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
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For calculation questions, marks are typically awarded for correct substitution of values into formulas, accurate arithmetic, and providing the final answer with appropriate units. For conceptual questions, marks are given for clear and concise explanations that demonstrate a deep understanding of the topic.
What the command words demand
- Determine
- Find or derive a specific value or quantity based on given information.
- Plot
- Create a graph using given data and appropriate axes.
- Interpret
- Analyze and explain the meaning of data or graphs.
- Derive
- Use known information to develop a mathematical expression or equation.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, perform calculations, and check your work.
- Identify the given data.0 marks[A] 1 = 0.1 mol dm -3 , Rate 1 = 0.02 mol dm -3 s -1[A] 2 = 0.2 mol dm -3 , Rate 2 = 0.04 mol dm -3 s -1
- Use the rate equation to set up a ratio.1 markRate 2 / Rate 1 = ([A] 2 / [A] 1 ) m0.04 / 0.02 = (0.2 / 0.1) m
- Simplify the ratio.1 mark2 = 2 m
- Solve for m.1 markm = 1The reaction is first-order with respect to A.
Final answer: First-order with respect to A
Work through every step correctly and you earn all 3 marks.
Another worked example
A concentration-time graph of a reactant A shows an exponential decay. Determine the order of the reaction with respect to A.
- Identify the shape of the graph.1 markThe graph shows an exponential decay.
- Interpret the shape of the graph.1 markAn exponential decay indicates a first-order reaction.
- Confirm by plotting ln[A] vs. time.1 markPlotting ln[A] vs. time should give a straight line with a negative slope equal to -k.The reaction is first-order with respect to A.
Final answer: First-order with respect to A
Work through every step correctly and you earn all 3 marks.
Common mistakes
Misinterpreting a straight line on a concentration-time graph as first-order.
Why it happens: Students may confuse the shape of the graph for different orders. A zero-order reaction also shows a straight line, but with a constant rate.
Fix: 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.
Why it happens: Students may plot [A] vs. time instead of ln[A] vs. time, leading to incorrect conclusions about the order.
Fix: Always use natural logarithms (ln) when plotting concentration-time data for first-order reactions.
Incorrectly setting up the ratio for initial rate data.
Why it happens: Students may set up the ratio incorrectly, leading to incorrect orders. For example, they might divide rates by concentrations instead of using powers.
Fix: 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.
Why it happens: Students may confuse the shape of the graph for different orders. A zero-order reaction shows a horizontal line, while a first-order reaction shows a straight line passing through the origin.
Fix: 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.
Why it happens: Students may forget to convert concentrations to mol dm -3 and rates to mol dm -3 s -1 , leading to incorrect calculations.
Fix: 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.
Why it happens: Students may combine the orders incorrectly, leading to an incorrect rate equation. For example, they might add the orders instead of using them as exponents.
Fix: 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
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Determine Reaction Order | Compare rate and concentration data to find the order with respect to A. | 3 |
| Determine Reaction Order | Deduce the order with respect to A from the shape of a concentration-time graph. | 3 |
| Determine Reaction Order | Interpret a rate versus concentration graph to deduce the reaction order with respect to A. | 3 |
| Determine Reaction Order | Compare rate and concentration data to find the order with respect to reactant B. | 3 |
| Determine Reaction Order | Deduce the order with respect to A from a rate versus concentration graph. | 3 |
| Total across these question types | 15 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.