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CAIE A-Level Chemistry A2 23.3 Entropy Change S Questions

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

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 ]

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 14KI(s)+2CO2( g)+O2( g)→2 K2CO3( s)+2I2( s)ΔH⊖=−203.4 kJ mol−11 \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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