AQA A-Level Chemistry Paper 3, 2024: Question 14

Difficulty: Medium

Maxwell–Boltzmann Curve: Meaning of Total Area

AQA A-Level Chemistry • Kinetics • Multiple choice (1 mark)

🧪 MB distributions

What this question tests

Core idea

Interpreting axes on a Maxwell–Boltzmann distribution: y = number of molecules at a given energy.
Knowing what area under the curve represents in this context.
Avoiding the common confusion between number of molecules and energy.

Part (a) Multiple choice (1 mark)

What is represented by the total area under the curve?

✅ Correct answer (from mark scheme)

C — The total number of molecules in the reaction mixture.

Mark breakdown: 1 mark (AO1) for selecting C.

💡 Key knowledge: Why area = total number of molecules

The y-axis is “number of molecules with a given energy”.
Adding up “number of molecules” across all energies is done by the area under the curve.
Therefore the total area corresponds to the total number of molecules in the sample (constant for a fixed amount of gas).

🧠 Exam technique: Fast elimination

If you see “area under curve” on a distribution where the y-axis is a “number/frequency”, area usually means total amount.
Options mentioning energy (A or B) are traps: energy is on the x-axis, but the graph is not “energy vs energy”.
“Reacting molecules” (B or D) is also a trap: the graph shown is a general distribution, not specifically molecules above Eₐ.

❌ Common errors (and why they lose the mark)

Choosing A: thinking “more area = more energy”. Area is summing numbers of molecules, not summing energies.
Choosing D: confusing “area under entire curve” with “area to the right of Eₐ”. Only the area beyond Eₐ would represent molecules able to react.
Choosing B: mixing both misconceptions: “reacting” and “energy”.

Examiner insight: This 1-mark MCQ often distinguishes students who correctly read axes from those who rely on memorised phrases (“area = energy”).

Link to wider Maxwell–Boltzmann ideas (useful for longer questions)

Increasing temperature changes the shape (peak lower and shifted right) but the total area stays the same (same number of molecules).
The fraction of molecules with energy ≥ Eₐ is the area under the curve to the right of Eₐ (not asked here, but a very common extension).

Mini-check (to secure the mark next time)

🧠 10-second checklist

Read the y-axis: is it number/frequency?
If yes, then area = total number.
Only a selected region (e.g., beyond Eₐ) would represent “reacting molecules”.

💡 Key phrase to remember

Total area under MB curve = total number of molecules