Introduction to Kinetics and EquilibriaEdexcel International A Level Chemistry: Topic test

20 questions, 54 marks

Edexcel International A Level Chemistry

Introduction to Kinetics and Equilibria topic test

Total 54 marks

Name

Class

Date

1
A student reacts 0.50 g of zinc powder and, in a separate experiment, a single 0.50 g zinc granule with equal volumes of the same excess dilute hydrochloric acid at 25 °C: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g).
(a)
Why does the zinc powder react faster than the zinc granule?
[1 mark]
AThe powder has a larger surface area exposed to the acid, so collisions between acid particles and zinc atoms are more frequent
BThe powder has a lower activation energy than the granule
CThe powder particles move faster than the granule at the same temperature
DThe powder raises the concentration of the acid
(b)
The granule experiment is repeated at 35 °C. What is the main reason for the increase in rate?
[1 mark]
AThe activation energy of the reaction decreases
BThe concentration of hydrochloric acid increases
CA greater proportion of collisions has energy equal to or greater than the activation energy
DThe zinc granule has a larger surface area
(c)
The granule takes 250 s to disappear and the powder takes 40 s. Calculate the rate, in s⁻¹, for each experiment using rate = 1/time, and state how many times faster the reaction with the powder is.
[2 marks]
Total for question 1: 4 marks
2
A car exhaust catalytic converter contains a solid platinum–rhodium catalyst that converts harmful gases: 2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g). The reaction is very slow in the hot exhaust gas without the catalyst.
(a)
How does the catalyst increase the rate of this reaction?
[1 mark]
AIt increases the kinetic energy of the colliding molecules
BIt provides an alternative reaction route with a lower activation energy
CIt makes the reaction more exothermic
DIt increases the activation energy so that fewer collisions are needed
(b)
On a Maxwell–Boltzmann distribution of molecular energies for the exhaust gases, what is the effect of adding the catalyst?
[1 mark]
AThe whole curve shifts to higher energy, with a lower peak
BThe total area under the curve increases
CThe activation energy moves to a higher energy so more molecules are above it
DThe position of the activation energy moves to a lower energy, so a larger area under the curve lies to its right
(c)
State two ways in which the use of catalysts makes industrial processes more sustainable.
[2 marks]
Total for question 2: 4 marks
3
Dinitrogen pentoxide decomposes in an inert solvent at constant temperature: 2N₂O₅ → 4NO₂ + O₂. A student collects the oxygen in a gas syringe and plots the volume of oxygen against time. A tangent drawn to the curve at 60 s passes through the points (20 s, 22 cm³) and (140 s, 58 cm³).
(a)
Use the tangent to calculate the rate of production of oxygen at 60 s. Give the units.
[3 marks]
(b)
Explain, using collision theory, why the rate at 60 s is lower than the initial rate. In a second experiment at the same temperature with a more concentrated solution of N₂O₅, 40 cm³ of oxygen is produced in 80 s, compared with 125 s in the first experiment. Calculate how many times faster the second experiment is, using rate = 1/time.
[4 marks]
Total for question 3: 7 marks
4
Hydrogen for fuel cells is made by the steam reforming of methane in a closed reactor: CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g), ΔH = +206 kJ mol⁻¹. The reactor uses a nickel catalyst at 1100 K and about 20 atm.
(a)
Explain the effect of increasing the temperature and of increasing the pressure on the position of equilibrium and on the rate of reaction, and hence why an operator chooses a high temperature but only a moderate pressure of 20 atm.
[6 marks]
(b)
Explain, with reference to the Maxwell–Boltzmann distribution, how the nickel catalyst increases the rate of reaction. State and explain its effect on the equilibrium yield of hydrogen, and explain how its use makes the process more sustainable.
[6 marks]
Total for question 4: 12 marks
5
Ethanoic acid reacts with ethanol in a sealed flask containing a few drops of concentrated sulfuric acid: CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l). The mixture is kept at 60 °C until its composition no longer changes.
(a)
Which statement describes the mixture once its composition no longer changes?
[1 mark]
ABoth reactions have stopped
BThe concentrations of all four substances are equal
CThe forward reaction is faster than the backward reaction until all the ethanoic acid is used up
DBoth the forward and backward reactions continue, at equal rates
(b)
Extra ethanol is added to the equilibrium mixture at constant temperature. What happens as a new equilibrium is established?
[1 mark]
AThe position of equilibrium shifts to the right, so the concentrations of the ester and of water both increase
BThe position of equilibrium shifts to the left, so the concentrations of the ester and water decrease
CThere is no change in position because the sulfuric acid fixes the composition
DThe position of equilibrium shifts to the right, so the ester concentration increases but the water concentration decreases
(c)
Explain why the sulfuric acid catalyst shortens the time taken to reach equilibrium but does not change the composition of the equilibrium mixture.
[2 marks]
Total for question 5: 4 marks
6
Nitrogen monoxide reacts with oxygen in the gas phase at room temperature: 2NO(g) + O₂(g) → 2NO₂(g). A chemist compresses a mixture of the gases into half its original volume at constant temperature.
(a)
Why does the rate of reaction increase when the gases are compressed?
[1 mark]
AA greater proportion of the molecules has energy equal to or greater than the activation energy
BThe activation energy of the reaction falls
CThe molecules are closer together, so collisions between them are more frequent
DThe molecules move faster because they are compressed
(b)
A small rise in temperature increases the rate of a reaction by a large factor. What is the best explanation?
[1 mark]
AThe number of collisions per second doubles
BThe proportion of collisions with energy at least equal to the activation energy rises sharply
CThe activation energy falls by 10 kJ mol⁻¹
DThe total number of molecules in the mixture increases
(c)
State what is meant by the term activation energy and explain why most collisions between NO and O₂ molecules at room temperature do not lead to reaction.
[2 marks]
Total for question 6: 4 marks
7
In an ammonia plant, nitrogen and hydrogen are mixed in a 1 : 3 mole ratio: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. At a fixed pressure of 200 atm the equilibrium percentage yield of ammonia is 52% at 350 °C, 28% at 450 °C and 13% at 550 °C. The reaction is too slow to be economic at 350 °C, even with an iron catalyst.
(a)
Use the data to explain why the plant operates at 450 °C rather than 350 °C or 550 °C.
[3 marks]
(b)
A reactor is fed with 100 mol of nitrogen and 300 mol of hydrogen and reaches equilibrium at 450 °C and 200 atm. Calculate the mass of ammonia (Mr = 17.0) in the equilibrium mixture, taking the yield as a percentage of the maximum possible. Explain why the unreacted gases are recycled.
[4 marks]
Total for question 7: 7 marks
8
Ethene is hydrogenated to ethane: C₂H₄(g) + H₂(g) ⇌ C₂H₆(g), ΔH = −137 kJ mol⁻¹. Without a catalyst the reaction is imperceptibly slow at 300 K, but with a finely divided nickel catalyst it is rapid at 400 K. The reaction is reversible, and above about 600 K the reverse reaction becomes significant.
(a)
Explain, in terms of the Maxwell–Boltzmann distribution, how raising the temperature and adding the nickel catalyst each increase the rate. Describe how the reaction profile for the catalysed reaction differs from that for the uncatalysed reaction.
[6 marks]
(b)
A manufacturer wants a faster production of ethane. Evaluate whether it is better to raise the temperature to 700 K or to use the nickel catalyst at 400 K, referring to yield, rate and sustainability.
[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).