Syllabus Edition

First teaching 2020

Last exams 2024

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Chemical Energetics (A Level Only) (CIE A Level Chemistry)

Exam Questions

2 hours12 questions
1a3 marks

This question is about the formation of calcium oxide.

Explain the meaning of the term lattice energy.

1b2 marks

The Born- Haber cycle In Fig. 1.1 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
Fig. 1.1
1c3 marks

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

  • Step B .....................................................................

  • Step D ...................................................................

  • Step F ...................................................................

1d2 marks

Step C represents the enthalpy change of atomisation of oxygen. 

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

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2a3 marks

This question is about lattice enthalpy. 

Write one equation to represent the following changes:

  • Atomisation of sodium
  • Second ionisation energy of magnesium
  • First electron affinity of chlorine

2b3 marks

The Born-Harber cycle for the formation of potassium fluoride is shown in Fig. 2.1. 

 
 
IB Chem SQ HL 15.1 E Q1c KF Born-Haber cycle
Fig. 2.1
 

Complete Table 2.1 by naming the enthalpy changes associated with the identified steps. 

 
Table 2.1
 

Step

Name of the Enthalpy Change 

1

 

2

Atomisation of potassium 

3

 

4

First ionisation energy of potassium 

5

 

6

Lattice enthalpy of formation

 

2c3 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.

2d
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3 marks

Use the data in Table 2.2 to calculate the enthalpy of solution of potassium fluoride.

 
Table 2.2
 

Enthalpy change 

Enthalpy change (kJ mol-1

ΔHθlatt KF

-830

ΔHθhyd K+ 

-351

ΔHθhyd F 

-504

 

ΔHθsol KF = ............................. kJ mol-1

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3a
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3 marks

This question is about enthalpy changes and calculations.

Table 3.1 shows the enthalpy data for the formation of sodium chloride.

Table 3.1

Enthalpy change 

Enthalpy change 

Enthalpy change / kJ mol-1 

ΔHθf NaCl

Na (s) + ½Cl2 (g) → NaCl (s)

-411

ΔHθat Cl

½Cl2 (g) → Cl (g)

121

ΔHθat Na

Na (s) → Na (g)

108

Eea Cl

Cl (g) → Cl - (g)

-349

ΔHθie Na

Na (g) → Na+ (g)

496

ΔHθlatt NaCl

Na+ (g) + Cl (g) → NaCl (s)

To be calculated

Calculate the enthalpy of lattice formation of sodium chloride. 

ΔHθlatt NaCl = ...................... kJ mol-1
3b3 marks

State the definition of electron affinity.

3c3 marks

A section of the Born-Haber cycle for the formation of magnesium oxide is shown in Fig. 3.1.

 
 
5-1-3c-e-mgo-born-haber-cycle-fig--3-1
 
Fig. 3.1 

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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1a3 marks

Lattice energies are always negative showing that they represent exothermic changes.

i)
 Explain what is meant by lattice energy.

[2]
ii)
Explain why lattice energy is an exothermic process.

[1]

Table 5.1

energy change value / kJ mol–1
standard enthalpy change of atomisation of potassium +89
electron affinity of O(g) –141
electron affinity of O(g) +798
standard enthalpy change of formation of potassium oxide –361
first ionisation energy of potassium +418
second ionisation energy of potassium +3070
first ionisation energy of oxygen +1310
second ionisation energy of oxygen +3390
O=O bond energy (diatomic molecule) +496
O–O bond energy (polyatomic molecule) +150
1b
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4 marks
i)
Use relevant data from Table 5.1 to calculate the lattice energy, increment H subscript latt superscript straight ɵ, of potassium oxide, K2O (s).
Show your working.

begin mathsize 14px style increment H subscript latt superscript ɵ end style, of K2O (s) = ............................................. kJ mol–1 [3]

ii)
State how begin mathsize 14px style increment H subscript latt superscript ɵ end style Na2O (s) differs from begin mathsize 14px style increment H subscript latt superscript ɵ end style K2O (s).
Indicate this by placing one tick (✓) in the appropriate box in Table 5.2.

