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CAIE A-Level Chemistry 23.3 Entropy change, ΔS Question Bank

Practise entropy definitions, sign predictions and ΔS calculations from equations, state changes and standard entropy tables.

Syllabus
2028–2030
Course
Chemistry 9701
Level
A2

Exam points

  • predict ΔS signs from gas mole changes, dissolving, melting, boiling or condensing
  • calculate ΔS from standard entropy tables using product minus reactant totals
  • explain large positive or negative ΔS values by referring to particles and states

Covered syllabus points

23.3.1—Terms: entropy, S, as the number of possible23.3.2—Predict/explain the sign of the entropy23.3.3—The entropy change for a reaction, ΔS

23.3 Entropy change, ΔS question 1

[Maximum number: 2]

Potassium chloride, KCl , and magnesium chloride, MgCl2\mathrm{MgCl}_{2}, are both ionic solids.

Table 1.1

Table 1.1

Question (a)

(a)

Explain what is meant by entropy, S.

[ 1 ]

Question (b)

(b)

Potassium chloride is very soluble in water at 20∘C20^{\circ} \mathrm{C}.

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

[ 1 ]

23.3 Entropy change, ΔS question 2

[Maximum number: 1]

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

KI slowly oxidises in air, forming I2\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

Table 1.2

Calculate the standard entropy change, ΔS⊖\Delta S^{\ominus}, of reaction 1 .

ΔS⊖=…………………………………..JK−1 mol−1\Delta S^{\ominus}=\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . . \mathrm{JK}^{-1} \mathrm{~mol}^{-1}
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