Equilibria (A Level only): Practice Questions

Consider the homogeneous gas-phase equilibrium:
\(PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)\)
What is the correct expression for the equilibrium constant, \(K_c\)?

A mixture of 2.0 mol of \(SO_2\) and 2.0 mol of \(O_2\) is placed in a \(2.0 \text{ dm}^3\) flask. At equilibrium, 0.8 mol of \(SO_3\) is present.
\(2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)\)
What is the concentration of \(O_2\) at equilibrium?

For the gas-phase reaction \(PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)\), the total pressure at equilibrium is \(2.0 \text{ atm}\). If the mole fraction of \(PCl_5\) is 0.4 and the mole fraction of \(PCl_3\) is 0.3, what is the value of \(K_p\)?

The mathematical definition of \(\text{pH}\) is given by which expression?

A weak monobasic acid, \(HA\), has an acid dissociation constant \(K_a = 1.00 \times 10^{-5} \text{ mol dm}^{-3}\) at 298 K.
What is the \(\text{pH}\) of a \(0.100 \text{ mol dm}^{-3}\) aqueous solution of \(HA\)?

Predict and explain the effect of increasing the pressure on the position of equilibrium for the following reaction:
\(N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)\)

For the equilibrium \(PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)\) at a specific temperature, the partial pressures are recorded as \(p(PCl_3) = 0.20\,atm\), \(p(Cl_2) = 0.20\,atm\), and \(p(PCl_5) = 0.40\,atm\). Calculate the value of the equilibrium constant, \(K_p\), and provide its units.

A reversible reaction involving gaseous reactants P and Q forms gaseous product R as shown below:

\(2P(g) + Q(g) \rightleftharpoons 2R(g)\)

At a certain temperature, 2.0 mol of P and 1.0 mol of Q are mixed in a 5.0 dm³ sealed container. At equilibrium, 0.80 mol of R is formed.

(a) Calculate the equilibrium concentrations of P, Q, and R.

(b) Calculate the value of the equilibrium constant, \(K_c\), at this temperature, including its units.

(c) State and explain how each of the following changes would affect the position of equilibrium and the value of \(K_c\):

(i) Increasing the pressure by decreasing the volume of the container.

(ii) Adding a catalyst to the reaction mixture.

(iii) Increasing the temperature of the system, given that the forward reaction is exothermic.