OCR A Level Chemistry

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A LevelChemistryOCRTopic Questions5. Physical Chemistry & Transition Elements (A Level Only)5.4 Thermodynamics

5.4 Thermodynamics

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1a3 marks

This question is about the formation of calcium oxide.

Define the term lattice enthalpy. 

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1b2 marks

The Born- Haber cycle below can be used to determine the lattice enthalpy of calcium oxide. 

Steps A-G includes the values for enthalpy changes. 

Complete the Born Haber cycle by adding the species present on the two dotted lines. 

Include state symbols. 

5-4_q1b-ocr-a-as--a-level-easy-sq

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1c3 marks

Name the enthalpy changes for the following steps in the Born-Haber cycle. 

i) B .....................................................................

ii) D ...................................................................

iii) F ...................................................................

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1d2 marks

C represents the enthalpy change of atomisation of oxygen. 

Why is the enthalpy change of atomisation always a positive value?  

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2a1 mark

The feasibility of a reaction is determined by the relationship between enthalpy change, and entropy change.

Write the equation that shows the relationship between ΔG, ΔH and ΔS.

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2b1 mark

What is the requirement for a reaction to be feasible?

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2c
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Chlorine can be formed in the following reversible reaction

4HCl(g)  +  O2(g)  rightwards harpoon over leftwards harpoon  2Cl2(g)  +  2H2O(g)

Use the following data to calculate the standard entropy change, ΔS for this reaction. 

  HCl (g) O2 (g) Cl2 (g) H2O (g)
S/ J K–1 mol–1 187 205 223 189

The equation to calculate standard entropy change is:   


ΔS = ΣSproducts – ΣSreactants 

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2d
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The standard enthalpy change for the reaction in (c) is -116 kJ mol-1

Calculate the minimum temperature at which this reaction is feasible. 

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3a
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This question is about enthalpy changes and calculations.

Table 1 shows the enthalpy data for the formation of sodium chloride.

Table 1

Enthalpy change 

Enthalpy change 

Enthalpy change (kJ mol-1) 

ΔHӨf NaCl

Na (s) + ½Cl2 (g) → NaCl (s)

-411

ΔHӨat Cl

½Cl2 (g) → Cl (g)

121

ΔHӨat Na

Na (s) → Na (g)

108

ΔHӨEA Cl

Cl (g) → Cl - (g)

-349

ΔHӨIE Na

Na (g) → Na+ (g)

496

ΔHӨlatt NaCl

Na+ (g) + Cl– (g) → NaCl (s)

To be calculated

Calculate the enthalpy of lattice formation of sodium chloride. 

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3b3 marks

State the definition of electron affinity.

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3c3 marks

A section of the Born-Haber cycle for the formation of magnesium oxide is shown in Figure 1.

 
Figure 1
 
mgo-bh-cycle-part
 

The first electron affinity of oxygen has a negative value so the arrow points downwards. The second electron affinity of oxygen has a positive value so the arrow points upwards. 

Explain why the arrows point in different directions.

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3d
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Using your answer to part (a) and the data in Table 2 calculate the enthalpy of solution for sodium chloride. 

Table 2
 

Enthalpy change 

Enthalpy change (kJ mol-1) 

ΔHӨlatt NaCl

To be calculated

ΔHӨhyd Na+ 

-404

ΔHӨhyd Cl– 

-381

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4a3 marks

Ammonium chloride, NH4Cl, can dissociate to form ammonia, NH3, and hydrogen chloride, HCl. 

NH4Cl (s)    →   NH3(g)  +   HCl(g)

Explain whether entropy increases or decreases in this reaction. 

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4b
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At 298K, ΔH = +176 kJ mol–1 and ΔS = +285 J mol–1 K-1. 

Calculate ΔG for this reaction at 298K. 

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4c2 marks

Explain whether the reaction is feasible under standard conditions.

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4d2 marks

State and explain how the feasibility of the reaction will change with increasing temperature. 

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5a3 marks

This question is about lattice enthalpy. 

Write one equation to represent the following changes:

i) Atomisation of sodium. 

ii) Second ionisation energy of magnesium.

iii) First electron affinity of chlorine.

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5b3 marks

The Born-Harber cycle for the formation of potassium fluoride is shown in Figure 1.  

Figure 1
 
IB Chem SQ HL 15.1 E Q1c KF Born-Haber cycle
 

Complete Table 1 by naming the enthalpy changes associated with the identified steps.  

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

 

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5c3 marks

The enthalpies of lattice formation for potassium fluoride and caesium fluoride are –830 kJ mol-1 and -730 kJ mol-1 respectively. 

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

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5d
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Use the data in Table 2 to calculate the enthalpy of solution of potassium fluoride. 

Table 2
 

Enthalpy change 

Enthalpy change (kJ mol-1) 

ΔHӨlatt KF

-830

ΔHӨhyd K+ 

-351

ΔHӨhyd F– 

-504

 

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1a
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Carbon dioxide and hydrogen can react to form methanol. 

CO2 (g) + 3H2 (g) ⇌ CH3OH (g) + H2O (g)

Data for enthalpy of formation and entropy is shown in the table.

  CO2 (g) H2 (g) CH3OH (g) H2O (g)
ΔHf / kJ mol–1 -394 0 -201 -242
S / J K–1 mol–1 214 131 238 189

Calculate the Gibbs free-energy change (ΔG), in kJ mol–1, for this reaction at 790 K.

