30

CHAPTER 4: THERMOCHEMISTRY

THERMOCHEMISTRY

Energy changes in chemical reaction

Heat of formation

Heat of displacement

Heat of neutralization

4.1 4.2

4.3

4.5

Heat of combustion

4.4

Several source of energy

1. Define the exothermic reaction. 2. .Define the endothermic reaction. 3. Give 3 examples of exothermic reaction and 3 examples of endothermic reaction 4. Compare and contrast between exothermic and endothermic reaction

1. Define the precipitation reaction. 2. Define the heat of precipitation. 3. Give 3 example of precipitation reaction and then write the chemical equation respectively.

1. Define the displacement reaction. 2. Define the heat of displacement. 3. Give 3 example of displacement reaction and then write the chemical equation respectively. 4. Describe an experiment to determine the heat of displacement for a certain displacement reaction.

1. Define the neutralization reaction. 2. Define the heat of neutralization. 3. Give 3 examples of neutralization reaction and then write their chemical equation respectively. 4. . Describe an experiment to determine the heat of neutralization for a certain displacement reaction between an acid and an alkali.

1. Define the combustion reaction. 2. Define the heat of combustion. 3. Give 3 examples of combustion reaction and then write their chemical equation respectively. 4. . Describe an experiment to determine the heat of combustion of ethanol.

31

B.4.1

ENERGY CHANGE IN CHEMICAL REACTIONS

Type of thermo chemical reaction

Exothermic reaction

Formulae of reactant

Energy

Endothermic reaction

Formulae of reactant

Endothermic reaction

Exothermic reaction

Examples of exothermic reaction; Examples of endothermic reaction;

Heat of reaction, ∆ H

Definition:

Energy level diagram

∆ H = - (Exothermic) ∆ H = + (Endothermic)

Energy change and chemical bond

Application of exothermic and endothermic reactions

Definition:

Definition:

Formulae of product

Formulae of product

All chemical reaction should involved breaking and formation of bond Heat is absorbed as the bond is breaking in the reactant. Heat is releases as the bond is forming in the product. If the total of heat released > total of heat absorbed… Exothermic If the total of heat realeased < total of heat absorbed… Endothermic

Change of heat, (Q)

Q = mcθθθθ

Assumption: 1. Density of solution

= 1g cm-3 2. Concentration of

solution is very low

Energy

32

B. 4.2

HEAT OF PRECIPITATION

Definition

Experiment B1: To determine heat of precipitation of silver chloride

Lab Report for the above experiment:

Diagram

33

B. 4.3

HEAT OF DISPLACEMENT

Definition

Experiment: To determine copper's heat of displacement by zinc

Lab. Report for the above experiment:

Diagram

34

B. 4.4

HEAT OF NEUTRALIZATION

Definition

To determine Heat of Neutralization

Experiment

Examples of calculation/ question

Calculation involved heat of neutralization;

The heat evolved when one mole of H+ reacted with one mole of OH- to produce one mole of water.

1 Strong acid with strong alkali

2 Weak acid with strong alkali

Plastic Acid

Alkali

Thermometer

Comparing heat of neutralization; 1. Why heat of neutralization

of sulphuric acid is double compare to nitric acid?

2. Why heat of neutralization of acetic acid is lower than hydrochloric even both of them are monobasic?

35

B.4.5

HEAT OF COMBUSTION

Definition

Determination of the heat of combustion of alcohols

Experiment; Heat of combustion of Ethanol

Comparison of heat of combustion for several substances

Alcohol

Food

Calculation

Calculate heat of combustion for ethanol if 0.8 g ethanol is burnt to rise the temperature of 100 g of water as much as 30o C.

Metal tin

Water

Lamp filled with ethanol

Thermometer

Quantity of heat evolved when one mole fuel is burnt completely.

Alcohol Heat kJ/mol CH3OH C2H5OH C3H7OH

Food Heat kJg-1 Rice 15 Margarine 34 Orange

Effectiveness of any fuel is depend to the heat evolved per gram

36

1. Write down the type of reaction ( exothermic or endothermic reaction ) in the column provided.

Statements Type of reaction 1. A chemical reaction that gives out heat to the surrounding

2. A chemical reaction that absorbs heat energy from the surrounding

3. The temperature of reaction increases

4. The temperature of reaction decreases

5. Energy required to form bonds of the products are higher than the energy required to break bonds of the reactants.

6. Energy required to form bonds of the products are lower than the energy required to break bonds of the reactants

7 The products level of the energy level diagram is lower than the reactants level.

8 The reactants level of the energy level diagram is lower than the products level.

2. Examples of exothermic reactions

(a) Neutralization reactions.

