IB Chemistry HL 2.3.7 Hl K and G Questions

Use real HL exam questions to convert between standard Gibbs free energy and K at a stated temperature, then interpret the sign and equilibrium direction.

Exam points

Calculate or convert between K and ΔG° using ΔG° = −RT ln K at the stated temperature, with logarithms, exponentials and consistent energy units.
Interpret the sign of ΔG° and the magnitude of K to identify the thermodynamic direction and equilibrium position of the reaction.

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Study 2.3.7 (HL)—K and ΔG conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

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Urea, $\left(\mathrm{H}_{2} \mathrm{~N}\right)_{2} \mathrm{CO}$ , is excreted by mammals and can be used as a fertilizer.

Urea can also be made by the direct combination of ammonia and carbon dioxide gases.

2 \mathrm{NH}_{3}(\mathrm{~g})+\mathrm{CO}_{2}(\mathrm{~g}) \rightleftharpoons\left(\mathrm{H}_{2} \mathrm{~N}\right)_{2} \mathrm{CO}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) \quad \Delta H<0

Determine an approximate order of magnitude for $K_{\mathrm{c}}$ , using sections 1 and 2 of the data booklet. Assume $\Delta G^{\ominus}$ for the forward reaction is approximately +50 kJ at 298 K .

$\ln K=\frac{-\Delta G^\ominus}{RT} =\frac{-50\times10^{3}\ \mathrm{J}}{8.31\ \mathrm{J\ K^{-1}\ mol^{-1}}\times298\ \mathrm{K}} \approx -20$ $K_c\approx2\times10^{-9}$

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IB Chemistry HL 2.3.7 Hl K and G Questions question 1