OxfordAQA A-Level Chemistry: Kinetics (9620) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover section 3.1.6 Kinetics of the OxfordAQA International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards: points 3.1.6.1 to 3.1.6.5. All of it is International AS content, so nothing here is International A-level only. The specification assesses it in Unit 2 (Organic 1 and Physical 1), where questions may also draw on amount of substance (3.1.2).

Learn the ideas first in the study guide and revision notes.

Questions

1. Define the term activation energy. [1]

2. State what is meant by the rate of a reaction, and give its unit when concentration is measured in mol dm⁻³ and time in seconds. [2]

3. Nitrogen and oxygen molecules in the air collide constantly at 25 °C, but they do not react to a noticeable extent. Explain why most of these collisions do not lead to a reaction. [2]

4. A student’s Maxwell–Boltzmann sketch starts above zero on the y-axis, is symmetrical about its peak, and meets the energy axis at high energy. Correct each fault, giving a reason. [3]

5. Curves A and B are drawn on squared paper for one gas sample at two temperatures. Each encloses 1600 small squares. To the right of Ea, curve A encloses 12 squares and curve B encloses 34 squares.

(a) Calculate the percentage of molecules with E ≥ Ea for curve A. [1] (b) State which curve is at the higher temperature, and calculate by what factor the proportion of molecules with E ≥ Ea has increased. [2] (c) Explain why the rate increases by roughly this factor, and not just by the change in collision frequency. [2]

6. Bromine reacts with methanoic acid in aqueous solution, and the brown colour fades:

Br₂(aq) + HCOOH(aq) → 2Br⁻(aq) + 2H⁺(aq) + CO₂(g)

Time / s	0	240	480
[Br₂] / mol dm⁻³	0.0150	0.0102	0.0072

(a) Calculate the mean rate of reaction over the first 240 s. Give the unit. [2] (b) Calculate the mean rate between 240 s and 480 s. [1] (c) Explain, in terms of collisions, why the rate in (b) is lower than the rate in (a). [2]

7. A student adds 0.300 g of zinc to 50 cm³ of sulfuric acid at a fixed temperature and collects the hydrogen in a gas syringe: one run with 1.0 mol dm⁻³ acid, one with 0.50 mol dm⁻³. The acid is in excess.

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

After 30 s the 1.0 mol dm⁻³ run has produced 54 cm³ of hydrogen and the 0.50 mol dm⁻³ run 29 cm³.

(a) Calculate the mean rate over the first 30 s for the 1.0 mol dm⁻³ run. [1] (b) Calculate the same for the 0.50 mol dm⁻³ run. [1] (c) Explain the difference between the two rates. [3] (d) Calculate the maximum volume of hydrogen, in cm³, that 0.300 g of zinc can produce at 100 kPa and 293 K. Ar of Zn = 65.4; R = 8.31 J K⁻¹ mol⁻¹. [3] (e) Explain why both runs finally collect the same volume of hydrogen. [1]

8. A gas decomposes slowly on heating, but much faster at the same temperature over a metal surface that is unchanged at the end. Use a Maxwell–Boltzmann distribution to explain how the metal increases the rate. [4]

9. For a gas-phase reaction at a fixed temperature, a distribution curve encloses 2000 small squares. The area beyond Ea for the uncatalysed reaction is 3 squares. The area beyond Ea for the catalysed route is 57 squares.

(a) Calculate the percentage of molecules with enough energy to react without and with the catalyst. [2] (b) Calculate the factor by which the catalyst increases this proportion. [1] (c) The student weighs 0.250 g of the solid catalyst before the reaction. State the mass she should recover afterwards and why. [1] (d) Explain, using an enthalpy profile, whether the catalyst changes the enthalpy change of the reaction. [2]

10. For a reaction with Ea = 60 kJ mol⁻¹, warming the mixture from 293 K to 303 K increases the mean speed of the molecules by 1.7 % and makes the proportion of collisions with E ≥ Ea 2.25 times larger. A student says: “The rate more than doubles mainly because the particles collide more often.” Evaluate this statement, using the data and the Maxwell–Boltzmann distribution. [5]

11. A mixture of hydrogen and oxygen can be kept at room temperature without reacting, but it explodes when a spark passes through it.

(a) Explain both observations in terms of collision theory. [4] (b) The mixture is compressed to a higher pressure before the spark, at the same temperature. Explain why the reaction is then faster. [2]

Answers

1. The minimum energy that particles must have when they collide for a reaction to occur [1]. [1] Examiner insight: Answers without the word “minimum”, or that say “energy to start a reaction”, are unlikely to earn this mark.

2. The change in concentration of a reactant or product per unit time [1]; unit mol dm⁻³ s⁻¹ [1]. [2] Examiner insight: A unit with no time part, such as mol dm⁻³, does not describe a rate, so make sure the s⁻¹ is there.

3. Most of the molecules have energy less than the activation energy [1]; so the collisions do not supply enough energy to break the bonds, and the molecules move apart unchanged [1]. [2] Examiner insight: This is about energy, not frequency; “they do not collide often enough” misses the point.

4. The curve starts at the origin, because no molecule has zero energy [1]; it is not symmetrical, with a long high-energy tail, so the mean energy lies to the right of the peak [1]; it approaches but never meets the energy axis, because there is no maximum energy [1]. [3] Examiner insight: Give a reason with each correction; “start at zero” alone is weaker than “no molecule has zero energy”.

