IB Chemistry HL 2.3.7 K and ΔG Question Bank

Practise IB Chemistry HL 2.3.7 by applying k and δg to exam-style questions.

Exam points

identify the relevant chemical structure, process or evidence
apply the correct chemical model or equation
evaluate the result using chemical reasoning and data

[Maximum number: 2]

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 .

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

Accept $1.69\times10^{-9}$ or $10^{-9}$ .
Accept a range of 20-20.2 for M1.
Award [2] for the correct final answer.

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2.3.7 (HL)—K and ΔG question 1