Table 5.2

bold increment bold italic H subscript bold latt superscript bold ɵ Na2O (s) is less negative than bold increment bold italic H subscript bold latt superscript bold ɵK2O (s) bold increment bold italic H subscript bold latt superscript bold ɵ Na2O (s) is the same as bold increment bold italic H subscript bold latt superscript bold ɵK2O (s) bold increment bold italic H subscript bold latt superscript bold ɵ Na2O (s) is more negative than bold increment bold italic H subscript bold latt superscript bold ɵ K2O (s)
     

Explain your answer.

[1]

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2a4 marks

One use of pure crystals of lithium fluoride are in X-ray monochromators. 

The Born- Haber cycle for lithium chloride is shown in Fig. 2.1 

q1a_15-1_sq_medium_ib_hl

Fig. 2.1 

i)
Define the term enthalpy of atomisation.

[1]

ii)
Explain why the enthalpy of atomisation of fluorine is positive.

[1]

iii)
Complete the Born-Haber cycle for lithium fluoride in Fig 2.1 by adding the missing species on the lines.

[2]

2b
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5 marks
Use the data in the Table 2.1 and your completed Born–Haber cycle from part (a) to answer the questions below.

Table 2.1

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) + ½F2 (g) → LiF (s)

-594

i)
Calculate the enthalpy of lattice formation of lithium fluoride.
[2]

ii)
Explain and justify how the enthalpy of lattice formation of LiBr compares with that of LiF.

You must refer to the size of the ions in your answer.

[3]
2c3 marks
This question is about enthalpy changes in solution.

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

[1]

ii)
Use the data in Table 2.2 to calculate the standard enthalpy of solution of potassium fluoride.

[2]

Table 2.2

Name of enthalpy change in solution

Enthalpy change (kJ mol-1)

Enthalpy of lattice dissociation potassium fluoride

+829

Enthalpy of hydration of potassium ions

-340

Enthalpy of hydration of fluoride ions

-504



2d2 marks

Explain why the value for the enthalpy of hydration, ΔHθhyd, of Group 1 ions increases from lithium to caesium.

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3a1 mark

Calcium chloride, CaCl2, is an important industrial chemical used in refrigeration plants, for de-icing roads and for giving greater strength to concrete.

Use an equation to show what is meant by the lattice energy of calcium chloride.

3b3 marks

Explain how the lattice energies of the following salts compare in magnitude with that of calcium chloride. 

  • calcium fluoride
  • calcium sulfide 

3c3 marks

Use the data in Table 3.1 to calculate the lattice energy of CaCl2.

Table 3.1

 standard enthalpy change of formation   –796 kJ mol–1 
 standard enthalpy change of atomisation of Ca(s)   +178 kJ mol–1 
 electron affinity per mole of chlorine atoms    –349 kJ mol–1 
 first ionisation energy of Ca   +590 kJ mol–1 
 second ionisation energy of Ca  +1150 kJ mol–1 
 standard enthalpy change of atomisation Cl2 (g)  +244 kJ mol–1 

3d
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3 marks

When a solution of CaCl2 is added to a solution of the dicarboxylic acid, malonic acid, the salt calcium malonate is precipitated as a white solid.

The solid has the following composition by mass: Ca, 28.2%; C, 25.2%; H, 1.4%; O, 45.2%.

i)
Calculate the empirical formula of calcium malonate.
[2]

ii)
Suggest the structural formula of malonic acid.
[1]

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4a1 mark

This question is about enthalpy change of hydration.

Explain what is meant by the term enthalpy change of hydration, ΔHθhyd?

4b2 marks
i)
Write an equation that represents the ΔHθhyd of the Mg2+ ions. 

[1]

ii)
Suggest a reason why the ΔHθhyd  of the Mg2+ ions is greater than ΔHθhyd  of the Ca2+ ion. 
[1]

4c4 marks

The enthalpy change of solution for MgCl2, (MgCl2(s)), is represented by the following equation.

MgCl2(s) + aq → Mg2+(aq) + 2Cl (aq)

Describe the simple apparatus you could use, and the measurements you would make, in order to determine a value for (MgCl2(s)) in the laboratory.

