modul kimia ting 5 bab 13

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Chapter 13 Thermochemistry

Concept Map

13.1 Energy Changes in Chemical Reactions

1. Chemical reactions take place with changes in energy.

2. This energy change is symbolized as H is called the heat of reaction.

3. Chemical reactions are classified as exothermic and endothermic reactions.

4. In exothermic reactions, heat is released to the surrounding. If the reaction takes place in aqueous solution, there will be a

increase in temperature of the solution. The products have lower energy content than the reactants.5 In endothermic reactions, heat is absorbed from the surrounding. If the reaction takes place in aqueous solution, there will be

a drop in temperature of the solution. The products have higher energy content than the reactants.

13.2 Energy Level Diagram

1. An energy level diagram of a chemical reaction shows

(a) the relative energy content of the reactants and products

(b) the heat change, H , of the reaction

H = H products - H reactants

2. Exothermic reaction:

Reactants

Thermochemistry

Exothermic reactions

Endothermic reactions

Energy level diagram

Heat of reaction, H

in kJ mol-1

Heat evolved or

absorbed = m c

Heat of

precipitation

Example:

Precipitation of

Heat of

displacement

Example:

Displacement

copper from

copper(II)

sulphate

Heat of

combustion

Example:

Combustion

of ethanol

Heat of

neutralization

Strong acid

strong base

Weak acid

strong base

Strong acid

Selection of

suitable fuels

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H reactants

Energy H = - x kJ mol 1

Hproducts

3. Endothermic reaction:

Products

H reactants

Energy H = + y kJ mol 1

reactants

H products

13.2 Heat of Precipitation

1. Heat of precipitation is the heat change when 1 mole of an insoluble salt is formed from its ions in solution understandard conditions.

2. When aqueous sodium sulphate is added to aqueous lead(II) nitrate, white precipitate of lead(II) sulphate is

formed. Experiment shows that the heat of precipitation of this reaction is42 kJ mol1

.

Pb(NO3)2(aq) + Na2SO4(aq) PbSO4(s) + 2NaNO3(aq) ; H = -42 kJ mol1

3. Energy level diagram:

Pb2+

(aq) + SO4(aq)

Energy H = - 42 kJ mol 1

PbSO4(s)

4. The following steps should be followed in the calculation of heat of precipitation:

(a) Write a balanced equation for the chemical reaction.

(b) Determine the mass of the precipitate formed.

(c) Calculate the number of moles of the precipitate.

(d) Calculate the heat change for the reaction by using formula

Heat absorbed or released = m c

where m = mass of the solution

c = specific heat capacity of the solution

= change in temperature

(e) Calculate the heat change, H, for 1 mol of the precipitate

(f) Convert the heat change to kJ mol1

5. An experimental result:

50 cm3

of 1.0 mol dm3

lead(II) nitrate was added to 50 cm3

of 1.0 mol dm3

sodium sulphate solution. The

temperature rise is 6oC.

Calculation:

Pb(NO3)2(aq) + Na2SO4(aq) PbSO4(s) + 2NaNO3(aq)

Heat evolved = m c

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= (50 + 50) c 4.2 x 6

= 2520 J

Number of moles of lead nitrate =MV

1000=

1.0 x 50

1000

= 0.05

Number of moles of sodium sulphate = 0.05

Since 1 mol of Pb(NO3)2 reacts with 1 mol of Na2SO4, hence number of mol of lead sulphate formed is 0.05.

H = -2520

0.05x1000= - 50.4 kJ mol

1

13.3 Heat of Displacement

1. Heat of displacement is the heat change when 1 mole of metal is displaced from its salt solution by a more

electropositive metal under standard condition.

2. When copper is displaced from aqueous copper(II) sulphate by iron filing, -150 kJ mol1

of energy is released.

