Examkin

Question

Difficulty: MediumActivation Energy and Energy Profile Diagrams

For the endothermic synthesis reaction X2(g)+Y2(g)→2XY(g)\text{X}_2(g) + \text{Y}_2(g) \rightarrow 2\text{XY}(g), the potential energy of the reactants is +40 kJ mol−1+40\text{ kJ mol}^{-1} and the potential energy of the products is +80 kJ mol−1+80\text{ kJ mol}^{-1}. If the activation energy for the forward reaction is 60 kJ mol−160\text{ kJ mol}^{-1}, what is the activation energy for the reverse reaction?

  1. 20 kJ mol−120\text{ kJ mol}^{-1}Answer
  2. B
    40 kJ mol−140\text{ kJ mol}^{-1}
  3. C
    100 kJ mol−1100\text{ kJ mol}^{-1}
  4. D
    140 kJ mol−1140\text{ kJ mol}^{-1}

Answer

20 kJ mol−120\text{ kJ mol}^{-1}
The peak energy of the transition state is obtained by adding the forward activation energy (60 kJ mol−160\text{ kJ mol}^{-1}) to the reactant energy (40 kJ mol−140\text{ kJ mol}^{-1}), giving 100 kJ mol−1100\text{ kJ mol}^{-1}. Subtracting the product energy level (80 kJ mol−180\text{ kJ mol}^{-1}) from this peak value yields an activation energy for the reverse reaction of 20 kJ mol−120\text{ kJ mol}^{-1}.

Step-by-Step Solution

1
Calculate the potential energy of the transition state (activated complex).
Energy of transition state=Energy of reactants+Ea,forward=40 kJ mol−1+60 kJ mol−1=100 kJ mol−1\text{Energy of transition state} = \text{Energy of reactants} + E_{a,\text{forward}} = 40\text{ kJ mol}^{-1} + 60\text{ kJ mol}^{-1} = 100\text{ kJ mol}^{-1}.
The forward activation energy is the energy barrier that reactants must overcome to reach the peak transition state.
2
Calculate the activation energy for the reverse reaction.
Ea,reverse=Energy of transition state−Energy of products=100 kJ mol−1−80 kJ mol−1=20 kJ mol−1E_{a,\text{reverse}} = \text{Energy of transition state} - \text{Energy of products} = 100\text{ kJ mol}^{-1} - 80\text{ kJ mol}^{-1} = 20\text{ kJ mol}^{-1}.
The reverse activation energy is the energy difference between the product level and the activated complex peak.

Key Concept

Activation energy relationship in energy profile diagrams
Estimated Time:1m 0s
Rate this question

Topics

Activation Energy and Energy Profile Diagrams