Question

For a chemical reaction : $\text{A} \rightarrow \text{D}$. Mechanism is Step-1: $\text{A} \rightarrow \text{B} : \Delta H = +\text{ve}$. Step-2: $\text{B} \rightarrow \text{C} : \Delta H = -\text{ve}$. Step-3: $\text{C} \rightarrow \text{D} : \Delta H = -\text{ve}$. Select the correct energy plot

• A. 1
• B. 2
• C. 3
• D. 4

Hint:

An energy profile diagram must display a number of peaks equal to the number of elementary steps in the reaction mechanism. $\Delta H$ for each step dictates the relative energy levels of consecutive intermediates.

Answer 1

The Correct Option is B

The reaction has 3 steps, thus the plot must show 3 transition states (peaks).
Step 1 ($\text{A} \rightarrow \text{B}$): $\Delta H = +\text{ve}$. Energy of $\text{B}$ is higher than $\text{A}$ ($E_B>E_A$).
Step 2 ($\text{B} \rightarrow \text{C}$): $\Delta H = -\text{ve}$. Energy of $\text{C}$ is lower than $\text{B}$ ($E_C<E_B$).
Step 3 ($\text{C} \rightarrow \text{D}$): $\Delta H = -\text{ve}$. Energy of $\text{D}$ is lower than $\text{C}$ ($E_D<E_C$).
Plot (2) shows three peaks and satisfies the energy relationships $E_B>E_A$, $E_C<E_B$, and $E_D<E_C$.