Thermodynamics (A Level only) (AQA A Level Chemistry)

Exam Questions

4 hours31 questions
1a5 marks

Lattice enthalpy is a measure of the strength of the forces between the ions in an ionic solid.

i)
Give the definition of the term enthalpy of lattice formation.

ii)

Write one equation to represent each the following changes:

Atomisation of sodium

Second ionisation energy of magnesium

First electron affinity of chlorine

1b3 marks

Complete Table 1 for the following Born-Harber cycle for formation of potassium fluoride shown in Figure 1. Include equations and state symbols in your answer.

Figure 1

pYAKIGd-_5

Table 1

Step

Name of the Enthalpy Change

1

 

2

Atomisation of potassium

3

 

4

First ionisation energy of potassium

5

 

6

Lattice enthalpy of formation

1c3 marks

The enthalpy of lattice formation of potassium fluoride and caesium fluoride is -830 kJ mol-1 and -730 kJ mol-1 respectively.

Explain why the enthalpy of lattice formation is more exothermic for potassium fluoride.

1d3 marks

Use the data in Table 2 to calculate the enthalpy of solution of potassium fluoride.

Table 2

ΔHӨlattKF (kJ mol-1)

+830

ΔHӨhydK+  (kJ mol-1)

-351

ΔHӨhydF-  (kJ mol-1)

-504

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2a3 marks

Using the data in Table 1 calculate the enthalpy of lattice formation of sodium chloride.

Table 1

Enthalpy change

Enthalpy change

Enthalpy change (kJ mol-1)

ΔHӨfNaCl

Na (s) + ½Cl2 (g) → NaCl (s)

-411

ΔHӨatCl

½Cl2 (g) → Cl (g)

121

ΔHӨatNa

Na (s) → Na (g)

108

ΔHӨEACl

Cl (g) → Cl- (g)

-349

ΔHӨIENa

Na (g) → Na+ (g)

496

ΔHӨlattNaCl

Na+ (g) + Cl- (g) → NaCl (s)

To be calculated

2b3 marks

State the definition of electron affinity.

2c3 marks

Figure 1 shows a part of a Born-Haber cycle for the formation of magnesium oxide. The arrow for the first electron affinity of oxygen points down so has a negative value, and the arrow for the second electron affinity of oxygen points upwards, so has a positive value.

Figure 1

Ynxmcfgd_4

Explain why the arrows point in different directions.

2d3 marks

Using your answer to part (a) and the data in Table 2 calculate the enthalpy of solution for sodium chloride.

If you did not get an answer for part (a) use the value -800 kJ mol-1. This is not the correct value.

Table 2

ΔHӨlattNaCl (kJ mol-1)

To be calculated

ΔHӨhydNa+ (kJ mol-1)

-404

ΔHӨhydCl- (kJ mol-1)

-381

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3a2 marks

Figure 1 shows how the entropy of compound Y varies with temperature.

Figure 1

kql~NIW6_2

State the changes occurring at T1 and T2.

3b4 marks

Using the data in Table 1 calculate the entropy changes for the following reactions:

i)

1 halfO2 (g) + H2 (g) → H2O (g)

ii)
CH4 (g) + 2O2 (g) → 2H2O (g) + CO2 (g)

Show your working for each reaction.

Table 1

Substance

CH4 (g)

O2 (g)

H2O (g)

H2 (g)

CO2 (g)

S (J K-1 mol-1)

196

205

189

131

214

3c3 marks

Using Table 2 calculate the enthalpy of reaction for the combustion of methane

Table 2

Substance

CH4 (g)

O2 (g)

H2O (g)

CO2 (g)

ΔHf (kJ mol-1)

-74.0

0

-241.8

-393.5

3d3 marks

Using your answers to parts (b) and (c) calculate the gibbs free energy change for the combustion of methane at 400 K.

If you did not get an answer for part (b) or (c), use the values:

ΔS = -19 J K-1 mol-1

ΔH = -750 kJ mol-1

These are not the correct values.

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4a3 marks

This question is about the lattice enthalpy of lithium oxide.

Complete the Born-Haber cycle for the formation of lithium oxide, Li2O, shown in Figure 1 by writing the missing equations on the energy levels.

Figure 1

LiqCLf9v_3

4b2 marks

Give the definition of ionisation enthalpy, ΔHIE.

4c3 marks

Using the data in Table 1, calculate the enthalpy of lattice formation of lithium oxide.

