Question

$A \rightarrow D$ is an endothermic reaction occurring in three elementary steps:
(i) $A \rightarrow B \quad \Delta H_i = +\text{ve}$ 
(ii) $B \rightarrow C \quad \Delta H_{ii} = -\text{ve}$ 
(iii) $C \rightarrow D \quad \Delta H_{iii} = -\text{ve}$ 
Which of the following graphs between potential energy (y-axis) versus reaction coordinate (x-axis) correctly represents the reaction profile of $A \rightarrow D$? 

• A.
• B.
• C.
• D.

Hint:

For multi-step reactions, the number of peaks equals the number of elementary steps, and the relative heights of reactants and products decide whether the reaction is endothermic or exothermic.

Answer 1

The Correct Option is B

Step 1: Understanding the nature of the overall reaction.
The reaction $A \rightarrow D$ is stated to be endothermic.
Therefore, the potential energy of product $D$ must be higher than that of reactant $A$.
Step 2: Analysing the individual steps.
Step (i): $A \rightarrow B$ with $\Delta H_i = +\text{ve}$
This means the energy level of $B$ is higher than that of $A$.
Step (ii): $B \rightarrow C$ with $\Delta H_{ii} = -\text{ve}$
This indicates that the energy level of $C$ is lower than that of $B$.
Step (iii): $C \rightarrow D$ with $\Delta H_{iii} = -\text{ve}$
This shows that the energy level of $D$ is lower than that of $C$.
Step 3: Drawing conclusions for the energy profile.
Since the reaction occurs in three elementary steps, the energy profile must show three peaks (three transition states).
The intermediate energy changes must follow the order:
\[ A \uparrow B \downarrow C \downarrow D \] with the final energy of $D$ still higher than $A$ because the reaction is endothermic.
Step 4: Matching with the given graphs.
Among the given options, Graph 2 correctly shows:
- Three energy maxima (three transition states)
- An initial rise in energy
- Subsequent decrease in intermediate steps
- Final energy of product higher than the reactant
Step 5: Final conclusion.
Hence, the correct reaction profile for the given endothermic multistep reaction is represented by Option (2).