Edexcel A-Level Chemistry: Kinetics & Equilibrium

A reaction between three gaseous species, X, Y and Z, was investigated. Using the initial-rates method, the experimentally determined rate equation was found to be

$\text{rate} = k[\text{X}]^{2}[\text{Y}]$rate=k[X]2[Y]

with the reaction being zero order with respect to Z.

(a) Describe how a student could use the initial-rates method to determine the order of reaction with respect to X. (3 marks)

(b) Explain what the rate equation reveals about the rate-determining step of the reaction, including the role of Z. (3 marks)

The reaction between three species, P, Q and S, was studied at a constant temperature using the initial-rates method. The initial concentrations were varied and the initial rate of reaction measured for each experiment. The results are shown below.

Experiment	$[\text{P}]$[P] / mol dm⁻³	$[\text{Q}]$[Q] / mol dm⁻³	$[\text{S}]$[S] / mol dm⁻³	Initial rate / mol dm⁻³ s⁻¹
1	0.20	0.10	0.10	$1.2 \times 10^{-3}$1.2×10−3
2	0.40	0.10	0.10	$2.4 \times 10^{-3}$2.4×10−3
3	0.20	0.20	0.10	$4.8 \times 10^{-3}$4.8×10−3
4	0.20	0.10	0.20	$1.2 \times 10^{-3}$1.2×10−3

(a) Deduce the order of reaction with respect to P, Q and S, 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)

Propanoic acid reacts with methanol in the presence of an acid catalyst to reach the homogeneous equilibrium

$\text{CH}_3\text{CH}_2\text{COOH}(\text{l}) + \text{CH}_3\text{OH}(\text{l}) \rightleftharpoons \text{CH}_3\text{CH}_2\text{COOCH}_3(\text{l}) + \text{H}_2\text{O}(\text{l})$CH3​CH2​COOH(l)+CH3​OH(l)⇌CH3​CH2​COOCH3​(l)+H2​O(l)

A mixture of 0.800 mol of propanoic acid and 0.500 mol of methanol was placed in a flask of volume 250 cm³ with a trace of acid catalyst and left to reach equilibrium at a constant temperature. At equilibrium the mixture contained 0.400 mol of the ester, methyl propanoate.

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

(b) Calculate the equilibrium amounts, in moles, of propanoic acid, methanol and water, and hence calculate the value of $K_\text{c}$Kc​. State its units (or state that it has none). (4 marks)

A chemical company manufactures a gaseous product Z from two gaseous reactants, X and Y, by the homogeneous equilibrium

$2\text{X}(\text{g}) + \text{Y}(\text{g}) \rightleftharpoons 2\text{Z}(\text{g}) \qquad \Delta H = -95\ \text{kJ mol}^{-1}$2X(g)+Y(g)⇌2Z(g)ΔH=−95 kJ mol−1

The plant operates at a moderately high temperature and a high total pressure.

(a) Explain, for the yield of Z, the rate of reaction and the value of $K_\text{p}$Kp​, the effect of operating at a higher pressure. (2 marks)

(b) A trainee suggests running the plant at a much lower temperature to increase the yield of Z. Explain why this would improve the equilibrium yield but is not used in practice, referring to the effect on rate and on $K_\text{p}$Kp​. (3 marks)

A compound W decomposes in solution at a constant temperature. The concentration of W was monitored by colorimetry and the following data obtained.

Time / s	$[\text{W}]$[W] / mol dm⁻³
0	0.80
50	0.40
100	0.20
150	0.10

(a) Use the half-life of the reaction to deduce the order of reaction with respect to W. Show your reasoning. (2 marks)

(b) Calculate the value of the rate constant $k$k for the decomposition and give its units. (2 marks)

Dinitrogen tetroxide and nitrogen dioxide exist in the homogeneous gaseous equilibrium

$\text{N}_2\text{O}_4(\text{g}) \rightleftharpoons 2\text{NO}_2(\text{g})$N2​O4​(g)⇌2NO2​(g)

The volume of the container is decreased at a constant temperature.

State and explain the effect of this change on the position of equilibrium and on the value of $K_\text{p}$Kp​. (3 marks)