Fe(s) + CuSO4(aq) Cu + FeSO4(aq) ; H = -150 kJ mol1

3. Energy level diagram:

Cu2+

(aq) + Fe(s)

Energy H = - 150 kJ mol 1

Cu(s) + Fe 2+(aq)

4. An experimental result:

An excess of zinc powder is added with stirring to 50 cm3

of 0.2 mol dm3

copper(II) sulphate solution in a

polystyrene cup. It was found that the temperature increased by 9.5 oC.

Calculation:

Zn(s) + CuSO4(aq) Cu + ZnSO4(aq)

Number of moles of CuSO4 =50 x 0.2

1000

= 0.01

0.01 mol of zinc reacts with 0.01 mol of CuSO4 to form 0.01 mol of metallic copper

Heat evolved = m c

= 50 x 4.2 x 9.5 = 1995 J

H = - 19950.01

= - 199500 J

= -199.5 kJ mol1

13.4 Heat of Neutralization

1. Neutralization occurs when an acid reacts with an alkali or a base to form a salt and water only.

2. Water is formed by the reaction between hydrogen ions from the acid and hydroxide ions from the base. The

ionic equation for neutralization is

H+

(aq) + OH

(aq) H2O(l)

3. Heat of neutralization is the amount of heat energy released when 1 mole of hydrogen ion from an acid solution

reacts with 1 mole of hydroxide ion from an alkali to form 1 mole of water molecules under standard conditions.

4. Theoretically, the heat of neutralization of a strong acid by a strong base is always

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57.2 kJ mol1

. This is because strong acids and strong bases are completely ionized in solution and 1 mole of water

molecules are formed. The other ions are just spectators.

HCl(aq) + NaOH(aq) NaCl(aq) + H 2O (l) ; H =57.2 kJ mol1

HNO3(aq) + KOH(aq) KNO3(aq) + H 2O (l) ; H =57.2 kJ mol1

The net ionic equation for the above two reactions are just

H+

(aq) + OH

(aq) H2O(l) ; H =57.2 kJ mol1

5. Sulphuric acid is a strong dibasic acid. Hence when 1 mole of sulphuric acid is fully neutralized by 2 moles of

NaOH, the total heat energy released should be

(2x57.2) kJ or114.4 kJ.

H 2 SO 4 (aq) + 2NaOH(aq) Na2SO4(aq) + 2 H2O (l)

6. Energy level diagram:

HCl(aq) + NaOH(aq)

Energy H = - 57.2 kJ mol 1

NaCl(aq) + H2O(l)

7. The heat of neutralization of a weak acid and a strong base is lower than that between a strong acid and a strong

base because the weak acid is only partially ionized in solution. For example, the heat of neutralization between

ethanoic acid and NaOH is only 55.0 kJ mol1

..

8. Ethanoic acid is a weak acid and is partially ionized. Some heat is absorbed for breaking the OH bond of the

acid.

CH3COOH(aq) + NaOH(aq) CH3COONa(aq) + H2O (l) ; H =57.2 kJ mol1

9. Experimental result:

50 cm3

of 1.0 mol dm3

sodium hydroxide was added with stirring to 50 cm3

of 1.0 mol dm3

hydrochloric acid

in a polystyrene cup. The temperature rise is 6.5oC.

Calculation:

HCl(aq) + NaOH(aq) NaCl(aq) + H 2O (l)

Number of moles of NaOH =1.0 x 50

1000

= 0.05

Number of moles of HCl =1.0 x 50

1000

= 0.05

0.05 mol of HCl exactly neutralises 0.05 mol of NaOH

Heat evolved = m c

= 100 x 4.2 x 6.5 = 2730 J

Heat of neutralization, H = -2730

0.05= - 4600

= - 54.6 kJ mol1

13.5 Heat of Combustion

1. The heat of combustion of a substance is the amount of heat energy released when 1 mol of the substance is

completely burnt in excess of oxygen under standard conditions.

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2. The heat of combustion of 1 mole of ethanol in excess oxygen is 1376 kJ mol1

CH3CH2OH + 3O2 2CO2 + 3 H 2O ; H = - 1376 kJ mol1

3. Energy level diagram:

CH3CH2OH + 3O2

Energy H = - 1376 kJ mol 1

2CO2 + 3 H 2O

4. Heat of combustion of ethano the set-up of the whole experiment.

(b) Determine the mass of ethanol burnt.

