Equilibria (AS): Practice Questions

Which statement about a system at dynamic equilibrium is correct?

Methanol is produced industrially by the following reversible reaction:
\( CO(g) + 2H_2(g) \rightleftharpoons CH_3OH(g) \)
A mixture at equilibrium contains 0.20 mol of \( CO \), 0.30 mol of \( H_2 \), and 0.10 mol of \( CH_3OH \) in a total volume of \( V \). If the total pressure of the system is \( P \), which expression represents the partial pressure of hydrogen, \( pH_2 \)?

A weak acid, \( HX \), has a concentration of 0.100 \( mol \text{ } dm^{-3} \). If the acid is 1% dissociated at this concentration, what is the equilibrium concentration of the unionised acid, \( [HX] \)?

A reaction is represented by the following equation:
\( 2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g) \)
At a constant temperature, the equilibrium partial pressures are \( pSO_2 = 0.20 \) atm, \( pO_2 = 0.10 \) atm, and \( pSO_3 = 0.40 \) atm. What is the value of the equilibrium constant, \( K_p \), for this reaction?

A mixture of 2.0 mol of \( H_2 \) and 2.0 mol of \( I_2 \) is placed in a 1.0 \( dm^3 \) flask and allowed to reach equilibrium at a certain temperature. At equilibrium, the concentration of \( HI \) is 3.0 \( mol \text{ } dm^{-3} \).
\( H_2(g) + I_2(g) \rightleftharpoons 2HI(g) \)
What is the value of the equilibrium constant \( K_c \)?

A closed system contains the equilibrium:
\(2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)\)    \(\Delta H = -197\text{ kJ mol}^{-1}\)
Deduce and explain the effect on the position of equilibrium if the temperature is increased.

Nitrogen dioxide exists in equilibrium with dinitrogen tetroxide:
\(2NO_2(g) \rightleftharpoons N_2O_4(g)\)    \(\Delta H = -57\text{ kJ mol}^{-1}\)
Predict and explain the change in the numerical value of \(K_p\) if the temperature is decreased.

Write an expression for the equilibrium constant, \(K_p\), for the reaction:
\(3H_2(g) + N_2(g) \rightleftharpoons 2NH_3(g)\)
Include the units for \(K_p\) in your answer, assuming pressure is measured in Pa.

The reaction for the formation of ammonia in the Haber process is:
\(\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \rightleftharpoons 2\text{NH}_3\text{(g)}\)     \(\Delta H = -92\text{ kJ mol}^{-1}\)
(a) Use Le Chatelier's principle to explain why a high pressure is used in this process.
(b) Explain why, despite the exothermic nature of the reaction, a relatively high temperature (around \(450\,^{\circ}\text{C}\)) is used in industry instead of a low temperature.
(c) State the catalyst used in the Haber process and explain how it affects the position of equilibrium.

Methanol, \(\text{CH}_3\text{OH}\), can be manufactured by the reaction of carbon monoxide and hydrogen in a reversible reaction:
\(\text{CO(g)} + 2\text{H}_2\text{(g)} \rightleftharpoons \text{CH}_3\text{OH(g)}\)     \(\Delta H_r = -91\text{ kJ mol}^{-1}\)
(a) Write an expression for the equilibrium constant, \(K_c\), for this reaction and state its units.
(b) Describe and explain the effect on the value of \(K_c\) if the temperature is increased.
(c) A mixture of \(0.50\text{ mol}\) of \(\text{CO}\) and \(1.00\text{ mol}\) of \(\text{H}_2\) is allowed to reach equilibrium in a \(2.00\text{ dm}^3\) vessel at a specific temperature. At equilibrium, the mixture contains \(0.15\text{ mol}\) of \(\text{CH}_3\text{OH}\). Calculate the value of \(K_c\).