Edexcel A-Level Chemistry Paper 2, November 2020: Question 9

✅ Correct Answer

Fraction / proportion / number of molecules (or particles) with energy, E

❌ Common Errors

Writing just "number of molecules" without referencing energy or probability. The vertical axis represents a distribution relative to energy, not a static count.

✅ Correct Answer

• Peak for T₂ shifted to the left (lower modal energy) compared to T₁ .
• Peak for T₂ must be higher than T₁ (maintaining area under the curve).
• Asymptotic tail on the right must be lower than T₁ and MUST NOT touch or cross the x-axis.

🧠 Exam Technique

When drawing a lower temperature curve, remember: taller peak, shifted left, lower right-hand tail. Examiners strictly penalize curves that cross the original curve at unusual angles or touch the axis at the high-energy end.

💡 Key Knowledge

• At a lower temperature, molecules have lower kinetic energy / move more slowly.
• Fewer molecules have energy greater than or equal to the activation energy ( E ≥ Eₐ ).
• Consequently, fewer successful/effective collisions occur per unit time.

❌ Common Errors

Students often lose marks by simply stating "there are fewer collisions overall". Examiners require specific reference to the activation energy and the proportion of molecules possessing sufficient energy to react.

✅ Correct Answer

Label Eₙₜ marked clearly to the left of the original Eₐ , specifically positioned underneath the falling curve (between the highest point of the peak and the original Eₐ ).

🧠 Exam Technique

Ensure your label is clearly placed on the energy axis. A catalyst provides an alternative reaction pathway with a lower activation energy, shifting the threshold to the left.

📐 Step-by-Step Calculation

Step 1: Substitute values into the provided Arrhenius expression

ln(4.87 × 10⁻³ / 1.50 × 10⁻³) = (-Eₐ / 8.31) × ((1 / 338) - (1 / 328))

Step 2: Evaluate both sides

ln(3.2467) = 1.1776

(1 / 338) - (1 / 328) = -9.0201 × 10⁻⁵ K⁻¹

Step 3: Rearrange to isolate Eₐ

Eₐ = (1.1776 × 8.31) / (9.0201 × 10⁻⁵)

Step 4: Calculate final value

Eₐ = +108 493 J mol⁻¹

Step 5: Apply significant figures and units

108 000 J mol⁻¹ OR 108 kJ mol⁻¹ (to 3 SF) or 110 kJ mol⁻¹ (to 2 SF)

❌ Common Calculation Traps

• Sign Errors: Forgetting that subtracting inverted temperatures (1/T₂ - 1/T₁) yields a negative value, which cancels out the negative sign in the Arrhenius formula -Eₐ/R . Never report a negative activation energy!
• Unit Omission: Failing to include J mol⁻¹ or kJ mol⁻¹ loses the final mark.
• Significant Figures: Data is provided to 3 SF, so answers should be given to 2 or 3 significant figures.