IB Chemistry HL R1.4 Entropy and spontaneity questions

Exam-style multiple choice and written questions with full markschemes. Check multiple choice answers straight away, or type, photograph or draw a written answer and get it marked.

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Question 1

Multiple choiceMedium1 mark

For the reaction shown, the standard molar entropies are S°(H₂O₂(l)) = 109.6, S°(H₂O(l)) = 69.9 and S°(O₂(g)) = 205.0 J K⁻¹ mol⁻¹. Which option gives ΔᵣS° per mole of reaction as written and the resulting change in the system’s entropy?

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Answer: C. For a reaction, ΔᵣS° = ΣνS°(products) − ΣνS°(reactants). Using the coefficients read from the figure: [2(69.9) + 205.0] − [2(109.6)] = +125.6 J K⁻¹ mol⁻¹. The positive value means the system’s entropy increases.

A: reverses products minus reactants, giving −125.6 J K⁻¹ mol⁻¹ and a decrease.
B: omits the coefficient 2 for water, giving +55.7 J K⁻¹ mol⁻¹.
D: omits the coefficient 2 for hydrogen peroxide, giving +235.2 J K⁻¹ mol⁻¹.

Remember: multiply each standard molar entropy by its stoichiometric coefficient before summing.

Question 2

Paper 2Medium7 marks

Calcium carbonate decomposes on heating according to the reaction

\ce{CaCO3(s) -> CaO(s) + CO2(g)}.

The standard entropy values and standard enthalpy change for this reaction are given below. Figure 1 shows how $\Delta_\mathrm r G^\circ$ varies with temperature.

Substance or quantity	Value
$S^\circ(\ce{CaCO3(s)})$	$92.9\ \mathrm{J\,K^{-1}\,mol^{-1}}$
$S^\circ(\ce{CaO(s)})$	$39.8\ \mathrm{J\,K^{-1}\,mol^{-1}}$
$S^\circ(\ce{CO2(g)})$	$213.7\ \mathrm{J\,K^{-1}\,mol^{-1}}$
$\Delta_\mathrm r H^\circ$	$+178.3\ \mathrm{kJ\,mol^{-1}}$

(a)

Calculate $\Delta_\mathrm r S^\circ$ for the reaction. Include the sign and units.

[2]

(b)

Using Figure 1, determine the approximate temperature at which $\Delta_\mathrm r G^\circ$ changes sign. State whether the forward reaction is thermodynamically favoured under standard-state conditions at $1000\ \mathrm K$

[2]

(c)

Using your answer to (a), calculate $\Delta_\mathrm r G^\circ$ at $1200\ \mathrm K$ State whether the forward reaction is thermodynamically favoured under standard-state conditions at this temperature.

[3]

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