(i) (ii) (iii)

(b) Reaction between metals and acids

(i) (ii) (iii)

(c) Reaction between metal carbonates and acids

(i) (ii) (iii)

(d) Precipitation reactions

(i) (ii) (iii)

37

(e) Displacement reactions

(i) (ii) (iii)

(f) Combustion of fuels.

(i) (ii) (iii)

3. Examples of endothermic reactions:

(i) Dissolving of ammonium salts such as ammonium chloride, ammonium sulphate, ammonium nitrate.

(a) NH4Cl(p) + H2O � NH4

+ (ak) + Cl- (ak)

(b) ……………………………………………………….

(c) ………………………………………………………

(ii) Heat decomposition of metal carbonates.

(a) (b) (c)

(iii) Heat decomposition of metal nitrates.

(a) (b)

38

4. Write down the ‘products’, ‘reactants’, ‘positive or negqtive’ and the type of reaction ( endothermic or exothermic reaction) in the space provided. Energy ? ∆H = ____?_______ ?

Type of reaction : _______________________

Energy ∆H = ___________

Type of reaction : _______________________ 5. Construct an energy level diagram for each of the following chemical reactions. eg. Mg (s) + H2SO4 (aq) � MgSO4 (aq) + H2 (g) ∆H = - 467kJ Energy Mg (s) + H2SO4 (aq) ∆H = -467kJ MgSO4 (aq) + H2 (g)

39

Chemical reactions Energy level diagram 1. 2HgO (s) � 2Hg (l) + O2 ∆H = +182 kJ mol-1

2. Zn (s) + CuSO4 (aq) � ZnSO4 (aq) + Cu (s) ∆H = - 190 kJ mol-1

3. Pb 2+ (aq) + SO4

2- (aq) � PbSO4 (s) ∆H = - 50 kJ mol-1

40

6. Fill in the boxes with the correct answers.

7. Match column A to column B

A B • m • Temperature change • C • joule • θ • Heat of reaction

• mc θ • Mass of solution • ∆H • Specific heat capacity of solution • J • Amount of heat given out.

8. In an experiment, to determine the heat of precipitation of lead(II) sulphate, 50 cm3 of 0.5 mol dm-3 lead(II) nitrate solution is added to 50 cm3 of 0.5 mol dm-3 sodium sulphate solution in a plastic cup. The following results are obtained.

Initial temperature of lead(II) nitrate solution = 28.0oC Initial temperature of sodium sulphate solution = 28.0oC Highest temperature reached when the two solutions are mixed = 30.5oC

Definitions Type of Heat of Reaction Examples The heat change when one mole of a metal is displaced from its salt solution by a more electropositive metals.

1) 2)

Heat of Precipitation

1) AgNO3 (aq) + NaCl (aq) � AgCl (s) + NaNO3 (aq)

2)

The heat change when one mole of water is formed frm the reaction between an acid and an alkali.

1) 2)

The heat change when one mole of a substance is completely burnt in oxygen under standard conditions.

1) 2)

41

(a) Calculate the number of mole of the following ions

(i) Pb2+ ions (ii) SO42- ions (iii) the number of mole of lead(II) sulphate

precipitated

(b) Calculate the heat given out in this experiment m = c = 4.2 Jg-1oC-1 θ = ( highest temperature - lowest temperature ) = Then, mcθ =

(c) Calculate the heat of precipitation. 8. In an experiment to determine the heat of neutralization between hydrochloric acid and ammonia solution, 50cm3 of 1.0 mol dm-3 hydrochloric acid is added to 50cm3 of 1.0 mol dm-3 ammonia solution. The temperature increases from 29.0oC to 35.5oC. Calculate the heat of neutralization.

(d) Calculate the number of mole of the following ions

(i) H+ ions (ii) OH- ions (iii) the number of mole of H2O formed

(e) Calculate the heat given out in this experiment m = c = 4.2 Jg-1oC-1 θ = ( highest temperature - lowest temperature ) = Then, mcθ =

42

(f) Calculate the heat of neutralization

(g) Draw an energy level diagram for the above reaction.

43

Objective Questions 1 Which of the following absorbs heat from the surroundings? A Adding acid to alkali. B Adding concentrated acid to water. C Dissolving solid ammonium chloride in water D Dissolving anhydrous copper(II) sulphate in water. 2 Figure below shows an energy level diagram

OH-

(aq) + H+(aq)

Energy

∆ H = - 57 kJ mol-1

H2O (l)

Based on the figure, it can be concluded that A the heat of neutralization is – 57 kJ mol-1

B 57 kJ of energy is needed for the reaction. C the products of reaction contain more energy than the reactants. D the temperature at the end of the reaction is lower than that at the begining of the reaction.

