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Difficulty: MediumActivation Energy and Energy Profile Diagrams

In an energy profile diagram for a reversible chemical reaction, the potential energy of the reactants is 85 kJ mol−185\text{ kJ mol}^{-1}, the potential energy of the products is 150 kJ mol−1150\text{ kJ mol}^{-1}, and the peak potential energy of the activated complex is 235 kJ mol−1235\text{ kJ mol}^{-1}. What is the activation energy of the reverse reaction in kJ mol−1\text{kJ mol}^{-1}?

Answer: 85 kJ mol^{-1}

Answer

The activation energy of the reverse reaction is 85 kJ mol−185\text{ kJ mol}^{-1}.
The activation energy for the reverse reaction is the difference in potential energy between the activated complex at the peak and the products. Subtracting the potential energy of the products (150 kJ mol−1150\text{ kJ mol}^{-1}) from the peak energy (235 kJ mol−1235\text{ kJ mol}^{-1}) gives 85 kJ mol−185\text{ kJ mol}^{-1}.

Step-by-Step Solution

1
Identify key energy values from the reaction energy profile
Energy of activated complex = 235 kJ mol−1235\text{ kJ mol}^{-1}, Energy of products = 150 kJ mol−1150\text{ kJ mol}^{-1}.
The activation energy of the reverse reaction is defined as the energy required for products to reach the transition state (activated complex).
2
Calculate the reverse activation energy (Ea,reverseE_{a,\text{reverse}})
Ea,reverse=235 kJ mol−1−150 kJ mol−1=85 kJ mol−1E_{a,\text{reverse}} = 235\text{ kJ mol}^{-1} - 150\text{ kJ mol}^{-1} = 85\text{ kJ mol}^{-1}.
Subtracting the potential energy of the products from the peak potential energy yields the minimum energy barrier for the backward process.

Key Concept

Calculating reverse activation energy from potential energy levels
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Topics

Activation Energy and Energy Profile Diagrams