A LevelChemistryCIETopic Questions23. Chemical Energetics (AL)23.2 Enthalpies of Solution & Hydration

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First exams 2025

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Enthalpies of Solution & Hydration (CIE A Level Chemistry)

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1a
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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+ = ...........................

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1b4 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.

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1c5 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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1a1 mark

This question is about enthalpy change of hydration.

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

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1b2 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]

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1c4 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.

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1d
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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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2a4 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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2b3 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.

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2c2 marks

Use the enthalpy values in Table 2.1 to suggest why there is a difference between the hydration values of calcium ions and sodium ions.

Table 2.1

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

 

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3a3 marks

The ionic compound calcium bromide, CaBr2, is used in photography and medicine.

Explain, in terms of electrons, how it is formed from its atoms.

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3b4 marks

Table 3.1 below gives data for the ionic compound calcium bromide, CaBr2. 

i)

Define the term enthalpy of lattice formation.

[2]

Table 3.1 

enthalpy change enthalpy change / kJ mol–1
enthalpy change of hydration bromide –348
enthalpy change of lattice formation calcium bromide –2176
enthalpy change of solution calcium bromide –99

 

ii)

Using Table 3.1, calculate the enthalpy of hydration of the calcium ion.

enthalpy of hydration of the calcium ion = ........................ kJ mol–1

 [2]

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3c3 marks

Potassium fluoride is also an ionic compound with a wide range of uses.

Use the data in Table 3.2 to calculate the enthalpy of solution of potassium fluoride. 

Table 3.2

enthalpy change 

enthalpy change / kJ mol–1

ΔHlattθ KF

-830

ΔHhydθ K+ 

-351

ΔHhydθ F– 

-504

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