The questions are on the rate equations test. Take it under real conditions before reading the mark scheme.
Quick answer: A rate equation, rate = k[A]^m[B]^n, shows how concentration controls reaction speed; orders come from experiments, not from the balanced equation. This mark scheme gives the model answers and the marks for the 8 questions of the Chemistry 11-12 test on rate equations, from units of k to changing two concentrations, with the equivalent answers to accept.
Model answers and mark schemes follow. Give credit for correct reasoning even if a later numerical slip occurs, and check the units of every rate constant.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R2.2
Question 1 Units of k (2 marks)
(a) Second order overall.
(b) mol dm⁻³ s⁻¹ = k × (mol dm⁻³)², so k has units dm³ mol⁻¹ s⁻¹.
Marking: 1 mark per part.
Question 2 Initial rates (3 marks)
Tripling [P] triples the rate: first order in P. Doubling [Q] has no effect: zero order in Q. Rate = k[P]. k = 3.0 × 10⁻⁴ / 0.10 = 3.0 × 10⁻³ s⁻¹.
Marking: 1 for both orders; 1 for the rate equation; 1 for k with units.
Question 3 Mechanism (3 marks)
Rate = k[H₂O₂][I⁻]. Overall: 2H₂O₂ → 2H₂O + O₂. I⁻ is the catalyst (used in step 1, regenerated in step 2) and IO⁻ is the intermediate.
Marking: 1 for the rate equation; 1 for the overall equation; 1 for catalyst and intermediate.
Question 4 Arrhenius graph (3 marks)
Gradient = (−7.2 − (−4.0)) / (0.00340 − 0.00300) = −3.2 / 0.00040 = −8000 K. Ea = 8000 × 8.314 = 66 512 J/mol = 66.5 kJ/mol.
Marking: 1 for the gradient; 1 for Ea = −gradient × R; 1 for the value in kJ/mol.
Question 5 Half-life (2 marks)
t½ = 0.693 / 2.31 × 10⁻² = 30.0 min.
Marking: 1 for the expression; 1 for the value and unit.
Question 6 Temperature and rate (3 marks)
At a higher temperature more particles have energy at least equal to the activation energy, and the fraction rises exponentially. A small increase in collision frequency also helps, but the larger fraction of successful collisions is the main reason the rate increases greatly.
Marking: 1 for more particles with E ≥ Ea; 1 for a large (exponential) change in the fraction; 1 for more frequent collisions as a smaller effect.
Question 7 Catalysts (2 marks)
A catalyst provides an alternative pathway with a lower activation energy, so a larger fraction of collisions are successful. It is regenerated at the end of the mechanism, so it is not used up.
Marking: 1 for lower activation energy via an alternative route; 1 for regeneration.
Question 8 Changing two concentrations (2 marks)
Factor for A: 2¹ = 2. Factor for B: (½)² = ¼. Overall = 2 × ¼ = ½, so the rate halves.
Marking: 1 for the factors for A and B; 1 for the combined factor.