5. (a) 12 ÷ 1600 × 100 = 0.75 % [1]. (b) Curve B is at the higher temperature [1]; factor = 34 ÷ 12 = 2.8 [1]. (c) B has 2.8 times the proportion of molecules with E ≥ Ea, so 2.8 times as many collisions can succeed [1]; the rise in collision frequency is tiny by comparison [1]. [5] Examiner insight: In (b), pick the hotter curve by its larger area beyond Ea, not by peak height; the hotter peak is lower.

6. (a) Change in [Br₂] = 0.0150 − 0.0102 = 0.0048 mol dm⁻³; rate = 0.0048 ÷ 240 [1] = 2.0 × 10⁻⁵ mol dm⁻³ s⁻¹ [1]. (b) (0.0102 − 0.0072) ÷ 240 = 0.0030 ÷ 240 = 1.25 × 10⁻⁵ mol dm⁻³ s⁻¹ (1.3 × 10⁻⁵) [1]. (c) Bromine and methanoic acid are used up, so their concentrations are lower in the second interval [1]; so collisions between reactant particles are less frequent and there are fewer successful collisions per second [1]. [5] Examiner insight: Rates are positive even though [Br₂] falls; a negative answer or a missing s⁻¹ in the unit can cost credit.

7. (a) 54 ÷ 30 = 1.8 cm³ s⁻¹ [1]. (b) 29 ÷ 30 = 0.97 cm³ s⁻¹ [1]. (c) Each cm³ of the 1.0 mol dm⁻³ acid holds more H⁺ ions [1]; so they strike the zinc more often [1]; the same fraction of strikes reaches Ea in both runs (same temperature), so more succeed each second in the stronger acid [1]. (d) n(Zn) = 0.300 ÷ 65.4 = 4.59 × 10⁻³ mol, and n(H₂) = n(Zn) = 4.59 × 10⁻³ mol (1 : 1) [1]; V = nRT/p = 4.59 × 10⁻³ × 8.31 × 293 ÷ 100 000 = 1.12 × 10⁻⁴ m³ [1] = 112 cm³ [1]. (e) Zinc is the limiting reagent and its amount is the same in both runs, so the same amount of hydrogen forms [1]. [9] Examiner insight: In (d), p must be in Pa, so V comes out in m³; remember to multiply by 10⁶ for cm³.

8. The metal is a catalyst, offering a different pathway whose energy barrier is lower [1]. With no change in temperature, a single, unmoved distribution curve applies [1]. Ea(cat) sits at lower energy than Ea, so a bigger slice of the tail lies beyond it: more molecules can react [1]; hence more successful collisions per second and a faster rate [1]. [4] Examiner insight: A sketch should show one curve with two Ea lines; a second, shifted curve shows a temperature change.

9. (a) Without: 3 ÷ 2000 × 100 = 0.15 % [1]; with: 57 ÷ 2000 × 100 = 2.85 % [1]. (b) 57 ÷ 3 = 19 times [1]. (c) 0.250 g, because a catalyst is not changed in amount (or chemical composition) by the reaction [1]. (d) No change: the catalysed route has a lower energy hump [1], but it starts and ends at the same reactant and product enthalpies, so ΔH is the same [1]. [6] Examiner insight: In (d), draw both routes between the same two levels, or the sketch contradicts your written answer.

10. Collision frequency rises by a factor of only about 1.017 [1]; the proportion with E ≥ Ea rises 2.25 times [1]. Overall the rate rises about 1.017 × 2.25 ≈ 2.3 times, so it does more than double [1]. But the reason given is wrong: almost all of the rise comes from the larger proportion with E ≥ Ea [1]. Ea lies in the tail of the curve, so a shift to higher energy makes the area beyond Ea grow sharply [1]. [5] Examiner insight: “Evaluate” needs a judgement backed by data: say which part is right (the rate) and which is wrong (the reason).

11. (a) At room temperature very few molecules have E ≥ Ea, so almost no collisions succeed [1]. The spark gives nearby molecules energy above Ea, so they react [1]. The exothermic reaction heats the nearby gas [1]; so a much larger proportion there have E ≥ Ea, and the reaction spreads rapidly as an explosion [1]. (b) At higher pressure there are more molecules per unit volume, so collisions are more frequent [1]; the proportion with E ≥ Ea is unchanged, but there are more successful collisions per second [1]. [6] Examiner insight: In (a), link each step with “so” or “because”; separate facts with no chain to rate are harder to credit.

Where marks are usually lost

• An Ea definition with no “minimum”.
• Picking the warmer curve because its peak is taller (it is shorter).
• Putting the temperature effect down to more frequent collisions only.
• Linking concentration or pressure to a change in molecular energy.
• Redrawing the curve for a catalyst instead of just adding a lower Ea marker.
• Omitting the new pathway, or claiming a catalyst alters ΔH.
• Negative rates, or rate units with no time part.
• Leaving a gas volume in m³ when cm³ was asked for.

Next steps

• Kinetics revision notes
• Kinetics study guide
• Amount of substance
• OxfordAQA A-Level Chemistry hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

OxfordAQA, International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards: section 3.1.6 Kinetics (International AS), points 3.1.6.1 to 3.1.6.5.