IB Chemistry HL 1.4.4 Hl Equilibrium and Gibbs Energy Questions

Use real HL exam questions to connect equilibrium with Gibbs energy, calculate K or ΔG and determine direction from Q and K.

Exam points

Use ΔG = 0 to identify the equilibrium condition and relate it to ΔH − TΔS when the thermodynamic values are supplied.
Calculate ΔG° = −RT ln K with kelvin temperature, the gas constant, correct units and the correct sign relationship between K and Gibbs energy.
Use ΔG = ΔG° + RT ln Q and compare Q with K to determine reaction direction, equilibrium position or whether the forward/reverse reaction is favoured.

[Maximum number: 2]

Sulfur trioxide is an important compound in industry.

$\quad \mathrm{SO}_{3}(\mathrm{~g})$ is made using the contact process.

2 \mathrm{SO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{SO}_{3}(\mathrm{~g}) \quad \Delta H^{\ominus}<0

Calculate the value of the equilibrium constant for the formation of $\mathrm{SO}_{3}(\mathrm{~g})$ at 773 K . Use sections 1 and 2 of the data booklet. If you did not obtain an answer to (d)(viii), use $-25.0 \mathrm{~kJ} \mathrm{~mol}^{-1}$ , although this is not the correct answer.

$\begin{aligned} <-25.8 \times 10^{3}=-8.31 \times 773 \times \ln K> \ln K=<\frac{-25.8 \times 10^{3}}{-8.31 \times 773}=-4.016 K=55.5 \end{aligned}$

Award [2] for correct final answer.

Accept answers in the range 55-60.

Award [2] for K=49.0 if $-25.0 \mathrm{~kJ} \mathrm{~mol}^{-1}$ was used.

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IB Chemistry HL 1.4.4 Hl Equilibrium and Gibbs Energy Questions question 1