AQA A-Level Chemistry: Kinetics & Equilibrium

The rate of a chemical reaction can be increased by raising the temperature or by adding a catalyst.

Using collision theory and the Maxwell-Boltzmann distribution of molecular energies, describe and explain how each of the following increases the rate of reaction:

(i) increasing the temperature;

(ii) adding a catalyst.

In your answer you should refer to the activation energy $E_\text{a}$Ea​, the proportion of molecules with energy $E \geq E_\text{a}$E≥Ea​, and the frequency of collisions. (Describe the shape of the distribution in words - you do not need to draw it.)

(6 marks)

The reaction between two species A and B was studied at a constant temperature using the initial-rates method. The initial concentrations of A and B were varied and the initial rate of reaction was measured for each experiment. The results are shown below.

Experiment	$[\text{A}]$[A] / mol dm⁻³	$[\text{B}]$[B] / mol dm⁻³	Initial rate / mol dm⁻³ s⁻¹
1	0.10	0.10	$2.0 \times 10^{-3}$2.0×10−3
2	0.20	0.10	$8.0 \times 10^{-3}$8.0×10−3
3	0.10	0.20	$4.0 \times 10^{-3}$4.0×10−3

(a) Deduce the order of reaction with respect to A and the order with respect to B, and hence write the overall rate equation. (3 marks)

(b) Calculate the value of the rate constant $k$k and give its units. (3 marks)

Hydrogen and iodine react in a sealed container to reach the homogeneous equilibrium

$\text{H}_2(\text{g}) + \text{I}_2(\text{g}) \rightleftharpoons 2\text{HI}(\text{g})$H2​(g)+I2​(g)⇌2HI(g)

A 0.500 mol sample of $\text{H}_2$H2​ and a 0.500 mol sample of $\text{I}_2$I2​ were placed in a sealed flask of volume 1.00 dm³ and allowed to reach equilibrium at a constant temperature. At equilibrium the flask was found to contain 0.600 mol of $\text{HI}$HI.

(a) Write the expression for the equilibrium constant $K_\text{c}$Kc​ for this reaction. (1 mark)

(b) Calculate the equilibrium amount, in moles, of $\text{H}_2$H2​ and of $\text{I}_2$I2​, and hence calculate the value of $K_\text{c}$Kc​. State its units (or state that it has none). (4 marks)

An industrial plant manufactures the gaseous fuel additive dimethyl ether (DME), $\text{CH}_3\text{OCH}_3$CH3​OCH3​, from carbon monoxide and hydrogen according to the homogeneous gaseous equilibrium

$2\text{CO}(\text{g}) + 4\text{H}_2(\text{g}) \rightleftharpoons \text{CH}_3\text{OCH}_3(\text{g}) + \text{H}_2\text{O}(\text{g}) \qquad \Delta H = -205\ \text{kJ mol}^{-1}$2CO(g)+4H2​(g)⇌CH3​OCH3​(g)+H2​O(g)ΔH=−205 kJ mol−1

For each of the following changes, predict and explain the effect on (1) the position of equilibrium / yield of DME and (2) the value of $K_\text{p}$Kp​.

(a) Increasing the total pressure at constant temperature. (2 marks)

(b) Increasing the temperature at constant pressure. (2 marks)

(c) Adding a catalyst. (1 mark)

(5 marks)

The rate constant $k$k for a gas-phase reaction was measured at two temperatures:

Temperature / K	Rate constant $k$k / s⁻¹
300	$2.0 \times 10^{-3}$2.0×10−3
320	$8.0 \times 10^{-3}$8.0×10−3

The Arrhenius equation may be written in the logarithmic form

$\ln k = \ln A - \frac{E_\text{a}}{RT}$lnk=lnA−RTEa​​

Use the data to calculate the activation energy $E_\text{a}$Ea​ for this reaction, in kJ mol⁻¹. (Take $R = 8.31\ \text{J K}^{-1}\,\text{mol}^{-1}$R=8.31 J K−1mol−1.) (4 marks)

A catalyst increases the rate of a chemical reaction without being used up overall.

Explain, in terms of activation energy and the Maxwell-Boltzmann distribution, how a catalyst increases the rate of a reaction. (3 marks)