Kinetics IEdexcel A-Level Chemistry: Topic test

20 questions, 54 marks

Edexcel A-Level Chemistry

Kinetics I topic test

Total 54 marks

Name

Class

Date

1
A student reacts 0.12 g of magnesium ribbon with an excess of 1.0 mol dm⁻³ hydrochloric acid in a conical flask at 24 °C. The hydrogen gas is collected in a gas syringe and its volume is recorded at intervals: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).
(a)
The experiment is repeated with the same mass of magnesium powder instead of ribbon, all other conditions unchanged. Which statement explains why the initial rate is higher?
[1 mark]
AThe magnesium particles have a higher average kinetic energy
BThe activation energy of the reaction is lower
CMore magnesium is exposed to the acid, so there are more collisions per second
DA greater proportion of collisions have energy greater than or equal to the activation energy
(b)
A tangent is drawn to the volume–time curve at t = 0. It passes through the origin and through the point (15 s, 36 cm³). What is the initial rate of reaction?
[1 mark]
A0.42 cm³ s⁻¹
B2.4 cm³ s⁻¹
C21 cm³ s⁻¹
D540 cm³ s⁻¹
(c)
Explain, using collision theory, why the gradient of the volume–time curve decreases as the reaction proceeds.
[2 marks]
Total for question 1: 4 marks
2
Ethene reacts with hydrogen to form ethane in the presence of a solid nickel catalyst: C₂H₄(g) + H₂(g) → C₂H₆(g), ΔH = −137 kJ mol⁻¹. The gases are passed over the nickel at about 420 K. Without the catalyst the activation energy is so high that the reaction does not occur at a measurable rate at this temperature.
(a)
Which statement describes the effect of the nickel catalyst on the Maxwell–Boltzmann distribution for the gas mixture at 420 K?
[1 mark]
AThe curve moves to the right because the molecules gain kinetic energy
BThe curve becomes lower and broader because the number of molecules decreases
CThe curve moves to the left because the molecules lose energy to the surface
DThe curve is unchanged, but the lower activation energy means more molecules have energy greater than or equal to it
(b)
Which row correctly shows the effect of the nickel catalyst on the activation energy and on the enthalpy change of the reaction?
[1 mark]
AActivation energy lower, enthalpy change unchanged
BActivation energy lower, enthalpy change less negative
CActivation energy unchanged, enthalpy change more negative
DActivation energy lower, enthalpy change more negative
(c)
The nickel is used as a fine powder spread over a porous solid. Explain why this increases the rate of reaction per gram of nickel.
[2 marks]
Total for question 2: 4 marks
3
An effervescent tablet contains citric acid and sodium hydrogencarbonate. When it is dropped into 200 cm³ of water in an open flask on a balance, carbon dioxide escapes and the total mass of the flask and its contents decreases. A student records the mass lost at regular intervals at 20 °C.
(a)
Describe how the rate of reaction at 30 s can be found from a graph of mass lost against time. Calculate this rate using a tangent that passes through the points (10 s, 0.10 g) and (50 s, 0.34 g), and give its units.
[3 marks]
(b)
The experiment is repeated at 40 °C with an identical tablet. The initial rate is higher. Explain this, with reference to the distribution of molecular energies.
[4 marks]
Total for question 3: 7 marks
4
In the manufacture of nitric acid, ammonia and air are passed at about 1100 K over a gauze of platinum–rhodium alloy: 4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g), ΔH = −905 kJ mol⁻¹. The alloy is very expensive, but it is not used up in the process. A plant manager asks whether the cost of the gauze is justified.
(a)
Describe how the reaction profiles for the uncatalysed and catalysed reactions differ, and explain how the gauze increases the rate of reaction.
[6 marks]
(b)
Evaluate whether using the expensive gauze is likely to be economically better than raising the operating temperature to speed up the reaction.
[6 marks]
Total for question 4: 12 marks
5
Nitrogen monoxide reacts with oxygen in the gas phase: 2NO(g) + O₂(g) → 2NO₂(g). The reaction is carried out in a sealed cylinder fitted with a movable piston, at constant temperature.
(a)
The piston is pushed in so that the volume of the gas mixture is halved. Why does the rate of reaction increase?
[1 mark]
AThe gas particles are closer together, so collisions occur more frequently
BThe gas particles move faster, so more collisions have energy above the activation energy
CThe activation energy decreases because the pressure is higher
DThe number of gas particles in the cylinder increases
(b)
Which statement defines the activation energy of this reaction?
[1 mark]
AThe energy released when nitrogen dioxide forms
BThe average kinetic energy of the gas molecules
CThe energy needed to break all the bonds in the reactants
DThe minimum energy that colliding molecules must have for a reaction to occur
(c)
Explain why only a very small fraction of collisions between nitrogen monoxide and oxygen molecules results in the formation of nitrogen dioxide.
[2 marks]
Total for question 5: 4 marks
6
Two identical sealed containers each hold 0.10 mol of nitrogen dioxide gas, one at 400 K and one at 500 K. Nitrogen dioxide decomposes slowly: 2NO₂(g) → 2NO(g) + O₂(g). The distribution of molecular energies in each container is represented by a Maxwell–Boltzmann curve.
(a)
Which statement about the Maxwell–Boltzmann curves for the two containers is correct?
[1 mark]
AThe curve at 500 K has a higher peak, at a higher energy
BThe curve at 500 K has a lower peak, at a higher energy, and the same area
CThe curve at 500 K has the same peak height, moved to a higher energy
DThe curve at 500 K has a lower peak, at a lower energy, and a smaller area
(b)
What is the main reason the decomposition is faster at 500 K than at 400 K?
[1 mark]
AThe activation energy is lower at 500 K
BThere are more molecules of nitrogen dioxide at 500 K
CA greater proportion of molecules have energy greater than or equal to the activation energy
DThe average energy of the molecules is greater than the activation energy at 500 K
(c)
Explain why the area under each Maxwell–Boltzmann curve is the same for the two containers.
[2 marks]
Total for question 6: 4 marks
7
A student studies the decomposition of hydrogen peroxide in aqueous solution at 25 °C: 2H₂O₂(aq) → 2H₂O(l) + O₂(g). A small amount of solid manganese(IV) oxide is added to 50 cm³ of the solution and the oxygen is collected in a gas syringe. 18 cm³ of oxygen is collected in the first 30 s. (Molar volume of gas under these conditions = 24 dm³ mol⁻¹.)
(a)
Calculate the mean rate of formation of oxygen over the first 30 s, in cm³ s⁻¹, and the amount, in mol, of hydrogen peroxide that has decomposed in this time.
[3 marks]
(b)
Describe the effect of manganese(IV) oxide on the activation energy and on the enthalpy change of the reaction. Explain why the same total volume of oxygen is obtained eventually with or without the manganese(IV) oxide.
[4 marks]
Total for question 7: 7 marks
8
Long-chain hydrocarbon vapour is cracked at about 720 K by passing it over a solid zeolite catalyst, which is a fine, porous powder. Without the catalyst, temperatures above 1100 K are needed for cracking to occur at a useful rate. A student claims: 'The catalyst increases the rate because it makes the hydrocarbon molecules move faster, so there are more collisions per second.'
(a)
Evaluate the student's claim, using the Maxwell–Boltzmann distribution of molecular energies.
[6 marks]
(b)
Explain how a solid catalyst such as zeolite speeds up a gas-phase reaction, and why using it as a fine, porous powder is better than using large pellets of the same mass.
[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).