CAIE A-Level Chemistry 23.3.2 Signs of Entropy Changes

Practise predicting the sign of ΔS for state, temperature and gas-mole changes and explaining the particle basis.

Exam points

predict positive ΔS for melting, boiling or many dissolving processes and negative for their reverse
predict higher entropy as temperature rises because more energy arrangements become accessible
compare gaseous mole totals in reactions and link an increase in gas particles to positive ΔS

[Maximum number: 1]

Potassium chloride, KCl , and magnesium chloride, $\mathrm{MgCl}_{2}$ , are both ionic solids.

Table 1.1

Potassium chloride is very soluble in water at $20^{\circ} \mathrm{C}$ .

Explain the solubility of potassium chloride by reference to change in entropy, $\Delta S$ .

$\Delta \mathrm{S}$ is positive
$\text { AND } \mathrm{KCl}(\mathrm{~s}) \rightarrow \mathrm{K}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq}) /$

ionic lattice solid forms aqueous ions OWTTE [1]

OR

$\Delta \mathrm{S}$ is positive

AND $\Delta \mathrm{G}$ is (therefore becomes) negative /

$\mathrm{T} \Delta \mathrm{S}$ is greater than $\Delta H_{\mathrm{sol}}$ OWTTE [1]

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23.3.2—Predict/explain the sign of the entropy question 1