3 Figure shows an energy level diagram.

Energy J (s) + T2+ (aq) ∆ H = - 220 kJ mol-1 J2+

(aq) + T(s) Based on the figure, what is the increase in temperature of the solution if excess J powder is added to 50 cm3 of T salt solution 0.2 mol dm-3? A 4.4oC B 5.5oC C 8.8oC D 11.0oC

44

4 The following equation shows the reaction between Ag+ and Cl- ions. Ag+

(aq) + Cl- (aq) � AgCl (s) ∆ H = - 65 kJ mol-1 Which of the following is true about the above equation? A Endothermic reaction occurs. B Heat is released to the surroundings. C The temperature of the product decreases. D 65 kJ of heat is absorbed when 1 mole of silver chloride is formed.

5 The following equation shows the combustion of heptane, C7H16 in excess oxygen. C7H16 (l) + 11O2 (g) � 7CO2 (g) + 8H2O (l), ∆ H = - 5512 kJ mol-1

The combustion of heptane in excess oxygen releases 1378 kJ of energy. What is the mass of heptane used? Use the information that thee relative atomic mass of H = 1 and C = 12 A 25.0 g B 36.0 g C 77.0 g D 88.0 g

Structure Questions 1. A student carried out an experiment to determine the heat of displacement for the reaction between copper and silver nitrate solution. In this experiment excess copper powder was added to 100cm3 of silver nitrate solution 0.5 mol dm-3. The heat of displacement in this experiment was -105 kJ mol-1. [ Specific heat capacity of the solution is 4.2 J g-1 oC-1, and the density of the solution is 1 gcm-1 ]

(a) What is meant by heat of displacement ?

……………………………………………………………………………………… ……………………………………………………………………………………… [ 1 mark ]

(b) Besides the data given above, state one other piece of data that is needed to calculate the heat of displacement.

……………………………………………………………………………………… ……………………………………………………………………………………… [ 1 mark ]

(c) State one precaution that must be taken while carrying out the experiment.

……………………………………………………………………………………… ………………………………………………………………………………………

45

[ 1 mark ]

(d) (i) State one observation of the experiment.

………………………………………………………………………………… [ 1 mark ]

(ii) State the reason for the observation in (d)(i) …………………………………………………………………………………

[ 1 mark ] (e) Based on the information from this experiment, calculate

(i) the number of moles of silver ions reacted. [ 1 mark ] (ii) the amount of heat released. [ 1 mark ] (iii) the change in temperature [ 1 mark ]

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

(g) The experiment is repeated using 100cm3 of 1.0 mol dm-3 silver nitrate solution and excess copper powder. Calculate the temperature change in this experiment. Explain why this change of temperature is different from that in (e)(iii). [ 3 marks ]

46

2.

Figure 4

Figure 4 shows the set-up of the apparatus of an experiment to determine the heat of precipitation. 25.0 cm3 of 0.5 mol dm-3 silver nitrate solution is reacted with 25.0 cm3 of 0.5 mol dm-3 sodium chloride solution. As a result there is a change in temperature of the mixture and a white precipitate is formed.

(a) Why is a polystyrene cup used in the experiment ?

……………………………………………………………………………………………… [ 1 mark ]

Before reaction

Sodium chloride solution Initial temperature = 29.0 0 C

Thermometer

Polystyrene cup

Silver nitrate solution Initial temperature = 29.0 0 C

Thermometer

Polystyrene cup

White percipitate

Mixture of the solution Maxsimum temperature = 31.5 o C

After reaction

47

(b) (i) Based on the change of temperature in the experiment, state the type of reaction that occurred. …………………………………………………………………………………………. [ 1 mark ]

(ii) How is the total energy of the products different from the total energy of the reactants? ………………………………………………………………………………………….. [ 1 mark ]

(c) State one step that should be taken while adding the two solutions in order to get a more accurate result..

……………………………………………………………………………………………….. [ 1 mark ]

(d) The ionic equation for the precipitation reaction of silver chloride is : Ag+ (aq) + Cl- (aq) � AgCl (s)

(i) What is the number of moles of Ag+ ions that reacted with Cl- ions ? [ 1 mark ]

(ii) Calculate the heat change of the precipitation reaction that has taken place.

Use the information that the specific heat capacity of water is 4.2 J g-1 oC-1 and density of water is 1 gcm-1. [ 2 marks ]

(iii) Calculate the heat of precipitation for this reaction. [ 2 marks ]

(e) The calculated value of the heat of precipitation for this reaction is less than the actual value. Give a reason. …………………………………………………………………………………………………. …………………………………………………………………………………………………

48

[ 1 mark ]