WJEC A-Level Chemistry AS Unit 2, June 2025: Question 3

Maxwell-Boltzmann Distributions & Catalysis

📌 What This Question Tests

Effect of Temperature on Energy Distributions: Sketching a Maxwell-Boltzmann distribution when temperature decreases.
Kinetic Principles: Recognizing how peak position (most probable energy) and peak height change with temperature while keeping the total area constant.
Mode of Action of Catalysts: Explaining how catalysts increase the rate of reaction using precise chemical vocabulary.

Question 3 (a) • 1 Mark

Drawing the Boltzmann Distribution Curve at a Lower Temperature

Draw the curve for the same reaction at 288 K (original curve is at 298 K).

✅ Mark Scheme Criteria

A curve of similar overall shape with:

A higher peak than the original 298 K curve, and
A peak positioned to the left of the original curve's peak.

🧠 Diagram Execution Guide

When drawing this curve on the exam paper:

Origin: Start exactly at (0,0) .
Initial Ascent: Rise steeper than the original line so the maximum is shifted to the left and higher up.
Intersection: Cross the original 298 K curve only once, on the downward slope.
Tail: The tail must stay below the 298 K curve and asymptote toward the energy axis (never touching or curling upwards).

💡 Key Knowledge: Why Does the Curve Shift This Way?

288 K is cooler than 298 K (a drop of 10 K):

Molecules have less average kinetic energy, shifting the most probable energy (the peak) to the left.
The total area under the curve represents the total number of particles, which has not changed.
Because the distribution narrows horizontally toward the left, the peak must become taller to maintain the same total area.

❌ Common Student Traps

Misreading the temperature: Assuming the new temperature is higher (e.g. sketching a lower, right-shifted peak) because textbooks usually show temperature increases. Always check whether T₂ > T₁ or T₂ < T₁!
Touching the horizontal axis: The distribution tail should approach the x-axis asymptotically; letting it touch or bounce off the axis loses marks in strict schemes.
Not starting at the origin: Starting above zero on the y-axis implies particles can have zero kinetic energy, which is impossible at temperatures above 0 K.

Examiner Insight: This is an "all-or-nothing" 1-mark question. Both conditions (peak higher and peak to the left) must be clearly visible to secure the mark.

Question 3 (b) • 1 Mark

Mechanism of Catalyst Action

State how a catalyst increases the rate of a reaction.

✅ Mark Scheme Answer

Provides an alternative reaction pathway (or route) with a lower activation energy.

💡 Key Knowledge

A catalyst increases reaction rate without being chemically changed or consumed at the end of the reaction.

It does not change the energies of the reacting molecules.
Instead, it opens a different mechanistic route where the threshold energy barrier (E_a) is lower.
Consequently, a significantly greater proportion of particles possess energy equal to or exceeding the activation energy (E ≥ E_a), leading to more frequent successful collisions.

🧠 Exam Technique: Two Essential Phrases

To guarantee the mark on WJEC papers, always include both parts of the standard definition:

"Alternative pathway / route"
"Lower activation energy"

❌ Common Errors

Incomplete statement: Writing only "it lowers the activation energy" without mentioning that it does so by providing an alternative route.
Misconception: Stating that a catalyst "gives particles more energy" or "increases the kinetic energy of particles". A catalyst has zero effect on particle kinetic energies!

Mark Allocation: [1 Mark] awarded for clearly expressing that an alternative reaction pathway/route with lower activation energy is provided.