Question

consider a hypothetical chemical reaction:

(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.

what is the heat of reaction?

is the reaction exothermic or endothermic?

can you determine the activation energy?

can you determine the activation energy of the reverse reaction?

Explanation:

Step1: Calculate heat of reaction

Heat of reaction \( \Delta H=H_{products}-H_{reactants}\). From the graph, \(H_{products}(C + D)\) is about \(200\space kJ/mol\) and \(H_{reactants}(A + B)\) is about \(100\space kJ/mol\). So \( \Delta H=200 - 100=100\space kJ/mol\).

Step2: Determine reaction type

Since \( \Delta H>0\) (products have higher energy than reactants), the reaction is endothermic.

Step3: Find activation energy

Activation energy is the energy difference between the reactants and the transition - state. From the graph, activation energy \(E_a\) (for \(A + B
ightarrow C + D\)): The energy of the transition - state is about \(400\space kJ/mol\) and \(H_{reactants}(A + B)\) is \(100\space kJ/mol\), so \(E_a = 400-100 = 300\space kJ/mol\).

Step4: Find reverse activation energy

For the reverse reaction \(C + D
ightarrow A + B\), activation energy \(E_{a(reverse)}\) is the energy difference between the products (\(C + D\)) and the transition - state. \(E_{a(reverse)}=400 - 200=200\space kJ/mol\)

Answer:

• What is the heat of reaction? \(100\space kJ/mol\)
• Is the reaction exothermic or endothermic? Endothermic
• Can you determine the activation energy? Yes, it's \(300\space kJ/mol\)
• Can you determine the activation energy of the reverse reaction \(C + D

ightarrow A + B\)? Yes, it's \(200\space kJ/mol\)