Table 1

Enthalpy change

Enthalpy change (kJ mol-1)

ΔHӨf Li2O

-598

ΔHӨat O

248

2 x ΔHӨat Li

322

ΔHӨEA1 O

-142

ΔHӨEA2 O

844

2 x ΔHӨIE Li

1040

ΔHӨlatt Li2O

To be calculated

4d2 marks

Explain why the value for the enthalpy of ionisation of Li is positive.

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5a3 marks

Use the data in Table 1 to calculate ΔH for the decomposition of sodium hydrogen carbonate, NaHCO3 (s).

2NaHCO3 (s) → Na2CO3 (s) + CO2 (g) + H2O (g)

Table 1

Compound

ΔHf Value (kJ mol-1)

NaHCO3 (s)

-951

Na2CO3 (s)

-1131

CO2 (g)

-394

H2O (g)

-242

5b3 marks

Use the data in Table 2 to calculate ΔS for the decomposition of sodium hydrogen carbonate, NaHCO3 (s).

Table 2

Compound

 S J K-1 mol-1

NaHCO3 (s)

102

Na2CO3 (s)

135

CO2 (g)

214

H2O (g)

189

5c3 marks

Use your answers to part (a) and (b) to calculate the temperature that the decomposition of sodium hydrogen carbonate will become feasible.

5d1 mark

If a reaction has a positive value for ΔH and a negative value for ΔS predict whether or not the reaction will be feasible at any temperature.

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1a5 marks

This question is about fluorine and the associated energy changes when it is reacted with magnesium to form magnesium fluoride.

i)
Define the term electron affinity for fluorine.

ii)

Use the data from Table 1 below to calculate a value for the electron affinity of fluorine.

Table 1

Name of enthalpy change

Energy change / kJ mol-1

Enthalpy of atomisation of magnesium

+150

Enthalpy of atomisation of fluorine

+121

First ionisation energy of magnesium

+736

Second ionisation energy of magnesium

+1450

Enthalpy of formation of magnesium fluoride

-642

Lattice enthalpy of formation of magnesium fluoride

-2493

1b4 marks

Table 2 below gives some values of standard enthalpy changes. Use these values to answer the questions.

Table 2

Name of enthalpy change

Energy change / kJ mol-1

Enthalpy of atomisation of bromine

+112

Electron affinity of bromine

-325

Enthalpy of atomisation of silver

+289

First ionisation enthalpy of silver

+732

Enthalpy of formation of silver bromide

-100

i)
Suggest why the electron affinity of bromine is an exothermic change.

ii)

Explain why the bond enthalpy of a Br–Br bond is greater than that of a I–I bond.

1c2 marks

This question is about lithium fluoride.

Complete the Born–Haber cycle for lithium fluoride by adding the missing species on the lines.

Figure 1

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1d5 marks

Use the data in Table 3 and your completed Born–Haber cycle from part (c) to answer the questions below.

Table 3

Name of enthalpy change

Energy change / kJ mol-1

Li (s) → Li (g)

+216

Li (g) → Li+ (g) + e-

+520

F2 (g) → 2F (g)

+158

F (g) + e- → F- (g)

-348

Li (s) + 1/2F2 (g) → LiF (s)

-594

i)
Calculate the enthalpy of lattice formation of lithium fluoride.

ii)

Explain and justify how the enthalpy of lattice formation of NaBr compares with that of NaF. You must refer to the size of the ions in your answer.

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2a5 marks

This question is about the energy changes represented in a Born-Haber cycle when forming silver chloride.

Figure 1

eX6im6MY_12

i)
Complete the Born–Haber cycle for silver chloride above.

ii)

Suggest why the electron affinity of chlorine has a negative value. You should refer to electrostatic forces in your answer.

2b4 marks

This question focuses on the difference between the theoretical enthalpy values and experimental values of ionic substances. A perfect ionic model can be used to calculate a theoretical value for the enthalpy of lattice dissociation.

i)

The theoretical enthalpy of lattice dissociation for sodium fluoride is +902 kJ mol–1.This is very different to the experimental value that can be calculated from a Born Haber cycle. Explain this difference.

ii)

The theoretical enthalpy of lattice dissociation value for sodium chloride is less than the theoretical enthalpy lattice dissociation of sodium fluoride. Explain why this difference exists.

2c7 marks

This question looks at the enthalpy steps of calcium sulfide and its Born-Haber cycle.

Figure 2

WcWxlC0I_13

i)
Name the enthalpy changes for Steps 1, 5 and 7 in this Born-Haber cycle for calcium sulfide.

ii)
Step 6 is an endothermic process. Explain why and identify Z

iii)

Explain why the value for Step 3 is smaller than step 4.