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1b1 mark

Explain whether this reaction is feasible at 790 K.

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1c
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Use your values of ΔH and ΔS from part (a) to calculate the temperature above which this reaction is feasible.

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1d1 mark

Explain why the reaction in part a) has a negative entropy change. 

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2a7 marks

This question is about Born-Haber cycles. 

Draw a fully labelled Born-Haber cycle for the formation of solid potassium sulfide, K2S, from its elements. 

Include state symbols for all species involved. 

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2b
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Use the data to calculate the lattice enthalpy of K2S.

  Enthalpy change
/ kJ mol–1
  Formation of potassium sulfide -381
  1st electron affinity of sulfur -200
  2nd electron affinity of sulfur +640
  Atomisation of sulfur +279
  1st ionisation energy of potassium +419
  Atomisation of potassium  +89

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2c3 marks

If lithium replaced potassium in the lattice, what would you expect to happen to the value of the lattice enthalpy?

Explain your answer.   

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2d2 marks

Explain the difference in values between the first and second electron affinities of sulfur. 

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3a
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This question is about the feasibility of a reaction. 

Calcium carbonate can be thermally decomposed to make calcium oxide. 

CaCO3 (s)  →   CaO (s)   +   CO2 (g)        ΔH= + 178 kJ mol-1

Table 1 below shows the standard entropies of each substance. 

Table 1 

Substance CaCO3 (s) CaO (s) CO2 (g)
S / J K–1mol–1 89 40 214

i) Use the information in the table to show that calcium carbonate is stable at room temperature (25 oC).
[4]

ii) Calculate the minimum temperature needed to decompose calcium carbonate.
[2]

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3b3 marks

Explain how the conditions can be changed for the reaction in a) to make the reaction feasible.

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3c4 marks

The evaporation of water is an endothermic reaction but also a spontaneous reaction.

Explain why.

Refer to the entropy change that occurs and the change in the arrangement of water molecules. 

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3d
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Use the information in the table to calculate the temperature above which the reaction between hydrogen and oxygen to form gaseous water is not feasible.

 

Sϴ / J K−1 mol−1

ΔH fϴ/ kJ mol−1

H2 (g)

131

0

O2 (g)

205

0

H2O (g)

189

−242

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3e2 marks

Explain what would happen to the sample of gaseous water if it was heated to a higher temperature than that calculated in part (d). 

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4a
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This question is about lattice enthalpy. 

Use the following data in Table 1 to calculate the lattice enthalpy 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

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4b
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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 the table, identify the Group 1 ion.

Group 1 ion

Enthalpy of hydration (kJ mol-1)

Li+

-519

Na+

-406

K+

-321

Rb+

-296

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4c4 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.

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4d5 marks

The enthalpy of hydration becomes less exothermic as you go down Group 1. Using the values 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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5a
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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) 

-829.0

SrCl2 (s) → Sr2+ (g) + 2Cl- (g) 

2156

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5b3 marks

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

Explain your choice of value.

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1a
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This question is about fluorine and the associated energy changes when it is reacted with magnesium to form magnesium fluoride.

MgF2 Born Haber cycle

i)

Define the term electron affinity for fluorine.

ii)

Use the data in the table below to calculate a value for the electron affinity of fluorine.

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1b4 marks

The table below gives some values of standard enthalpy changes. Use these values to answer the questions.

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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.

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1c2 marks

This question is about lithium fluoride.

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

1-7

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1d
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Use the data in the table below and your completed Born–Haber cycle from part (c) to answer the questions below.

6a4dcf6f-19d8-4c8a-ab83-3cde83b5c26c

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.

3-5

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.

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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.

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2c7 marks

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

4-3

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.

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2d
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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 the table below to calculate the standard enthalpy of solution of silver fluoride.

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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.

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3b3 marks

The enthalpy of hydration of the fluoride and chloride ions are -524 kJ mol-1 and -364 kJ mol-1. 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.

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3c2 marks

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

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3d
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The table below gives data for the ionic compound calcium bromide, CaBr2. 

i)

Define the term enthalpy of lattice formation.

ii)

Using the data from the table below, calculate the enthalpy of hydration of Ca2+.

80ff8e0d-3439-40af-b2a7-ef35d7ef04b6

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4a
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The table below shows some solution and hydration enthalpies.

84c96a75-d6c3-40cb-81e8-4d8f70aee93b

i)

Using the data from the table, calculate the enthalpy of hydration of calcium ions.

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.

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4b5 marks

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

5-2

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 the graph draw the boiling point (Tb) of water on the appropriate axis.

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4c
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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)

4904d5f2-5eea-4634-bfa0-b33c9936e772

i)

Use the data from the table 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.

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4d
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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)

f8ffd079-9809-4fa6-be77-126755ff9b8f

i)

Calculate the enthalpy change of the contact process reaction, using the data provided in the table. 

ii)

The standard entropy change of this reaction is –189 J K-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°C.

iii)

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

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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.

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5b
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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.

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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 

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i)

Using the data from the table to prove that this process is feasible at 25 ℃.

ii)

Use your knowledge of structure and bonding to explain why ΔH is positive for this

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5d3 marks

Diamond and graphite are both allotropes of carbon. The table 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)

21227249-b907-4c99-98a7-a37b8d44d1c8

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