(c) Calculate number of moles of ethanol burnt.

(d) Note of volume of water heated by ethanol.

(e) Record the rise in temperature of the water

(f) Calculate the amount of heat released by using m c

(g) Calculate the total heat evolved per mol of ethanol.

5. Experimental result:

Mass of ethanol burnt = 4.6 g

Volume of water used = 500 cm 3

Increase in temperature of water = 47.5 oC

Calculation:

Number of moles of ethanol =4.6

46= 0.1

Heat released = m c

= 500 x 4.2 x 47.5 = 99750 J

= 99.75 kJ

When 0.1 mol of ethanol is burnt, the heat evolved is 99.75 kJ

Hence when 1 mol of ethanol is burnt, the total heat released is99.75

0.1= 997.5 kJ mol 1

13.6 Selection of Fuels

1. Factors which must be considered when selecting a fuel:

(a) fuel value: the higher the fuel value, the higher is the energy released per gram of the fuel.

(b) effect on the surrounding: the fuel should not burnt with sooty flames or give out toxic gases when burnt, liquid fuels andgaseous fuels do not leave behind ashes or residue after combution, they are better fuels than solid fuels.

(c) low cost : the fuel should not be expensive; propane, butane, and petrol are expensive fuels.

(d) can be burnt easily or with low activation energy: fuels which can be burnt easily have low ignition temperatures

Assessment 13

Objective Question

1. Mangesium nitrate solution reacts with sodium carbonate solution under room conditions. The reaction is exothermic.

Which of the following statements is true of the reaction?

A. Activation energy of the reaction is zero.

B. Heat of reaction is a positive value.

C. Reactants have the same energy content as the products.

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D. A maximum temperature of reaction mixture will be obtained.

2. Which of the following is an endothermic reaction?

A. Reaction between acid and alkali. B. Potassium nitrate dissolves in water.

C. Reaction between acid with a metal carbonate.

D. The displacement of a metal from its salt solution.

3. The heat of precipitation of a substance is

A. the heat change when 1 mole of the substance is formed.

B. the heat change when 1 mole of the substance is consumed.

C. the heat change for 1oC increase in temperature. D. the heat change when 1 mole of water is formed.

4. When 50 cm 3 of 1.0 mol dm 3 lead(II) nitrate solution is added to 50 cm 3 of

1.0 mol dm 3 sodium chloride solution, a white precipitate is formed. The temperature of the mixture increased by 8.5 oC.

What is the heat energy released in this reaction?

[Specific heat capacity of water = 4.2 J g

1

o

C

1

;desnity of solution = 1 g cm

3

]A. 1785 J B. 3570 J C. 1785 kJ D. 3570 kJ

5. The figure below shows the set-up of apparatus used to determine the heat energy given out in the reaction between

magnesium and zinc sulphate solution.

The function of the plastic cup is to

A. reduce heat loss to the surrounding B. reduce evaporation of the mixture

C. maintain a constant temperature for the mixture

D. enable the temperature of the mixture to increase quickly.

6. Which of the following data are required to determine the heat of reaction between excess zinc powder and copper(II) nitrate

solution?

I Relative atomic mass of zinc II Volume of copper(II) nitrate solution

III Concentration of copper(II) nitrate solution IV The increase in temperature of the mixture.

A. I and III only B. II and IV only C. II, III, and IV only D. I, II, III, and IV.

7. The figure below shows the energy level diagram for the reaction between zinc and copper(II) nitrate solution.

Cu2+

(aq) + Zn(s)

Energy

H = - 190 kJ mol

1

Cu(s) + Zn 2+(aq)

What is the energy released if 2 g of copper is formed?

