KineticsAQA A-Level Chemistry: Topic test

20 questions, 54 marks

AQA A-Level Chemistry

Kinetics topic test

Total 54 marks

Name

Class

Date

1
A student reacts an excess of zinc granules with 50.0 cm³ of 1.0 mol dm⁻³ hydrochloric acid at 298 K and measures the volume of hydrogen collected at regular intervals. The rate of reaction is greatest at the start of the experiment.
(a)
The experiment is repeated with 50.0 cm³ of 2.0 mol dm⁻³ hydrochloric acid at 298 K. Which statement best explains why the initial rate is greater?
[1 mark]
AThe activation energy of the reaction is lower
BThe hydrogen ions have a greater mean kinetic energy
CThere are more hydrogen ions per unit volume, so collisions with the zinc surface are more frequent
DA greater proportion of collisions have energy greater than the activation energy
(b)
The original experiment is repeated using 1.0 mol dm⁻³ acid at 308 K. The rate is about twice the rate at 298 K. Which statement best explains this large effect of a 10 K rise?
[1 mark]
AThe activation energy decreases to about half its value
BA much greater proportion of collisions have energy equal to or greater than the activation energy
CThe collision frequency doubles because the particles move twice as fast
DThe number of hydrogen ions per unit volume doubles
(c)
Explain why the rate of the original reaction decreases as the reaction proceeds, even though the temperature stays constant.
[2 marks]
Total for question 1: 4 marks
2
Ethene reacts with hydrogen in a sealed reaction vessel at 450 K to form ethane: C₂H₄(g) + H₂(g) → C₂H₆(g). Without a catalyst the reaction is extremely slow, but in the presence of finely divided solid nickel it is rapid at the same temperature.
(a)
How does the nickel catalyst increase the rate of this reaction?
[1 mark]
AIt provides an alternative reaction route with a lower activation energy
BIt increases the mean kinetic energy of the gas molecules
CIt makes the reaction more exothermic
DIt is used up, supplying the energy needed to break bonds
(b)
In a separate experiment, without nickel, the gas mixture is compressed to half its original volume at constant temperature. Which statement is correct?
[1 mark]
AThe activation energy of the reaction halves
BThe mean kinetic energy of the molecules doubles
CThe proportion of molecules with energy above the activation energy doubles
DThe number of molecules per unit volume doubles, so collisions become more frequent
(c)
At the end of the reaction the nickel is filtered off, washed, dried and weighed. State what is found about the mass and chemical composition of the nickel, and explain why this means nickel is a catalyst.
[2 marks]
Total for question 2: 4 marks
3
Two samples of the same gas, each containing the same number of molecules, are held at 300 K and 600 K. A student draws Maxwell–Boltzmann distribution curves of molecular energies for the two samples on the same axes, with energy on the horizontal axis and the number of molecules with that energy on the vertical axis.
(a)
Describe three ways in which the curve for 600 K differs from, or is the same as, the curve for 300 K.
[3 marks]
(b)
The gas reacts with a second reactant with an activation energy of 90 kJ mol⁻¹, which is far greater than the mean molecular energy at either temperature. Explain why the rate of reaction at 600 K is much more than twice the rate at 300 K.
[4 marks]
Total for question 3: 7 marks
4
In the Contact process, sulfur dioxide reacts with oxygen to form sulfur trioxide: 2SO₂(g) + O₂(g) → 2SO₃(g). The reaction is carried out at about 700 K and a pressure of about 2 atmospheres, using a solid vanadium(V) oxide catalyst. A plant manager asks an engineer to explain why each of these three conditions increases the rate of reaction (the effect on equilibrium yield is to be ignored).
(a)
Explain how each of the following increases the rate of reaction: operating at 2 atm instead of 1 atm, operating at 700 K instead of 300 K, and using the vanadium(V) oxide catalyst.
[6 marks]
(b)
