CAIE A-Level Chemistry 23.4.3 Gibbs Energy and Feasibility

Practise deciding whether a reaction or process is thermodynamically feasible from the sign of ΔG.

[Maximum number: 4]

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

(a)

KI forms an ionic lattice that is soluble in water.

[ 2 ]

(i)

KI(s) has a high solubility in water although its enthalpy change of solution is endothermic.

Explain how this high solubility is possible.

[ 2 ]

there is a (large) increase in entropy $\mathbf{OR} \Delta \mathrm{S}$ is positive $\mathbf{OR} \mathrm{T} \Delta \mathrm{S}$ is positive

so $\Delta G$ is negative / $T \Delta S$ outweighs $\Delta H$

(b)

KI slowly oxidises in air, forming $\mathrm{I}_{2}$ .

$reaction $1 \quad 4 \mathrm{KI}(\mathrm{s})+2 \mathrm{CO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{~K}_{2} \mathrm{CO}_{3}(\mathrm{~s})+2 \mathrm{I}_{2}(\mathrm{~s}) \quad \Delta H^{\ominus}=-203.4 \mathrm{~kJ} \mathrm{~mol}^{-1}$$

Table 1.2 shows some data relevant to this question.

Table 1.2

[ 2 ]

(i)

Use your answer to (c)(i) to show that reaction 1 is spontaneous at 298 K .

[ 2 ]

$\Delta G=\Delta \mathrm{H}-\mathrm{T} \Delta \mathrm{S}$ AND use of 298 K for T OR clear working to represent this

$\Delta G=-203.4-298(-514.4 / 1000)$ $\Delta G=\mathbf{- 5 0 . 1}$ OR $\mathbf{- 5 0 . 2}\left(\mathrm{kJ} \mathrm{mol}^{-1}\right)$ OR $\Delta G=\mathbf{- 5 0 1 0 8 . 8 ~ J ~ m i n} 3 \mathrm{sf}$ ECF from (c)(i)

(negative so spontaneous)

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23.4.3—Feasibility from ΔG sign question 1

Question (a)

Question (i)

Question (b)

Question (i)