A-Level · Chemistry · AQA · Mark scheme decoded
AQA A-Level Chemistry: Rate Equation and Reaction Orders — mark scheme explained
The short answer
The rate equation is a fundamental concept in chemical kinetics that describes the relationship between the rate of a reaction and the concentrations of its reactants.
The question
A reaction has the following initial rate data: [A] (mol dm -3 ) = 0.1, Rate (mol dm -3 s -1 ) = 0.02; [A] = 0.2, Rate = 0.04. Determine the order of the reaction with respect to A. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the given data.
- S2
[A] 1 = 0.1 mol dm -3 , Rate 1 = 0.02 mol dm -3 s -1
- S3
[A] 2 = 0.2 mol dm -3 , Rate 2 = 0.04 mol dm -3 s -1
- S4
Step 2: Use the rate equation to set up a ratio.
- S5
Rate 2 / Rate 1 = ([A] 2 / [A] 1 ) m
- S6
0.04 / 0.02 = (0.2 / 0.1) m
- S7
Step 3: Simplify the ratio.
- S8
2 = 2 m
- S9
Step 4: Solve for m.
- S10
m = 1
- S11
The reaction is first-order with respect to A.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the given data.
- S2
[A] 1 = 0.1 mol dm -3 , Rate 1 = 0.02 mol dm -3 s -1
- S3
[A] 2 = 0.2 mol dm -3 , Rate 2 = 0.04 mol dm -3 s -1
- S4
Step 2: Use the rate equation to set up a ratio.
- S5
Rate 2 / Rate 1 = ([A] 2 / [A] 1 ) m
- S6
0.04 / 0.02 = (0.2 / 0.1) m
- S7
Step 3: Simplify the ratio.
- S8
2 = 2 m
- S9
Step 4: Solve for m.
- S10
m = 1
- S11
The reaction is first-order with respect to A.
Final answer: First-order with respect to A
Common mistakes
- Misinterpreting a straight line on a concentration-time graph as first-order. — Always check if the rate is constant (zero-order) or if it changes exponentially (first-order).
- Forgetting to use natural logarithms when plotting ln[A] vs. time for first-order reactions. — Always use natural logarithms (ln) when plotting concentration-time data for first-order reactions.
- Incorrectly setting up the ratio for initial rate data. — Use the correct form of the rate equation and ensure that the ratio is set up correctly: Rate 2 / Rate 1 = ([A] 2 / [A] 1 ) m .
- Misinterpreting a horizontal line on a rate vs. concentration graph as first-order. — Always check if the plot is a horizontal line (zero-order) or a straight line passing through the origin (first-order).
- Forgetting to use the correct units for rate and concentration. — Always ensure that concentrations are in mol dm -3 and rates are in mol dm -3 s -1 when substituting into the rate equation.
- Incorrectly deriving the rate equation from known orders. — Always use the correct form of the rate equation: Rate = k[A] m [B] n . Ensure that the orders are used as exponents.
Where the marks go
- Full worked solution (all marking points)3 marks