2 9 13 Thermochemistry

TUTORIAL 9: THERMOCHEMISTRY

1.

2.

3.

The following terms are often used in thermochemistry. Explain each one of them by giving an appropriate example:
(a)

Standard enthalpy change

(b)

Exothermic process

(c)

Endothermic process

Define and write an example of thermochemical equation for each of the following terms:
(a)

Enthalpy of formation

(b)

Enthalpy of combustion

(c)

Enthalpy of atomisation

(d)

Enthalpy of neutralisation

(e)

Enthalpy of hydration

(f)

Enthalpy of solution (dissolution)

(g)

Lattice energy

Consider the following reaction:
H2(g) + ½ O2(g)  H2O(g)

ΔH = 241.8 kJ

(a)

Sketch the energy profile diagram of the above reaction.

(b)

Which enthalpy is larger, that of the reactants or the product?

(c)

For the reaction,
2H2O(g)  2H2(g) + O2(g) what is the enthalpy change under the same conditions?

4.

(a)

The following reaction occurs at room temperature and pressure:
2Cl(g)  Cl2(g)

ΔH = -ve

State which has higher enthalpy under this condition, Cl(g) or Cl2(g)?
5.

From the following data,
2Fe(s) +

3

2 O2(g)

ΔHf = 822.2 kJ mol1

 Fe2O3(s)

determine the enthalpy of combustion of iron.
6.

A bomb calorimetric experiment is performed with powdered xylose, C 5H10O5 as the combustible substance. The data obtained are:
Mass of xylose burned

=

1.183 g

Heat capacity of calorimeter

=

4.728 kJ0C-1

Initial calorimeter temperature =

23.290C

Final calorimeter temperature

27.190C

=

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TUTORIAL 9: THERMOCHEMISTRY

i) ii) 7.

Calculate the heat of combustion of xylose, in kJmol -1
Write the thermochemical equation for combustion of xylose

A quantity of 1.922 g of methanol (CH3OH) was burned in a constant-volume bomb calorimeter. Consequently, the temperature of the water rose by 4.200C. If the quantity of water surrounding the calorimeter was exactly 2000g and the heat capacity of the calorimeter was 2.02 kJ0C-1, calculate the molar heat of combustion of methanol
[The specific heat of water is 4.18 J g1 C1].

8.

When 200 mL of 1.00 M NaOH was mixed with 150 mL of 1.00 M HCl in a styrofoam
‘coffee cup’ calorimeter, the temperature rose from 25.00C to 30.00C. Calculate the heat of neutralisation. Assume that the specific heat of solution is 4.18 J g1 C1 and density of solution is 1 g mL1.

9.

(a)

State Hess’s Law

(b)

The enthalpies of formation of ammonia, hydrogen chloride and ammonium chloride are -46.1 kJmol-1, -92.3 kJmol-1, -14.4kJmol-1 respectively
i)

Write the thermochemical equation for the formation of each of the above substances ii)

Calculate the enthalpy change for the reaction
NH3 (g) + HCl (g)  NH4Cl

10.

Nitrogen oxides undergo many interesting reactions. Calculate ΔH for the overall equation
2NO2 (g) + O2 (g)  N2O5 (s)
From the following equations:

11.

N2O5(s)  2NO(g) + O2(g)

ΔH = 223.7 kJ

NO (g) + O2 (g)  NO2 (g)

ΔH = -51.7 kJ

Given:
S(s) + O2(g)  SO2(g)

ΔH = 297 kJ

SO2(g) + ½O2(g)  SO3(g)

ΔH = 99 kJ

Calculate ΔH for the following reaction using energy cycle method:
S(s) +
12.

