For a reaction A→B, enthalpy of reaction is −4.2 kJ mol₋₁ and enthalpy of activation is 9.6 kJ mol₋₁. The correct potential energy profile for the reaction is shown in option.

The correct answer is Option 1.

To determine the correct potential energy profile for the reaction $A\to B$, we analyze the given thermodynamic parameters:

1. Enthalpy of the Reaction ($\Delta H$):
The enthalpy change of the reaction is defined as the difference between the enthalpy of the products and the enthalpy of the reactants:
$\Delta H=H_{\text{products}}-H_{\text{reactants}}=H_B-H_A$
Given:
$\Delta H=-4.2{\text{kJ mol}}^{-1}$
Since $\Delta H$ is negative ($\Delta H<0$), the reaction is exothermic. This means that the potential energy of the products (B) is lower than the potential energy of the reactants (A). Specifically, the level of B must be situated 4.2 units below the level of A on the potential energy axis.

2. Activation Energy ($E_a$):
The activation energy is the minimum energy required to initiate the chemical reaction, which corresponds to the energy barrier separating the reactants and products:
$E_a=E_{\text{transition state}}-H_A$
Given:
$E_a=9.6{\text{kJ mol}}^{-1}$
This means the energy level of the transition state (the peak of the potential energy curve) is 9.6 units higher than the potential energy level of reactant A.

3. Characteristics of the Correct Profile:
- The potential energy curve starts at the level of reactant A.
- It rises to a maximum peak (transition state) where the vertical distance from the reactant level A to the peak is $9.6{\text{kJ mol}}^{-1}$.
- It then decreases to the level of product B, where the vertical distance from reactant level A down to product level B is $4.2{\text{kJ mol}}^{-1}$.
The diagram corresponding to these exact energy changes is represented in Option 1.