QUESTION 8 [14 marks]

Question

Consider the following potential energy diagram (shown in Figure 2 below)
which shows the change in potential energy for a reversible reaction.
Figure 2: Potential energy diagram for A + B ⇌ C + D
Question 8.1 [2 marks]
Define activation energy.
QUESTION 8.2
Which of the letters (from E1
, E2 and E3
) on the graph represent the following:
Question 8.2.1 [1 mark]
The energy state of the activated complex.
Question 8.2.2 [1 mark]
The energy of the reactants.
Question 8.2.3 [1 mark]
The energy of the products.
Question 8.2.4 [2 marks]
Use the symbols (from E1
, E2 and E3
) to write an equation to describe how you
would calculate the activation energy of the reverse reaction.
Question 8.2.5 [2 marks]
Use the symbols (from E1
, E2 and E3
) to write an equation to describe how you
would calculate the enthalpy of the forward reaction.
Question 8.3 [2 marks]
Is the forward reaction an endothermic or exothermic reaction? Explain your
answer.
QUESTION 8.4 [3 marks]
Draw a sketch graph of the reverse reaction. Include all the labels:● A+B, C+D
● Axes labels.
● E1
,E2 and E3

Answer 1

Sure! Let’s break down each part of the question step by step.

QUESTION 8.1 [2 marks]

Define activation energy. Activation energy is defined as the minimum amount of energy required to initiate a chemical reaction. It is the energy difference between the reactants and the transition state (activated complex) of the reaction, which must be overcome for the reaction to proceed.

QUESTION 8.2

Let’s assign the energies for each letter based on the usual potential energy diagram:

• E1: Represents the energy state of the reactants (A + B)
• E2: Represents the energy state of the activated complex (transition state)
• E3: Represents the energy state of the products (C + D)

Question 8.2.1 [1 mark]

The energy state of the activated complex is represented by E2.

Question 8.2.2 [1 mark]

The energy of the reactants is represented by E1.

Question 8.2.3 [1 mark]

The energy of the products is represented by E3.

Question 8.2.4 [2 marks]

To calculate the activation energy of the reverse reaction (C + D → A + B), the equation would be: \[ \text{Activation Energy (reverse)} = E2 - E3 \]

Question 8.2.5 [2 marks]

To calculate the enthalpy of the forward reaction (A + B → C + D), the equation would be: \[ \text{Enthalpy (forward)} = E3 - E1 \]

QUESTION 8.3 [2 marks]

Is the forward reaction an endothermic or exothermic reaction? Explain your answer. The forward reaction is an endothermic reaction. This conclusion can be drawn from the potential energy diagram: if the energy of the products (E3) is higher than the energy of the reactants (E1), it indicates that energy is absorbed during the reaction, which is characteristic of an endothermic process.

QUESTION 8.4 [3 marks]

Sketch of the reverse reaction: Here’s a simple text representation of what you would draw. You would want to visualize this graph with proper axes and labels in a physical or digital drawing format.

Potential Energy
      |        Transition State
      |          .
      |         / \      E2
      |        /   \
      |-------/     \------|-------- E1 (A + B)
      |      /       \
      |     /         \
      |    /           \
      |---/_____________\-----|------- E3 (C + D)
      |
      |
      +-----------------------------------
                          Reaction Progress

• Label the Y-axis as "Potential Energy".
• Label the X-axis as "Reaction Progress".
• Clearly label the points for A + B at E1, C + D at E3, and the transition state at E2.

This sketch is a basic representation and can be improved visually with relative heights and precise slopes as per the original diagram you are referencing.