3

2

O2(g)  SO3(g)

Acetylene, C2H2, is a gas used in welding torches. It is produced by the action of water on calcium carbide, CaC2. Calculate H f for acetylene in kJ mol1 by using the thermochemical equations given below.
CaO(s) + H2O(l)  Ca(OH)2(s)

∆H = 65.3 kJ

CaO(s) + 3C(s, graphite)  CaC2(s) + CO(g)

∆H = +462.3 kJ

CaC2(s) + 2H2O(l)  Ca(OH)2(s) + C2H2(g)

∆H = 126.0 kJ

C(s, graphite) + ½O2(g)  CO(g)

∆H = 220.0 kJ

2H2O(l)  2H2(g) + O2(g)

∆H = +572.0 kJ
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TUTORIAL 9: THERMOCHEMISTRY

13.

Which substance in the following pairs has larger lattice energy? Explain.
KCl or RbCl

14.

Based on the data given below:
ΔHohydration Na+

=

-390 kJmol-1

ΔHohydration Cl-

=

-380 kJmol-1

ΔHosolution NaCl

=

+6 kJmol-1

i) ii) 15.

16.

Construct an energy cycle diagram to represent the dissolution of NaCl
Calculate the lattice energy of NaCl

Use a Born Haber cycle for KF and the following values to calculate a value for the standard enthalpy of formation of KF(s)
K (s)  K (g)

ΔHo = 90kJ

K (g)  K+ (g) + ē

ΔHo = 419kJ

F2 (g)  2F (g)

ΔHo = 160kJ

F (g) + ē  F- (g)

ΔHo = -336kJ

K+ (g) + F- (g)  KF (s)

ΔHo = -821kJ

Use the data below to construct a Born-Haber cycle for copper(II) oxide and determine its lattice energy.
Enthalpy of atomisation of copper

: +339 kJ mol1

First ionisation energy of copper

: +745 kJ mol1

Second ionisation energy of copper

: +1960 kJ mol1

Enthalpy of atomisation of oxygen

: +248 kJ mol1

First electron affinity of oxygen

: –141 kJ mol1

Second electron affinity of oxygen

: +791 kJ mol1

Enthalpy of formation of copper(II) oxide

: –155 kJ mol1

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TUTORIAL 9: THERMOCHEMISTRY

OBJECTIVE QUESTIONS
1.

2.

3

4.

Which of the following processes is endothermic?
A.

H2O(s)  H2O(l)

B.

O2(g) + 2H2(g)  2H2O(g)

C.

CH4(g) + 2O2(g)  CO2(g) + 2H2O(l)

D.

3O2(g) + 2CH3OH(g)  2CO2(g) + 4H2O(g)

Which of the following is an exothermic process?
A.

ice melting

B.

boiling soup

C.

water evaporating

D.

condensation of water vapour

A __________ H corresponds to an __________ process.
A.

negative, endothermic

B.

negative, exothermic

C.

zero, endothermic

D.

zero, exothermic

Consider the following reaction:
2Na2O2(s) + 2H2O(l)  4NaOH(s) + O2(g)

H = 126 kJ.

The amount of heat released by the reaction of 25.0 g of Na2O2 with water is

5.

A.

20.2 kJ

B.

40.4 kJ

C.

67.5 kJ

D.

80.8 kJ

A sample of aluminium metal absorbs 9.86 J of heat when its temperature increases from
23.2C to 30.5C. If the specific heat capacity of aluminium is 0.90 J g1 C1, the mass of the sample is
A.

1.5 g

B.

7.2 g

C.

8.1 g

D.

65 g

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TUTORIAL 9: THERMOCHEMISTRY

6.

7.

8.

Choose a substance with H f  0
A.

I2(s)

B.

Na(s)

C.

Br2(l)

D.

CO2(g)

Enthalpy of neutralisation is the heat released when
I.

1 mol of acid reacts with 1 mol of base.

II.

1 mol of aqueous H+ reacts with 1 mol of aqueous OH.

III.

1 mol of water is formed from the reaction of an acid and a base.

Choose the correct statement(s).
I.

Ionisation energy is always positive.

II.

Electron affinity is always negative.

III.

Enthalpy of hydration can be positive or negative.

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