4d
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3 marks

Use the data in Table 4.1 to calculate the value for the enthalpy change of solution for magnesium chloride. 

Table 4.1

  ΔHθlatt MgCl2 (s)  –2526 kJ mol-1
   ΔHθhyd (Mg2+ (g))  –1890 kJ mol-1
   ΔHθhyd  (Cl(g))   –398 kJ mol-1

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5a2 marks

Magnesium reacts with fluorine to form magnesium fluoride. 

Explain what is meant by the term first electron affinity. 

5b2 marks
i)
Write an equation for the second electron affinity of fluorine.
[1]

ii)
Suggest why the second electron affinity for fluorine is endothermic. 
[1]

5c
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1 mark

Determine the electron affinity of a fluorine atom, ΔHθEA using Table 5.1.

[3]

Table 5.1

Name of enthalpy change

Energy change (kJ mol-1)

Enthalpy of atomisation of magnesium

+150

Second ionisation energy of magnesium

+1450

Enthalpy of formation of magnesium fluoride

-642

Lattice enthalpy of formation of magnesium fluoride

-2493

5d3 marks

Explain the trend in the first electron affinities going from fluorine to iodine. 

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1a7 marks

Potassium sulfide is a reagent used in analytical chemistry and pharmaceutical preparations. 

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

Include state symbols for all species involved.

1b
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3 marks

Use the data in Table 1.1 to calculate the lattice enthalpy of potassium sulfide, ΔHθlatt. Show your working.


Table 1.1

Enthalpy change 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







ΔHθlatt = ................................. kJ mol-1

1c2 marks

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

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2a1 mark

Magnesium oxide is often used in optical applications due to its light-reflecting properties in crystal form.

Write an equation to represent the lattice energy of MgO.

2b
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3 marks

Use the data in Table 2.1, to calculate a value for the second ionisation energy of magnesium. ΔHθie. Show your working.


Table 2.1

Enthalpy change Enthalpy change / kJ mol-1
Lattice energy of MgO (s) - 3791
Enthalpy change of atomisation of Mg  148
Enthalpy of atomisation of oxygen 248
Electron affinity of the oxygen atom -141
Electron affinity of the oxygen anion, O 798
First ionisation energy of Mg 736
Enthalpy of formation of MgO 552




second ionisation energy of Mg = .............................. kJ mol-1

2c1 mark

Magnesium oxide is generally insoluble in water whereas calcium oxide is sparingly soluble. Explain why there is no enthalpy of solution data for calcium oxide.

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3a3 marks

The incomplete Born-Haber cycle for sodium selenide is shown below in Fig. 3.1.

Write the equations for processes 1, 2 and 3. 

239d5e33-5fba-4b56-8a32-4882bf910f8f
Fig. 3.1

1 ............................................................

2 ............................................................

3 ............................................................
3b3 marks

If sulfur is used as opposed to selenium in the lattice, suggest the change in value of the lattice energy, ΔHθlatt. Explain your answer.

Change .....................................


Explanatiion .......................................................................................


                    ........................................................................................

3c
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3 marks

Use the data in Table 3.1 to calculate the lattice energy of aluminium oxide, ΔHθlatt. Show your working.


Table 3.1

Enthalpy Change Enthalpy 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
Electron affinity of O atom -141
Electron affinity of O +753
Formation of aluminium oxide -1670






ΔHθlatt = .................................... kJ mol-1

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4a
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2 marks

A Group 1 metal, M, formed 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 4.1, identify the Group 1 ion, M+. Show your working.

Table 4.1

Group 1 ion

Enthalpy of hydration (kJ mol-1)

Li+

-519

Na+

-406

K+

-321

Rb+

-296







Identity of M+ = ...........................

4b4 marks

The same Group 1 metal from part (a) 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.

4c5 marks

The enthalpy of hydration becomes less exothermic as you go down Group 1. Using the values in part (a):

 
i)
Explain why the enthalpy of hydration of Group 1 ions is negative.
 
[3]
 
ii)
Explain why the enthalpies of hydration become less negative as you go down the group.
 
[2]

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