2d3 marks

This question is about enthalpy changes in solution.

i)
Write the equation for the process showing the enthalpy of solution of silver fluoride. Include state symbols in your answer.

ii)

Use the data in Table 1 below to calculate the standard enthalpy of solution of silver fluoride.

Table 1

Name of enthalpy change in solution

Enthalpy change kJ mol-1

Enthalpy of lattice dissociation silver fluoride

+967

Enthalpy of hydration of silver ions

-464

Enthalpy of hydration of fluoride ions

-606

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3a4 marks

This question focuses on the enthalpy of hydration of fluoride compounds.

i)
Define the term enthalpy of hydration in relation to a fluoride ion.

ii)
State whether the hydration of a fluoride ion is an exothermic or endothermic process. Explain your answer.
3b3 marks

This question is about the enthalpy of hydration of halide ions.

Table 1

Name of enthalpy change

Enthalpy change kJ mol-1

Enthalpy of hydration of fluoride

-524

Enthalpy of hydration of chloride

-364


Using the data from Table 1 above, explain why the value for the enthalpy of hydration for the fluoride ion is more negative than that for the chloride ion. You must refer to the attractive forces involved in your answer.

3c2 marks

Use the enthalpy values below in Table 2 to suggest why there is a difference between the hydration values of calcium ions and sodium ions.

Table 2

Name of enthalpy change

Enthalpy change / kJ mol-1

Enthalpy of hydration of Cl- ions

-364

Enthalpy of hydration of Na+ ions

-406

Enthalpy of hydration of Ca2+ ions

-1579

3d4 marks
i)
Define the term enthalpy of lattice formation.

ii)

Using the data from Table 3 below, calculate the enthalpy of hydration of Ca2+ ions.

Table 3

Name of enthalpy change

Enthalpy change / kJ mol-1

Enthalpy change of hydration Br-

-348

Enthalpy change of lattice formation CaBr2

-2176

Enthalpy change of solution CaBr2

-99

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4a5 marks
i)

Table 1 below shows some solution and hydration enthalpies. Using the data from Table 1, calculate the enthalpy of hydration of calcium ions.

Table 1

Name of Enthalpy change

Enthalpy value/ kJ mol-1

Lattice dissociation enthalpy of calcium chloride

+2237

Enthalpy of solution of calcium chloride

-83

Enthalpy of hydration of chloride ions

-364



ii)

When a calcium ion is hydrated, calcium ions attract water molecules and energy is released. Explain why water molecules are attracted to calcium ions. You may use a labelled diagram to illustrate your answer.

4b5 marks

This question looks at how the entropy change of water varies with temperature.

Figure 1

14

i)
The entropy of water is zero when the temperature is zero Kelvin. Explain why, with reference to the water molecules in your answer.

ii)
Explain why the entropy change, ΔS, is larger at temperature T2 than at temperature T1

iii)

On Figure 1, draw the boiling point (Tb) of water on the appropriate axis.

4c3 marks

Standard entropies can be used to calculate the entropy change of a reaction, ΔS.

For example, for the reaction between nitrogen monoxide and oxygen which is shown below.

2NO (g) + O2 (g) → 2NO2 (g)

Table 2

Substance

Entropy value J K-1 mol-1

NO (g)

+211

O2 (g)

+205

NO2 (g)

+240

i)
Use the data from Table 2 to calculate the entropy change of the reaction between nitrogen monoxide and oxygen. 

ii)

Using your answer from part (i), explain what the sign of the entropy change indicates about the products (NO2) compared to the reactants (NO and O2) in this reaction.

4d6 marks

The contact process is a method used by industries to form sulfur trioxide, by reacting sulfur dioxide and oxygen together over a vanadium (V) oxide catalyst.

The equation for this reaction is shown below:

2SO2 (g) + O2 (g) 2SO3 (g)

Table 3

Substance

Enthalpy values/kJ mol-1

SO2 (g)

-297

O2 (g)

0

SO3 (g)

-395

i)
Calculate the standard enthalpy change of the contact process reaction, using the data provided in Table 3

ii)
The standard entropy change of this reaction is –189 JK-1 mol-1. Use this value and your enthalpy value from part (i) calculate a value for the free energy change for this reaction at 45 degreeC.
If you did not calculate an answer for part (i), use 70 degreeC as your value. Note that this is not the correct answer to part (i). 

iii)
Use your answer to part (ii) to explain whether the reaction is feasible at 45 degreeC.

