in breaking a covalent bond is called the bond energy. It is the same as the energy given out in making the bond. The bind energy of some common covalent bonds are given in Table 1 below.

Covalent bond Bond energy (kJ/mol of bonds)

Example of molecule containing bond

H-H 436 Hydrogen, H2

O-H 463 Water, H2OO=O 496 Oxygen, O2

Calculating heat changes in a reaction

There are three steps involved in calculating a bond energy change:

1. Add up the bond energies for all the bonds in the products. This is the heat given out in making bonds.

2. Add up the bond energies for all the bonds in the reactants. This is the heat absorbed in breaking bonds.

3. The overall heat change for the reaction (ΔH) = Heat given out in making bonds + Heat absorbed in breaking bonds.

If the answer is negative, it is an exothermic reaction.If the answer is positive, it is an endothermic reaction.

An example is the combustion of hydrogen.

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Using the bond energy from Table 1, calculate the overall heat change.

From the calculation you just did, is the reaction exothermic or endothermic? Explain your answer.

______________________________________________________________


ACTIVATION ENERGY AND ENERGY PROFILE DIAGRAMS

All chemical reactions needed to have a certain minimum energy to start off the reaction, which is called the activation energy.

The energy is needed to break the bonds in the reactants before the new bonds can form.

The activation energy is like a ‘barrier’ that must be overcome before the reaction can start.

To show the activation energy of a reaction, diagrams called energy profile diagrams are used.

The energy level diagrams for exothermic and endothermic reactions are compared below.

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COMBUSTION OF FUELS

Everyday we use an enormous amount of energy in our homes, schools and factories for different purposes.

Most of the energy requires is produced from the combustion of chemicals called fuels.

What are fuels? Fuels are substances that can burn easily in air to give out energy.

Therefore the combustion of fuels is what type of reaction? ____________

List down examples of fuels that you know.______________________________________________________________

The most commonly used fuels are fossil fuels such as coal, petroleum and natural gas.

They are formed from decayed plants and animals that lived millions of years ago. They are carbon compounds.

Fractional Distillation of Petroleum

Petroleum is a mixture of hydrocarbons molecules. Suggest a method to separate the mixture of hydrocarbons in petroleum. __________________

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The properties and uses of some of the fractions are summarised in the Table below.

What happens when fuels burn?

Coal is mainly carbon. It burns in excess of air to give carbon dioxide and energy.Equation: carbon + oxygen carbon dioxide + heat energy

C(s) + O2(g) CO2(g) + heat energy

Most fuels contain carbon and hydrogen, when these fuels burn, carbon dioxide, water and heat energy are produced.Write the equation for the complete combustion of natural gas, which contains mainly methane (CH4).___________________________________________________________

If a limited supply of air is used, carbon particles (in the form of soot) and a poisonous gas, carbon monoxide are produced. This is called incomplete combustion.Write the equation for the incomplete combustion of natural gas.___________________________________________________________

HYDROGEN AS A FUEL

Hydrogen burns cleanly in air. The product is steam, whish is a non-pollutant.Equation: hydrogen + oxygen water

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

Hydrogen produces at least twice as much heat energy per gram when burnt, than any other common fuels. This is why it is used as a fuel in space shuttles and other big rockets.

Hydrogen has great possibilities as a fuel for cars, replacing petrol.

Fuel Cells

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A fuel cell is a chemical cell in which reactants (usually a fuel and oxygen) are continuously supplied to produce electricity directly.

The best known example is the hydrogen-oxygen fuel call which is used as a source of electrical power in space vehicles.

The overall reaction is simply the conversion of hydrogen and oxygen to water.What do you notice about the overall reaction in the fuel cell?___________________________________________________________

Photosynthesis

Green plants absorb carbon dioxide and water, and convert them into glucose and oxygen using sunlight as energy, which is absorbed by the green pigment, chlorophyll.

Thus photosynthesis is what type of reaction?_______________________

Construct the overall reaction of photosynthesis.

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___________________________________________________________


WORKSHEET 1

1. When solid sodium carbonate is added to water the temperature of the mixture rises. What conclusion from this observation is correct?

A Dissolving sodium carbonate is exothermic.B A large amount of sodium carbonate dissolves in water.C Aqueous sodium carbonate has more energy than solid sodium carbonate.D The temperature rises when all solids dissolve in water.

( )

2, The dissolving of ammonium chloride in water is an endothermic change. Which graph shows how the temperature changes when solid ammonium chloride is stirred with water and the solution is left to stand?

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( )3. Which statement is true for all examples of combustion?A One product is carbon dioxide.B It is an exothermic reaction.C The combustion products have more energy than the original fuel and oxygen.D No bonds are broken in the reaction.

( )

4.

5.

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6.

7. Classify each of the following changes as exothermic or endothermic. (a) Changing water to steam. (b) Reacting sodium with cold water. (c) Reacting magnesium with dilute sulphuric acid.

