A Levels Chemistry (9701)•9701/13/M/J/19

Explanation
Energy Diagram Analysis for Reaction Kinetics
Steps:
- Identify ΔH_forward as energy difference between reactants and products; negative if exothermic (products lower).
- Determine Ea_forward as energy barrier from reactants to transition state; always positive.
- Determine Ea_backward as energy barrier from products to transition state; always positive.
- Compare signs: ΔH_forward negative, Ea_backward positive, confirming opposite signs.
Why C is correct:
- Activation energy is always positive by definition, while ΔH_forward is negative for exothermic reactions (ΔH = H_products - H_reactants < 0), yielding opposite signs per standard thermodynamics.
Why the others are wrong:
- A: Ea_forward (positive) and ΔH_backward (positive, as reverse of exothermic) have same sign, not opposite.
- B: No diagram scale provided to compare magnitudes; not enough information.
- D: ΔH_forward magnitude cannot exceed Ea_backward (always positive and larger barrier); violates energy conservation.
Final answer: C
Topic: Reaction kinetics
Practice more A Levels Chemistry (9701) questions on mMCQ.me