Analyzing Activation Energy and Enthalpy from a Reaction Energy Diagram

Q17. What is the activation energy in the graph to the right? What is the delta H of the reaction? Would the delta H be the same for the reverse reaction?

Background

Topic: Reaction Energy Diagrams (Potential Energy vs. Reaction Pathway)

This question tests your understanding of how to interpret a reaction energy diagram to determine activation energy, enthalpy change (ΔH), and how these values relate to the reverse reaction.

Key Terms and Formulas:

• Activation Energy (Ea): The minimum energy required for a reaction to occur, represented by the energy difference between the reactants and the peak of the curve.

• ΔH (Enthalpy Change): The difference in potential energy between the products and reactants. It indicates whether the reaction is exothermic (ΔH < 0) or endothermic (ΔH > 0).

• Reverse Reaction: The enthalpy change for the reverse reaction is equal in magnitude but opposite in sign to the forward reaction.

Step-by-Step Guidance

1. Identify the energy of the reactants (the starting point on the y-axis, PE in kJ).

2. Identify the energy of the products (the ending point on the y-axis, PE in kJ).

3. Find the peak of the curve (the highest point), which represents the transition state.

4. Calculate the activation energy ($E_a$) by subtracting the energy of the reactants from the energy at the peak: $E_a = \text{PE}_{\text{peak}} - \text{PE}_{\text{reactants}}$.

5. Calculate the enthalpy change ($\Delta H$) by subtracting the energy of the reactants from the energy of the products: $\Delta H = \text{PE}_{\text{products}} - \text{PE}_{\text{reactants}}$.

Try solving on your own before revealing the answer!

Final Answer:

Activation energy: 200 kJ

ΔH: +50 kJ (endothermic reaction)

Reverse reaction ΔH: -50 kJ (opposite sign)

The activation energy is the difference between the reactants and the peak, and ΔH is the difference between products and reactants. For the reverse reaction, ΔH is equal in magnitude but opposite in sign.