Kinetics IIEdexcel A-Level Chemistry: Topic test

20 questions, 54 marks

Edexcel A-Level Chemistry

Kinetics II topic test

Total 54 marks

Name

Class

Date

1
The gas-phase decomposition of azomethane, CH₃N₂CH₃(g) → C₂H₆(g) + N₂(g), is first order with respect to azomethane at 600 K, so rate = k[CH₃N₂CH₃]. The rate constant at 600 K is 3.6 × 10⁻⁴ s⁻¹.
(a)
What are the units of the rate constant, k, for this reaction?
[1 mark]
As⁻¹
Bmol dm⁻³ s⁻¹
Cdm³ mol⁻¹ s⁻¹
Ds
(b)
What is the half-life of azomethane at 600 K?
[1 mark]
A5.2 × 10⁻⁴ s
B2.8 × 10³ s
C1.9 × 10³ s
D2.5 × 10⁻⁴ s
(c)
Calculate the initial rate of decomposition when the concentration of azomethane is 0.200 mol dm⁻³. Include units.
[2 marks]
Total for question 1: 4 marks
2
A student follows the acid-catalysed hydrolysis of methyl ethanoate, CH₃COOCH₃ + H₂O → CH₃COOH + CH₃OH, at constant temperature. At intervals a 5.0 cm³ sample of the mixture is pipetted into ice-cold water and titrated with sodium hydroxide solution.
(a)
Why is each sample added to ice-cold water before titration?
[1 mark]
ATo dissolve the ester so that it can be titrated
BTo slow the reaction so that the composition does not change during the titration
CTo neutralise the acid catalyst
DTo increase the rate of reaction so the titration is faster
(b)
The volume of sodium hydroxide needed for each sample increases with time. Why?
[1 mark]
AThe acid catalyst is being used up
BSodium hydroxide is being used up
CMore ester is forming
DEthanoic acid is being formed and reacts with the sodium hydroxide as well as the acid catalyst
(c)
Describe how the student could use the titration results to find the initial rate of the reaction.
[2 marks]
Total for question 2: 4 marks
3
The reaction BrO₃⁻(aq) + 5Br⁻(aq) + 6H⁺(aq) → 3Br₂(aq) + 3H₂O(l) was studied at constant temperature using the initial-rate method. Experiment 1: [BrO₃⁻] = 0.10, [Br⁻] = 0.10, [H⁺] = 0.10 mol dm⁻³; initial rate 1.2 × 10⁻³ mol dm⁻³ s⁻¹. Experiment 2: [BrO₃⁻] = 0.20, [Br⁻] = 0.10, [H⁺] = 0.10; initial rate 2.4 × 10⁻³. Experiment 3: [BrO₃⁻] = 0.10, [Br⁻] = 0.30, [H⁺] = 0.10; initial rate 3.6 × 10⁻³. Experiment 4: [BrO₃⁻] = 0.10, [Br⁻] = 0.10, [H⁺] = 0.20; initial rate 4.8 × 10⁻³.
(a)
Deduce the order of reaction with respect to each of BrO₃⁻, Br⁻ and H⁺, giving your reasoning.
[3 marks]
(b)
Write the rate equation, calculate the rate constant, k, with its units, and calculate the initial rate when all three concentrations are 0.050 mol dm⁻³.
[4 marks]
Total for question 3: 7 marks
4
Nitrogen dioxide is oxidised by ozone in the atmosphere: 2NO₂(g) + O₃(g) → N₂O₅(g) + O₂(g). Experiments show that the rate equation is rate = k[NO₂][O₃]. A proposed mechanism has two steps. Step 1: NO₂ + O₃ → NO₃ + O₂ (slow). Step 2: NO₃ + NO₂ → N₂O₅ (fast).
(a)
Explain how this mechanism is consistent with both the rate equation and the overall equation, and identify the intermediate.
[6 marks]
(b)
A second student proposes a different mechanism. Step 1: NO₂ + NO₂ → N₂O₄ (slow). Step 2: N₂O₄ + O₃ → N₂O₅ + O₂ (fast). Show that this mechanism fits the overall equation, deduce the rate equation it predicts, and evaluate whether the experimental evidence supports it.
[6 marks]
Total for question 4: 12 marks
5
Nitryl chloride decomposes in the gas phase: 2NO₂Cl(g) → 2NO₂(g) + Cl₂(g). The rate equation is rate = k[NO₂Cl]. The proposed mechanism is: step 1, NO₂Cl → NO₂ + Cl (slow); step 2, NO₂Cl + Cl → NO₂ + Cl₂ (fast).
(a)
Which statement links the rate equation to the mechanism correctly?
[1 mark]
AStep 2 is rate-determining because it involves two reactants
BThe fast step is rate-determining because it forms the final product, Cl₂
CBoth steps are rate-determining because both involve NO₂Cl
DStep 1 is rate-determining because it contains one NO₂Cl molecule, matching the rate equation
(b)
Which species is an intermediate in this mechanism?
[1 mark]
ANO₂
BCl
CCl₂
DNO₂Cl
(c)
Explain why the reaction is first order with respect to NO₂Cl even though two molecules of NO₂Cl appear in the overall equation.
[2 marks]
Total for question 5: 4 marks
6
A student studies how the rate constant, k, of a reaction in solution varies with temperature, and plots ln k against 1/T, where T is in kelvin. The line has a gradient of −6.0 × 10³ K. (R = 8.31 J K⁻¹ mol⁻¹.)
(a)
What is the activation energy of the reaction?
[1 mark]
A+49.9 kJ mol⁻¹
B−49.9 kJ mol⁻¹
C0.722 kJ mol⁻¹
D6.0 kJ mol⁻¹
(b)
Which statement explains why k increases when the temperature is raised?
[1 mark]
AThe activation energy decreases
BThe concentrations of the reactants increase
CA greater proportion of molecules have energy equal to or greater than the activation energy
DThe collision frequency doubles for every 10 K rise
(c)
A catalyst is added to the reaction. State and explain the effect on the gradient of the ln k against 1/T line.
[2 marks]
Total for question 6: 4 marks
7
The decomposition of hydrogen peroxide in aqueous solution, 2H₂O₂(aq) → 2H₂O(l) + O₂(g), is catalysed by iodide ions. At constant temperature, the concentration of H₂O₂ was found by titrating samples: 0.800 mol dm⁻³ at 0 s, 0.400 mol dm⁻³ at 150 s, 0.200 mol dm⁻³ at 300 s and 0.100 mol dm⁻³ at 450 s.
(a)
Use the data to deduce the order of reaction with respect to hydrogen peroxide.
[3 marks]
(b)
Calculate the rate constant, k, with units, and the rate of reaction when the concentration of hydrogen peroxide is 0.600 mol dm⁻³.
[4 marks]
Total for question 7: 7 marks
8
A chemist studies the reaction between iodine monochloride and hydrogen in the gas phase: 2ICl(g) + H₂(g) → I₂(g) + 2HCl(g). She believes the rate equation is rate = k[ICl][H₂].
(a)
Describe how she could use the initial-rate method to test this rate equation and find the rate constant. Include a suitable technique for following the reaction.
[6 marks]
(b)
The rate constant was measured at four temperatures. Values of 1/T (in 10⁻³ K⁻¹) and ln k are: 3.33 and −3.51; 3.23 and −2.83; 3.13 and −2.15; 3.03 and −1.47. A graph of ln k against 1/T is a straight line. Calculate the activation energy using the first and last points (R = 8.31 J K⁻¹ mol⁻¹), and explain why the gradient is negative and what the intercept represents.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).