Deduce the equilibrium constant expression, ${K_{\text{c}}}$, for this reaction.

Explain the effect on the position of the equilibrium and on the value of ${K_{\text{c}}}$ when:

(i)     pressure is decreased and temperature is kept constant.

(ii)     temperature is increased and pressure is kept constant.

$({K_{\text{c}}} = )\frac{{{\text{[}}{{\text{N}}_2}{{\text{O}}_4}{\text{(g)]}}}}{{{{{\text{[N}}{{\text{O}}_2}{\text{(g)]}}}^2}}}$;

Ignore state symbols.

(i)     equilibrium shifts to left as there are more moles (of gas) on reactant side;

no change to ${K_{\text{c}}}$ as it is a constant at fixed temperature / OWTTE;

Award [1 max] for correct equilibrium shift and ${K_c}$ change if no explanation given.

(ii)     equilibrium shifts to left since reaction is exothermic/$\Delta H$ negative / reverse reaction is endothermic/absorbs heat;

value of ${K_{\text{c}}}$ decreases because less ${{\text{N}}_2}{{\text{O}}_4}$ and more ${\text{N}}{{\text{O}}_2}$ / OWTTE;

Award [1 max] for correct equilibrium shitft and ${K_c}$ change if no explanation given.

The equilibrium constant expression for the equilibrium between ${\text{N}}{{\text{O}}_{\text{2}}}$ and ${{\text{N}}_{\text{2}}}{{\text{O}}_{\text{4}}}$ was well done by most candidates in 3(a) although a few had the expression upside down and several separated the molecules into atoms. Students generally could state and explain the shift in equilibrium position but had more trouble with the effect on the equilibrium constant in both (b)(i) and (b)(ii). It was rare to find a response that stated the changes and explained them for both the equilibrium position and the equilibrium constant. Candidates should take care to answer all parts of the question.

The equilibrium constant expression for the equilibrium between ${\text{N}}{{\text{O}}_{\text{2}}}$ and ${{\text{N}}_{\text{2}}}{{\text{O}}_{\text{4}}}$ was well done by most candidates in 3(a) although a few had the expression upside down and several separated the molecules into atoms. Students generally could state and explain the shift in equilibrium position but had more trouble with the effect on the equilibrium constant in both (b)(i) and (b)(ii). It was rare to find a response that stated the changes and explained them for both the equilibrium position and the equilibrium constant. Candidates should take care to answer all parts of the question.