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 Chemistry 11-12 test on rate equations has 8 questions worth 20 marks, to do in 45 minutes: units of k, initial rates and mechanism. The mark scheme is on a separate page; free printable PDF.
This test covers rate equations, orders, units of k, half-life, activation energy, catalysts and mechanisms. Show your working, keep units, and use the marks as a guide to the detail needed.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R2.2
Question 1 Units of k (2 marks)
A reaction has rate = k[A][B].- State the overall order. (1 mark)
- Deduce the units of k. (1 mark)
Question 2 Initial rates (3 marks)
Initial rates for P + Q → products: (1) [P] = 0.10, [Q] = 0.20, rate 3.0 × 10⁻⁴; (2) [P] = 0.30, [Q] = 0.20, rate 9.0 × 10⁻⁴; (3) [P] = 0.10, [Q] = 0.40, rate 3.0 × 10⁻⁴ (mol/dm³ and mol dm⁻³ s⁻¹). Deduce the orders, write the rate equation and calculate k with units.Question 3 Mechanism (3 marks)
A two-step mechanism is: step 1 (slow) H₂O₂ + I⁻ → IO⁻ + H₂O; step 2 (fast) IO⁻ + H₂O₂ → I⁻ + H₂O + O₂. Write the rate equation, give the overall equation and identify the catalyst and the intermediate.Question 4 Arrhenius graph (3 marks)
Question 5 Half-life (2 marks)
A first order reaction has k = 2.31 × 10⁻² min⁻¹. Calculate the half-life.Question 6 Temperature and rate (3 marks)
Explain, using the idea of activation energy, why the rate of a reaction increases greatly with a small rise in temperature.Question 7 Catalysts (2 marks)
Explain how a catalyst increases the rate of a reaction without being used up.Question 8 Changing two concentrations (2 marks)
The rate equation is rate = k[A][B]². Calculate the factor by which the rate changes when [A] is doubled and [B] is halved. Show your working.