WORKSHEET 2

1. The reaction of iodine and hydrogen is an endothermic reaction:

Which deduction from this information is correct?A The products have more energy than the reactants.B The number of bonds formed in the reaction is less than the number of bonds broken.C Energy is absorbed when a H atom and an I atom form an H-I bond.D An H-I bond is stronger than an H-H bond or an I-I bond. ( )

2. The equation for the extraction of iron in the blast furnace is:Fe2O3 + 3CO 2Fe + 3CO2; ΔH=-27 kJ

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What heat energy change takes place when 1 mole of iron is produced in the reaction?A 27 kJ of heat energy absorbed.B 13.5 kJ of heat energy is absorbed.C 13.5 kJ of heat energy is given out.D 27 kJ of heat is given out. ( )

3. The equation for the reaction of sulphuric acid with sodium hydroxide is:H2SO4(aq) + 2NaOH(aq) Na2SO4(s) + 2H2O(l) ΔH=-114 kJ

a) Is this reaction exothermic or endothermic?b) How much heat energy is obtained from the reaction of 2moles of H2SO4?

4. a) Write the chemical equation for the thermit reaction? b) Is this reaction endothermic or exothermic? Give a reason. c) Will the enthalpy change for the reaction be positive or negative? Explain.

(HINT: Turn to Page 218 of Chemistry Insights)

5. When 1 mol of carbon is burnt in excess oxygen, 40.0 kJ of heat is liberated.C(s) + O2(g) CO2(g) ΔH= -40.0 kJ/mol

Calculate the mass of carbon required to provide 6.0 kJ of heat on burning in excess oxygen.

WORKSHEET 3

1. Which change is exothermic?

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( )

2. The formation of ammonia from nitrogen and hydrogen is an exothermic reaction.

This implies that

A the products possess more energy than the reactants.B the breaking of the bond is an exothermic process.C the number of bonds broken is less than the number of bonds formed.D the total energy change for bond formation is greater than that for bond breaking. ( )

3. The reaction between hydrogen and oxygen to form water occurs in three stages. In which stages are the signs of energy changes correctly shown?

A 1 and 2 only B 2 and 3 onlyC 1 and 3 only D 1,2 and 3. ( )

4.

5. In each change, state whether it is endothermic or exothermic.

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6. Hydrogen reacts with chlorine to produce hydrogen chloride. The equation for the reaction is:

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

a) What bonds are broken in the reaction?b) What bonds are made in the reaction?c) Δ H for the reaction is negative. What deduction can you make about the bonds in this reaction?

7. Hydrogen reacts with chlorine to produce hydrogen chloride. This reaction is exothermic. During the reaction, covalent bonds are broken and new covalent bonds are formed.

H2(g) + Cl2(g) 2HCl(g) ; ΔH=-184 kJ/mol

(a) (i) What type of energy change occurs when bonds are broken? (ii) What type of energy change occurs when bonds are formed? (iii) Which is the larger of the energy changes taking place in the reaction above, bond breaking or bond forming? Explain your answer.

(b) When 1 mole of chlorine gas reacts, the energy change, ΔH, is -184 kJ.Calculate the energy change when 14.2 g of chlorine reacts completely with hydrogen.

(c) (i) In forming ionic bonds, electrons are transferred between atoms. How is a covalent bond formed? (ii) Draw a ‘dot and cross’ diagram to show the bonding in a molecule of hydrogen chloride. You only need to draw the electrons in the outermost shells.

WORKSHEET 4

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Energy change calculations

bond bond energy (kJ/mol) bond bond energy (kJ/mol)

H-H 436 C=O 743

O=O 496 H-O 463

C-C 348 Br-Br 193

C=C 612 C-Br 276

C-H 412 S=O 435

O-O 146 Cl-Cl 242

H-Cl 431

Use the bond energy data in the table to calculate the energy change in each reaction (make sure your sign is correct).

Draw an energy profile diagram to show the relative position of the reactants and products in each one.

1)

1

1)

O S O2 + O OS

O

OO

2

2)

ii)

+ 2H H Cl Cl H Cl

3)

iii)

+ 5 O OH C

H

C

H

C

H

H

HHH

O C O HO

H3 + 4

4)

iv)

+ O O2 HO

H

H

O O

H

2

5)+ H C

H

C

H

C

H

H

H

Br Br H C

H

C

H

H Br Br

H

H

C

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WORKSHEET 5

1. Diesel is a hydrocarbon fuel.i) What is meant by the term hydrocarbon?ii) State one important use of diesel.iii) Name two products when diesel is completely burnt in air.iv) Name one pollutant that is obtained in the incomplete combustion of diesel.v) Another hydrocarbon fuel is petrol. Give one difference between a typical petrol molecule and a typical diesel molecule.

2. The bar chart compares the amounts of energy used by country Z in three years.

a) What was the main source of energy in 1975?b) Name one fuel obtained from petroleum.c) i) Name a major solid fuel (other than wood). ii) Suggest one reason for the change in the amount of solid fuel used after 1955.d) State one way in which burning large amounts of natural gas and fuels from petroleum can harm the environment.e) State one problem that may appear in the future if oil and natural gas continue to be consumed in large quantities.f) i) Name the type of energy obtained in large quantities from uranium. ii) What would you expect to happen to the relative amount of this energy used, by the year 2015? Give a reason.

3. Liquid hydrogen is used as a fuel in big rockets.a) Suggest a reason why liquid hydrogen is carried on big rockets in tanks rather than hydrogen gas.b) i) Write the equation for the combustion of hydrogen. ii) In terms of bonds broken and binds formed, explain why the burning of hydrogen is exothermic.

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iii) One mole of H2 releases 282 kJ of energy when burnt.What is the amount of energy released when 10 kg of hydrogen burns? c) It has been suggested that hydrogen could be developed as an alternative fuel to replace hydrocarbon fuels in the future. Suggest one advantage hydrogen has over fossil fuels.

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Energy From Chemicals