For a one step reaction, the activation energy for the forward reaction is 40.0 kJ mol-1, and the enthalpy of reaction is -20.0 kJ mol-1. Calculate the activation energy for the reverse reaction.

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The problem gives the forward activation energy Ea,f = 40.0 kJ/mol and the enthalpy change ΔH = -20.0 kJ/mol for a one-step reaction. To relate forward and reverse barriers, use the relation ΔH = Ea,f - Ea,r, which rearranges to Ea,r = Ea,f - ΔH.

Compute Ea,r: Ea,r = 40.0 - (-20.0) = 60.0 kJ/mol. Thus the reverse activation energy is positive and larger than the forward barrier by 20 kJ/mol.

Option a. +60.0 kJ mol-1: This matches the calculated Ea,r and is consistent with the formula above.

Option b. -20.0 kJ mol-1: Activation energies are barriers and are inherently positive; a negative value would be unphysical here.

Option c. -1200 kJ mol-1: An enormous negative value with three zeros is not compatible with the standard relationship between Ea,f, Ea,r, and ΔH for a simple reaction.

Option d. +20.0 kJ mol-1: This would correspond to Ea,r equaling the magnitude of ΔH but fails to account for the forward barrier; it ignores the subtraction needed in the formula Ea,r = Ea,f - ΔH.

Option e. +1200 kJ mol-1: This is an inflated, improbable value not supported by the given data or the fundamental relationship between forward and reverse activation energies.

CHM1051 MUM S1 2025 Week 12: Weekly quiz (1%)

For a one step reaction, the activation energy for the forward reaction is 40.0 kJ mol-1, and the enthalpy of reaction is -20.0 kJ mol-1. Calculate the activation energy for the reverse reaction.

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