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CAIE A-Level Chemistry 23.2 Enthalpies of Solution and Hydration

Practise defining solution and hydration enthalpies and linking them to formation lattice energy in cycles.

Syllabus
2028–2030
Course
Chemistry 9701
Level
A2

Exam points

  • define ΔHsol for one mole of solute forming an infinitely dilute aqueous solution
  • define ΔHhyd for one mole of gaseous ions becoming aqueous ions in water
  • use ΔHsol = ΣΔHhyd − ΔHlatt when ΔHlatt is defined for ionic-solid formation

Covered syllabus points

23.2.1—Hydration and solution enthalpies23.2.2—An energy cycle involving enthalpy change23.2.3—Calculations: the energy cycles in 23.2.223.2.4—Explain, in qualitative terms, the effect

23.2 Enthalpies of solution and hydration question 1

[Maximum number: 6]

Potassium iodide, KI, is used as a reagent in both inorganic and organic chemistry.

Question (a)

(a)

KI forms an ionic lattice that is soluble in water.

[ 1 ]

Question (i)

(i)

Define enthalpy change of solution, ΔHsol \Delta H_{\text {sol }}.

[ 1 ]

Question (b)

(b)

Table 1.1 gives some data about the halide ions, Cl−,Br−\mathrm{Cl}^{-}, \mathrm{Br}^{-}and I−\mathrm{I}^{-}, and their potassium salts.

Table 1.1

Table 1.1

[ 5 ]

Question (i)

(i)

Explain the trend in the enthalpy change of hydration of the halide ions.

[ 2 ]

Question (ii)

(ii)

The ΔHsol \Delta H_{\text {sol }} values of these potassium halides are almost constant.

Use the ΔHhyd \Delta H_{\text {hyd }} and ΔHlatt \Delta H_{\text {latt }} data in Table 1.1 to suggest why.

[ 1 ]

Question (iii)

(iii)

The enthalpy change of solution of KI(s) is +21.0 kJ mol−1+21.0 \mathrm{~kJ} \mathrm{~mol}^{-1}.

Use this information and the data in Table 1.1 to calculate the enthalpy change of hydration of the potassium ion, K+(g)\mathrm{K}^{+}(\mathrm{g}).

ΔHhyd  of K+(g)=……………………………………..kJ mol−1\Delta H_{\text {hyd }} \text { of } \mathrm{K}^{+}(\mathrm{g})=\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . . \mathrm{kJ} \mathrm{~mol}^{-1}
[ 2 ]
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