WJEC A-Level Chemistry Unit 3, June 2025: Question 1

✅ Correct Answer

Complete the right-hand side of the formula as:

Mark scheme: [1 mark] for Ae^−Ea/RT . The negative sign and correct positioning of all symbols are essential.

💡 Key Knowledge

Every term in the equation has a specific physical meaning and unit:

• k = Rate constant (units depend on overall order)
• A = Arrhenius pre-exponential factor (frequency factor; same units as k)
• e = Base of natural logarithms (mathematical constant ≈ 2.718)
• E_a = Activation energy (J mol⁻¹)
• R = Gas constant (8.314 J K⁻¹ mol⁻¹)
• T = Absolute temperature (Kelvin, K)

🧠 Exam Technique & Examiner Insight

• Recognise the prompt: The question begins with k = , indicating it requires the exponential form, not the logarithmic form ( ln k = ln A − E_a/RT ).
• Superscript clarity: Ensure the power −E_a/RT is clearly written as an exponent of e , not on the same baseline.
• Subscript clarity: Activation energy must have a lowercase subscript 'a' ( E_a ).

❌ Common Errors & Pitfalls

• Missing the negative sign: Writing Ae^+Ea/RT is incorrect and loses the mark. Rate constant k decreases as activation energy increases, which requires the minus sign.
• Inverting the exponent: Writing RT / Ea instead of Ea / RT .
• Writing the ln form: Providing ln A − Ea/RT next to k = invalidates the equation.
• Capitalisation errors: Writing a instead of A for the pre-exponential factor, or t (time/Celsius) instead of T (Kelvin).