A reaction has the following potential energy diagram - Scottish Highers Chemistry - Question 19 - 2015

To determine the activation energy for the forward reaction from the potential energy diagram, we need to calculate the difference in potential energy between the reactants (X) and the transition state (Y).

The activation energy (E_a) is given by:

$E_a = E_Y - E_X$

Based on the options provided, the correct choice corresponds to:

A: X - Y.