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5a3 marks

This question is about the enthalpy of solution of sodium chloride.

If the value of enthalpy of solution of sodium chloride is +4 kJ mol-1, explain why the free energy change for dissolving sodium chloride in water is negative, despite the enthalpy change being a positive value.

5b3 marks

Calcium carbonate thermally decomposes to form calcium oxide and carbon dioxide, as shown below:

CaCO3 (s) → CaO (s) + CO2 (g)

The enthalpy change of the above reaction is ΔH = +178 kJ mol-1 and the entropy change is ΔS = +161 J K-1 mol-1.

Calculate the temperature at which the free-energy change, ΔG, for this process is zero.

5c6 marks

Some ionic compounds such as potassium chloride, KCl, will dissolve in water at room temperature in an endothermic process.

KCl (s) → K+ (aq) + Cl- (aq)                 ΔH = +16 kJ mol-1      

Table 1

Substance

Entropy value J K-1 mol-1

KCl (s)

+83

K+ (aq)

+103

Cl- (aq)

+57

i)
Using the data from Table 1, prove that this process is feasible at 25 degreeC.

ii)

Use your knowledge of structure and bonding to explain why ∆H is positive for this process.

5d3 marks

Diamond and graphite are both allotropes of carbon.

Table 2 below shows the data for the conversion of graphite into diamond. Use this data to calculate values for ΔH and ΔS for the reaction. Use these values to explain why this reaction is not feasible under standard pressure at any temperature.

Carbon (graphite) → Carbon (diamond)

Table 2

 

C (graphite)

C (diamond)

ΔH / kJ mol-1

0

+1.9

ΔS / J K-1 mol-1

+5.7

+2.4

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1a3 marks

Use the following data in Table 1 to calculate the enthalpy of lattice formation of caesium oxide.

Table 1

Name of enthalpy change

Energy change (kJ mol-1)

Enthalpy of formation of caesium oxide

-233

Enthalpy of atomisation of caesium

+78

First ionisation energy of caesium

+375

Bond enthalpy of oxygen

+494

First electron affinity of oxygen

-141

Second electron affinity of oxygen

+845

1b2 marks

A different Group 1 metal forms an ionic compound with chlorine. The enthalpy of lattice dissociation for this compound is +773 kJ mol-1 and the hydration enthalpy of a chloride ion is -363 kJ mol-1.

The enthalpy of solution of the Group 1 chloride is +4 kJ mol-1.

Using the information in Table 2, identify the group one ion.

Table 2

Group 1 ion

Enthalpy of hydration (kJ mol-1)

Li+

-519

Na+

-406

K+

-321

Rb+

-296

1c4 marks

The same Group 1 metal from part (b) forms an ionic lattice with another halide ion. This new ionic compound has an enthalpy of lattice formation of -705 kJ mol-1.

Using M to represent the Group 1 metal, suggest a formula for the new ionic lattice and explain your answer.

1d5 marks

The enthalpy of hydration becomes less exothermic as you go down group 1. Using the values in Table 2 in part (b):

i)
Explain why the enthalpy of hydration of Group 1 ions is negative.

ii)
Explain why the enthalpies of hydration become less negative as you go down the group. 

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2a6 marks

Draw a fully labelled Born-Haber cycle for the formation of sodium selenide, Na2Se (s), from its elements. Include state symbols in all equations.

2b3 marks

If sulfur is used as opposed to selenium in the lattice, what would you expect to happen to the value of the enthalpy of lattice dissociation. Explain your answer.

2c3 marks

Use the data in Table 1 to calculate the value for the enthalpy of lattice formation of aluminium oxide.

Table 1

Enthalpy change

Energy change (kJ mol-1)

Atomisation of aluminium

+326

Atomisation of oxygen

+249

First ionisation energy of aluminium

+578

Second ionisation energy of aluminium

+1817

Third ionisation energy of aluminium

+2745

Formation of aluminium oxide

-1670

First electron affinity of oxygen

-142

Second electron affinity of oxygen

+844

 Lattice formation of aluminium oxide

To be calculated

2d1 mark

Aluminium oxide is insoluble in water, but sodium oxide is soluble. Explain why there is no enthalpy of solution data for sodium oxide.

