KineticsEdexcel International A Level Chemistry: Topic test

20 questions, 54 marks

Edexcel International A Level Chemistry

Kinetics topic test

Total 54 marks

Name

Class

Date

1
Bromate(V) ions oxidise bromide ions in acidic solution: BrO₃⁻(aq) + 5Br⁻(aq) + 6H⁺(aq) → 3Br₂(aq) + 3H₂O(l). Experiments show that the reaction is first order with respect to BrO₃⁻ and to Br⁻, and second order with respect to H⁺, so rate = k[BrO₃⁻][Br⁻][H⁺]².
(a)
What is the overall order of the reaction?
[1 mark]
A4
B3
C5
D12
(b)
The concentration of H⁺ is tripled and the concentrations of the other two ions are unchanged. By what factor does the rate change?
[1 mark]
A3
B6
C9
D27
(c)
In one experiment, the initial concentrations are [BrO₃⁻] = 0.010 mol dm⁻³, [Br⁻] = 0.020 mol dm⁻³ and [H⁺] = 0.10 mol dm⁻³, and the initial rate is 2.4 × 10⁻⁵ mol dm⁻³ s⁻¹. Calculate the rate constant, k, and state its units.
[2 marks]
Total for question 1: 4 marks
2
A pesticide in soil breaks down by a first-order process at constant temperature, with a half-life of 12.0 days.
(a)
Which observation shows that the breakdown is first order with respect to the pesticide?
[1 mark]
AThe concentration falls by the same amount every day
BThe time taken for the concentration to halve is the same whatever the starting concentration
CThe half-life doubles each time the concentration halves
DThe rate of reaction is independent of concentration
(b)
A second sample of soil starts with twice the concentration of pesticide, at the same temperature. What is the half-life of the pesticide in this sample?
[1 mark]
A24.0 days, because there is twice as much to break down
B6.0 days, because the rate is doubled
CIt cannot be predicted without the rate constant
D12.0 days, the same as before
(c)
A soil sample contains 6.4 mg kg⁻¹ of the pesticide. Calculate the time taken for the concentration to fall to 0.40 mg kg⁻¹.
[2 marks]
Total for question 2: 4 marks
3
Nitrogen monoxide reacts with chlorine in the gas phase at constant temperature: 2NO(g) + Cl₂(g) → 2NOCl(g). Initial-rate data are as follows. Experiment 1: [NO] = 0.020 mol dm⁻³, [Cl₂] = 0.010 mol dm⁻³, initial rate = 2.0 × 10⁻⁵ mol dm⁻³ s⁻¹. Experiment 2: [NO] = 0.040 mol dm⁻³, [Cl₂] = 0.010 mol dm⁻³, initial rate = 8.0 × 10⁻⁵ mol dm⁻³ s⁻¹. Experiment 3: [NO] = 0.020 mol dm⁻³, [Cl₂] = 0.030 mol dm⁻³, initial rate = 6.0 × 10⁻⁵ mol dm⁻³ s⁻¹.
(a)
Deduce the order of reaction with respect to NO and with respect to Cl₂, showing your reasoning, and write the rate equation.
[3 marks]
(b)
Calculate the rate constant, k, using experiment 1, with its units. Hence calculate the initial rate when [NO] = 0.050 mol dm⁻³ and [Cl₂] = 0.020 mol dm⁻³.
[4 marks]
Total for question 3: 7 marks
4
Bromine reacts with methanoic acid in aqueous solution: Br₂(aq) + HCOOH(aq) → 2Br⁻(aq) + 2H⁺(aq) + CO₂(g). Bromine solution is orange-brown and the other substances are colourless. A student follows the reaction using a colorimeter, with methanoic acid in large excess, and repeats the experiment at several temperatures to find the rate constant k at each. A graph of ln k against 1/T is a straight line passing through the points (3.20 × 10⁻³ K⁻¹, −5.20) and (3.40 × 10⁻³ K⁻¹, −6.80). The Arrhenius equation is ln k = ln A − Ea/RT, with R = 8.31 J K⁻¹ mol⁻¹.
(a)
Describe how the student could use the colorimeter to obtain concentration–time data and use the data to show that the reaction is first order with respect to bromine. Explain why continuous monitoring by colorimetry suits this reaction.
[6 marks]
(b)
Explain how the student would obtain the data for the graph of ln k against 1/T, and use the two points given to calculate the activation energy, in kJ mol⁻¹.
[6 marks]
Total for question 4: 12 marks
5
The reaction 2NO₂ + F₂ → 2NO₂F has the rate equation rate = k[NO₂][F₂]. A proposed mechanism has two steps. Step 1 (slow): NO₂ + F₂ → NO₂F + F. Step 2 (fast): NO₂ + F → NO₂F.
(a)
Which species react together in the rate-determining step?
[1 mark]
ATwo NO₂ molecules and one F₂ molecule
BOne NO₂ molecule and one fluorine atom
CTwo fluorine atoms
DOne NO₂ molecule and one F₂ molecule
(b)
Which species is an intermediate in the mechanism?
[1 mark]
AF, a fluorine atom
BNO₂F, because it is formed in both steps
CF₂, because it is used up in step 1
DNO₂, because it takes part in both steps
(c)
Show that the two steps are consistent with the overall equation, and explain why the rate equation does not contain a term for the species in step 2.
[2 marks]
Total for question 5: 4 marks
6
In the manufacture of ammonia, a mixture of nitrogen and hydrogen is passed over an iron catalyst in the form of small pellets.
(a)
What term describes the iron catalyst in this reaction?
[1 mark]
AHomogeneous, because it is in the same phase as the reactants
BAutocatalytic, because it is formed in the reaction
CHeterogeneous, because it is in a different phase from the reactants
DAn inhibitor, because it makes the reaction go further
(b)
Why is the catalyst used as small pellets rather than as a single large block?
[1 mark]
AIt increases the concentration of nitrogen and hydrogen in the reactor
BA large surface area provides many sites where reactant molecules can be adsorbed and react
CIt changes the enthalpy change of the reaction
DIt ensures the catalyst is used up more quickly
(c)
Describe how a solid catalyst such as iron provides a surface for a gas-phase reaction.
[2 marks]
Total for question 6: 4 marks
7
The Arrhenius equation is ln k = ln A − Ea/RT, where R = 8.31 J K⁻¹ mol⁻¹. Part 1: a first-order reaction has an activation energy of 75.0 kJ mol⁻¹ and a constant A = 2.00 × 10¹¹ s⁻¹. Part 2: for a different reaction, the rate constant doubles when the temperature is raised from 300 K to 310 K.
(a)
For the reaction in Part 1, calculate the rate constant at 310 K, with units.
[3 marks]
(b)
Calculate the activation energy, in kJ mol⁻¹, of the reaction in Part 2.
[4 marks]
Total for question 7: 7 marks
8
Nitrogen dioxide reacts with carbon monoxide: NO₂(g) + CO(g) → NO(g) + CO₂(g). At 500 K the rate equation is rate = k[NO₂]². When the concentration of NO₂ is 0.020 mol dm⁻³, the rate of reaction is 1.6 × 10⁻⁴ mol dm⁻³ s⁻¹.
(a)
Suggest a two-step mechanism that is consistent with the rate equation and the overall equation, identifying the rate-determining step and the intermediate. State the effect on the rate of doubling the concentration of CO and of doubling the concentration of NO₂.
[6 marks]
(b)
Describe the shape of a graph of rate against concentration for a reaction that is zero order, first order and second order with respect to a reactant. State which shape applies to NO₂ in this reaction, and calculate the rate constant, with units.
[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).