In a separate experiment, a student investigates how temperature affects the rate of reaction of magnesium ribbon with excess 0.50 mol dm⁻³ sulfuric acid. Pieces of ribbon of equal size are used, and the time taken for the ribbon to disappear is 160 s at 293 K, 80 s at 303 K, 40 s at 313 K and 20 s at 323 K. The student concludes: "the rate doubles for every 10 K rise because the particles move twice as fast". Evaluate this conclusion, using collision theory and the Maxwell–Boltzmann distribution.
[6 marks]
Total for question 4: 12 marks
5
A sealed flask contains hydrogen iodide gas at 700 K. The molecules have a range of energies, described by a Maxwell–Boltzmann distribution. Hydrogen iodide decomposes according to the equation 2HI(g) → H₂(g) + I₂(g), a reaction with a high activation energy.
(a)
Which statement about the Maxwell–Boltzmann distribution curve for this sample is correct?
[1 mark]
AThe curve is symmetrical about the most probable energy
BThe curve meets the energy axis at the maximum possible molecular energy
CThe curve starts at a positive value because every molecule has some kinetic energy
DThe area under the curve is proportional to the total number of molecules in the sample
(b)
Which statement correctly defines the activation energy of the decomposition?
[1 mark]
AThe minimum energy colliding particles must have for a reaction to occur
BThe mean kinetic energy of the molecules in the flask
CThe energy released when the products are formed
DThe difference in energy between the reactants and the products
(c)
Finely divided gold catalyses the decomposition. Explain, with reference to the Maxwell–Boltzmann distribution, how the gold increases the rate at 700 K.
[2 marks]
Total for question 5: 4 marks
6
Carbon monoxide reacts with nitrogen dioxide in a rigid sealed vessel at 500 K: CO(g) + NO₂(g) → CO₂(g) + NO(g). The activation energy of the reaction is 134 kJ mol⁻¹, and the mean kinetic energy of the molecules at 500 K is about 6 kJ mol⁻¹.
(a)
Helium, an unreactive gas, is added to the vessel at constant volume and temperature so that the total pressure rises. What happens to the rate of reaction?
[1 mark]
AIt increases because the total pressure is higher
BIt is unchanged because the concentrations of CO and NO₂ are unchanged
CIt decreases because helium atoms block collisions between the reactants
DIt increases because helium atoms raise the mean energy of the reactants
(b)
Which change would give the largest increase in the proportion of molecules with energy greater than the activation energy of the uncatalysed reaction?
[1 mark]
ADoubling the concentrations of both gases
BCompressing the gases to half the volume
CRaising the temperature from 500 K to 600 K
DAdding more helium at constant volume
(c)
Use the figures given to explain why most collisions between CO and NO₂ molecules at 500 K do not result in reaction.
[2 marks]
Total for question 6: 4 marks
7
Dinitrogen pentoxide decomposes in the gas phase at 330 K: 2N₂O₅(g) → 4NO₂(g) + O₂(g). In an experiment, the concentration of N₂O₅ is 0.0400 mol dm⁻³ at the start, 0.0250 mol dm⁻³ after 300 s and 0.0160 mol dm⁻³ after 600 s.
(a)
Calculate the mean rate of reaction, in terms of the concentration of N₂O₅, for the first 300 s and for the second 300 s. Include units.
[3 marks]
(b)
Explain why the mean rate is lower in the second 300 s, and why the initial rate would be much higher if the experiment were repeated at 350 K with the same initial concentration.
[4 marks]
Total for question 7: 7 marks
8
Ethanal vapour decomposes in a rigid vessel at 700 K: CH₃CHO(g) → CH₄(g) + CO(g). A student studying the reaction needs to explain the effect of a catalyst on the rate, and also writes: "Doubling the pressure of the gas and raising the temperature by 10 K both increase the rate for the same reason: the molecules collide more often."
(a)
Describe how you would use a labelled Maxwell–Boltzmann distribution diagram to explain how a catalyst increases the rate of this decomposition at 700 K.
[6 marks]
(b)
Evaluate the student's statement, referring to collision frequency and to the Maxwell–Boltzmann distribution.
[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).