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3a3 marks

The equations for two separate reversible reactions are as follows:

Reaction A      2SO2 (g) + O2 (g) ⇌ 2SO3 (g)

Reaction B      CO (g) + H2O (g) ⇌ CO2 (g) + H2 (g)

Use the information in Table 1 to calculate the temperature at which the free energy change for reaction A becomes feasible

Table 1

 

SO2

O2

SO3

CO

H2O

CO2

H2

ΔHӨf  / kJ mol-1

-296.8

0

-395.7

-110.5

-241.8

-393.5

0

SӨ / J K-1 mol-1

248.2

205.1

256.8

197.6

188.7

213.8

130.6

3b4 marks

By using the data in Table 1, deduce if reaction B is feasible at a low temperature.

3c4 marks

Magnesium carbonate decomposes at a relatively high temperature. Using the data in Table 2, determine if this reaction is feasible at 280 degreeC.

Table 2

 

CO2

MgCO3

O2

MgO

ΔHӨf  / kJ mol-1

-393.5

-1095.8

0

-601.7

SӨ / J K-1 mol-1

213.8

65.7

205.1

26.9

3d2 marks

Using your answer to part (c), determine the temperature in degreeC at which the decomposition of magnesium carbonate becomes feasible.

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4a2 marks

This question is about enthalpy changes. 

Strontium is used as a red colouring agent in fireworks as it provides a very intense red colour.

Use the data in Table 1 to calculate the atomisation energy for chlorine in strontium chloride.

Table 1

Enthalpy change

Enthalpy change (kJ mol-1)

Sr (s) → Sr (g)

164.0

Sr (g) → Sr+ (g)

549.5

Sr+ (g) → Sr2+ (g)

1064.3

Cl (g) → Cl- (g) 

-349.0

Sr (s) + Cl2 (g) → SrCl2 (s) 

-828.9

SrCl2 (s) → Sr2+ (g) + 2Cl- (g)  2156.0
4b3 marks

Using information in part (a) suggest a value for the lattice enthalpy of formation of rubidium chloride. Explain your choice of value.

4c3 marks

A different Group 1 metal, lithium, can be used in rechargeable batteries for mobile phones, laptops, digital cameras and electric vehicles. Lithium is also used in some non-rechargeable batteries for things like heart pacemakers, toys and clocks.     

Lithium carbonate will decompose on heating.

Figure 1 shows the graph of ΔG plotted against temperature, K for the decomposition of lithium carbonate.

i)
Use Figure 1 to calculate the entropy change, ΔS, for this reaction in J K-1 mol-1

ii)
Determine the temperature at which this reaction becomes feasible in K. 

Figure 1

73Ib0NdF_20

4d4 marks

Which of the following compounds would have the greatest difference between the theoretical and experimental value for lattice enthalpy. Explain your answer.

Silver fluoride

Silver iodide

Sodium fluoride

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5a4 marks

Butane can be converted into ethene and hydrogen which can be used in hydrogenation of vegetable oil in the presence of a nickel catalyst.

i)
Write a fully balanced chemical equation including state symbols for cracking of butane.

ii)
Using the data in Table 1, calculate the free energy change for this reaction. 

iii)

Using your answer to part (ii) explain under what conditions this reaction would be feasible.

Table 1

 

Butane (g)

Ethene (g)

H2 (g)

GӨ / kJ mol-1

-15.7

68.1

0

5b4 marks

The free energy change for this reaction varies with temperature.

i)
Use the data in Table 2 to plot a graph of free energy against temperature on the grid below.

ii)

Calculate the gradient of the line on your graph and hence calculate the entropy change, ΔS in J K mol-1 for the formation of the cracking of butane.

Table 2

T / K

400

600

800

1000

1200

1400

ΔG

126.0

74.2

22.4

-29.4

-81.2

-133.0


Figure 1

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5c3 marks

Hydrazoic acid is a colorless, volatile explosive liquid under standard conditions. The decomposition of the acid in a gaseous state is shown in the following equation.

HN3 (g) →  begin mathsize 16px style 1 half end styleH2 (g) + 3 over 2N2 (g)

Figure 1 shows a graph of graph of free energy against temperature on the grid below.

Figure 1

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i)
Use Figure 1 to calculate the value for ΔS for this reaction.

ii)
Determine a value for ΔH for this reaction.
5d3 marks

Use the data give in Table 2 to explain why the reaction for the formation of ethene from its elements is never feasible at any temperature. You must show your working.

Table 2

 

C (s)

H2 (g)

C2H4 (g)

ΔHӨf  / kJ mol-1

0

0

52.3

ΔGӨ / kJ mol-1

0

0

68.1

SӨ / J K-1 mol-1

5.7

131

219

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