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

To determine the correct energy plot for the chemical reaction \(\text{A} \rightarrow \text{D}\) with the given mechanism, we need to analyze the energy changes in each step:

1. Step 1: \(\text{A} \rightarrow \text{B}\)
   • This step has a positive enthalpy change \((\Delta H = +\text{ve})\), indicating it is endothermic. 
     The potential energy of the system increases as the reaction progresses from A to B.
2. Step 2: \(\text{B} \rightarrow \text{C}\)
   • This step has a negative enthalpy change \((\Delta H = -\text{ve})\), indicating it is exothermic. 
     The potential energy decreases as the reaction progresses from B to C.
3. Step 3: \(\text{C} \rightarrow \text{D}\)
   • This step also has a negative enthalpy change \((\Delta H = -\text{ve})\), so it is exothermic. 
     The potential energy further decreases as the reaction progresses from C to D.

Considering these energy changes, the overall potential energy diagram would show an initial increase in energy (endothermic) followed by two decreases (exothermic).

Option 2 displays this pattern: an up-down-down sequence of energy changes. Therefore, the correct answer is option 2.