Question

consider a hypothetical chemical reaction:

$a + b \to c + d$

(in this equation $a$, $b$, $c$ and $d$ stand for some unknown chemical formulas.)

here is an energy diagram for the reaction:

use the energy diagram to answer these questions.

Explanation:

Step1: Calculate the heat of reaction

The heat of reaction ($\Delta H$) is calculated as the energy of products minus the energy of reactants.
From the diagram, energy of reactants ($A + B$) is $200\ \text{kJ/mol}$ and energy of products ($C + D$) is $100\ \text{kJ/mol}$.
So, $\Delta H=100 - 200=- 100\ \text{kJ/mol}$.

Step2: Determine the type of reaction

Since $\Delta H=-100\ \text{kJ/mol}<0$, the reaction is exothermic (releases heat).

Step3: Determine the activation energy

The activation energy is the energy difference between the reactants and the transition - state (the peak of the energy curve).
From the diagram, the energy of reactants ($A + B$) is $200\ \text{kJ/mol}$ and the peak energy is $400\ \text{kJ/mol}$.
So, activation energy $E_a = 400 - 200=200\ \text{kJ/mol}$.

Step4: Determine the activation energy of the reverse reaction

The activation energy of the reverse reaction ($C + D
ightarrow A + B$) is the energy difference between the products ($C + D$) and the transition - state.
The energy of products ($C + D$) is $100\ \text{kJ/mol}$ and the peak energy is $400\ \text{kJ/mol}$.
So, $E_{a(\text{reverse})}=400 - 100 = 300\ \text{kJ/mol}$.

Answer:

• What is the heat of reaction? \(-100\ \text{kJ/mol}\)
• Is the reaction exothermic or endothermic? Exothermic
• Can you determine the activation energy? Yes, it's \(200\ \text{kJ/mol}\)
• Can you determine the activation energy of the reverse reaction \(C + D

ightarrow A + B\)? Yes, it's \(300\ \text{kJ/mol}\)