OxfordAQA International A Level Chemistry
OxfordAQA International A Level Chemistry 3.1.11 Rate Equations Exam Questions
Free OxfordAQA International A Level Chemistry revision for 3.1.11 Rate Equations: what you need to know, common mistakes and exam-style questions with answers. Covers rate equations, orders of reaction, the rate constant, the Arrhenius equation, initial rates and rate-determining steps.
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About OxfordAQA International A Level Chemistry 3.1.11 Rate Equations
Rate equations is an A2 physical chemistry topic examined in Unit 4. It builds on the qualitative kinetics from Unit 2 by measuring rate quantitatively. You need to write rate equations of the form rate = k[A]^m[B]^n with orders of 0, 1 or 2, calculate the rate constant and its units, and use the Arrhenius equation in both exponential and logarithmic forms. You also need to deduce orders from concentration-time graphs, initial rates data and rate-concentration graphs, and link the rate equation to the rate-determining step of a mechanism. Required practical 8 uses an initial rate method and a continuous monitoring method.
What you need to know
- Rate equation Rate = k[A]^m[B]^n, where m and n are the orders with respect to A and B and are found by experiment, not from the balanced equation. The overall order is m + n.
- Orders from data For order 0, changing concentration has no effect on rate. For order 1, doubling concentration doubles rate. For order 2, doubling concentration quadruples rate.
- Graphs On a rate-concentration graph, zero order is a horizontal line, first order a straight line through the origin and second order an upward curve. On a concentration-time graph zero order is a straight line, and first order has a constant half-life.
- Rate constant Rearrange the rate equation to find k, then work out units by cancelling. k increases with temperature.
- Arrhenius equation k = Ae^(-Ea/RT), which can be written as ln k = -Ea/RT + ln A. A plot of ln k against 1/T is a straight line with gradient -Ea/R. The equations are given when needed.
- Rate-determining step The rate-determining step is the slowest step in a mechanism. Species in the rate equation are involved in or before the rate-determining step.
Common mistakes in 3.1.11 Rate Equations
- Taking orders from the balanced equation. Orders can only be found experimentally.
- Getting the units of k wrong. Write mol dm-3 for each concentration, mol dm-3 s-1 for rate, then rearrange and cancel.
- Using temperature in degrees Celsius in the Arrhenius equation. T must be in kelvin.
- Forgetting that Ea from the gradient is in J mol-1. Multiply the gradient by -R, then divide by 1000 for kJ mol-1.
Try three 3.1.11 Rate Equations exam-style questions
1. Rate = k[A][B]^2. When [A] = 0.20 and [B] = 0.10 mol dm-3, rate = 4.0 x 10^-4 mol dm-3 s-1. Calculate k and give its units. (3 marks)
Show the mark scheme answer
k = 4.0 x 10^-4 / (0.20 x 0.10^2); = 0.20; mol-2 dm6 s-1.
2. Doubling [X] doubles the rate and doubling [Y] has no effect. Write the rate equation. (1 mark)
Show the mark scheme answer
Rate = k[X].
3. A plot of ln k against 1/T has a gradient of -6000 K. Calculate Ea in kJ mol-1. R = 8.31 J K-1 mol-1. (2 marks)
Show the mark scheme answer
Ea = -gradient x R = 6000 x 8.31 = 49 860 J mol-1; = 49.9 kJ mol-1.
3.1.11 Rate Equations FAQs
What does OxfordAQA International A Level Chemistry 3.1.11 Rate Equations cover?
It covers rate equations, orders of reaction, the rate constant, the Arrhenius equation, initial rates and rate-determining steps.
Which unit is 3.1.11 Rate Equations examined in?
It is examined in Unit 4, an International A2 paper covering organic and physical chemistry. The Unit 5 practical and synoptic paper can also assess it.
How do I find an order from initial rates data?
Choose two experiments where only one concentration changes. Compare the factor by which concentration changes with the factor by which rate changes: same factor means first order, factor squared means second order, no change means zero order.
