Enthalpy changes in reversible reaction pathways Steps:

• Identify reactants and products energy levels from the diagram to find ΔH_forward = E_products - E_reactants.
• For reversible reaction, ΔH_backward = - ΔH_forward.
• Compare diagram values to statement enthalpy figures.
• Activation energies may appear but are irrelevant for ΔH.

Why B is correct:

• Backward ΔH equals the negative of forward ΔH, per Hess's law; diagram shows forward ΔH = -90 kJ mol⁻¹, so backward is +90 kJ mol⁻¹.

Why the others are wrong:

• A: Diagram indicates forward ΔH = -90 kJ mol⁻¹, not -30 kJ mol⁻¹.
• C: Forward reaction is exothermic (negative ΔH), not endothermic (+30 kJ mol⁻¹).
• D: Not enough information (option incomplete).

Final answer: B