[Relative atomic mass: Cu = 64]

8. A mixture of 50 cm3

of 1.0 mol dm 3sodium hydroxide solution and 50 cm

3of 1.0 mol dm

3hydrochloric acid releases

2850 J of energy. Which of the following mixtures will produce the same amount of energy?

I 50 cm3

of 1.0 KOH and 50 cm3

of 1.0 mol dm 3

HCl

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II 50 cm3

of 1.0 mol dm 3

NaOH and 1 dm3

of 0.025 mol dm 3

H2SO4

III 50 cm 3 of 0.5 mol dm 3 NaOH and 1 dm 3 of 1.0 mol dm 3 HNO3

IV 50 cm 3 of 1.0 mol dm 3 KOH and 50 cm 3 of 1.0 mol dm 3 HNO3

A. I and II only B. I and IV only C. I, II, and IV only D. I, II, III, and IV

9. The figure below shows the energy level diagram for the combustion of ethanol in excess oxygen.

CH3CH2OH + 3O2

Energy H = - 1366 kJ mol 1

2CO2 + 3 H 2O

What is the mass of ethanol required to release 273.2 kJ of energy?

[Relative atomic mass: H=1 ; C=12; O=16]A. 4.6 g B. 9.2 g C. 13.1 g D. 46 g

10. The table below shows the heat of combustion of three alcohols.

Alcohol Heat of combustion/ kJ mol 1

Methanol -725

Ethanol -1376

Propanol -2015

Which of the following statements is not true?

A. Propanol is more flammable than methanol

B. The heat of combustion increases from methanol to propanol

C. Propanol releases the highest energy when it is burnt because it has the largest molecule

D. Molecule with higher percentage of carbon releases higher amount of energy when it is burnt.

Structured Question

1. An experiment was carried out to determine the heat of precipitation of barium sulphate, BaSO4. The initial temperature of 50 cm3 of

1.0 mol dm 3 barium nitrate solution was recorded and the solution was added to 50 cm 3 of 1.0 mol dm 3 potassium sulphate solution. The

highest temperature of the reaction mixture was recorded. The readings of the thermometer are shown in the figure below.

The mixture was stirred and the highest temperature recorded.

(a) Why is polystyrene cup used? [1 mark]

(b) What is the colour of the precipitate formed? [1mark]

(c) What type of reaction is this in terms of heat change? [1 mark]

(d) Calculate the change in temperature. [1 mark]

(e) Construct the equation for this reaction. [1 mark]

(f) Calculate the heat change for this reaction. [1 mark]

(g) Calculate the heat change to form one mole of barium sulphate in this experiment. [1 mark]

(h) Draw the energy level diagram for this reaction. [Density of solution = 1.0 g cm 3; Specific heat capacity of water = 4.2 J g 1oC 1]

2. An experiment was carried out by a student to determine the heat of displacement of iron from its salt solution by magnesium. About 4 g

of magnesium powder was added into a plastic cup containing 50 cm 3 of 1.0 mol dm 3 iron(II) sulphate solution. The mixture was stirred

by using a thermometer until the highest temperature was obtained. The table below shows the data of the experiment.

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Initial temperature of iron(II) sulphate 28.6 oC

Highest temperature of the mixture 49.6 oC

[Relative atomic mass: Mg , 24; density of solution, g cm 3 , Specific heat capacity of

water = 4.2 J g 1oC

1]

(a) Besides the change in temperature, state one other observation in this experiment. [1 mark]

(b) What are the data requried in calculating the heat of displacement of iron. [2 marks]

(c) Construct an overall ionic equation for this reaction. [1 mark]

(d) Calculate the heat change for this reaction. [1 mark]

(e) Calculate the heat of displacement of iron from iron(II) sulphate by magnesium. [2 marks]

(f) Draw an energy level diagram for this reaction. [2 marks]

Essay Question

1. (a) Define exothermic and endothermic reactions. Give one example for each type of reaction.

[4 marks]

(b) Outline an experiment to determine the heat of precipitation of lead(II) chloride. Your answer should include workings of

calculations, assumptions made, and energy level diagram for the reaction. [12 marks](c) Give the advantages of using liquid hydrogen as fuel for rockets. [4 marks]