Q.Describe, in words, the potential energy profile for an exothermic reaction $A \rightarrow B$, and use it to define activation energy and the activated complex, and to show how the profile reveals that the reaction is exothermic.

[!TLDR] The profile rises from reactants, peaks at the activated complex ($E_a$ above reactants), then falls to products; products below reactants shows the reaction is exothermic. [!ANSWER] The potential energy rises from the reactants, over a maximum at the unstable activated complex (this rise is $E_a$), then falls to the products …

On a graph of potential energy (vertical axis) against reaction progress (horizontal axis), the reactants $A$ start at their own energy level on the left. As the reaction proceeds, potential energy rises to a maximum at an unstable, transient arrangement of atoms called the activated complex (or transition state), in which old bonds are partially broken and new bonds are partially formed; this cannot be isolated, existing only for an instant. The height of this peak above the reactants' energy level is the activation energy, $E_a$ -- the minimum extra energy reactant molecules must have to cross over into products. Beyond the peak, potential energy falls to the level of the products $B$, on the right. Because the diagram is for an exothermic reaction, the products' final energy level is drawn LOWER than the reactants' starting level; the vertical drop between them is the reaction's enthalpy change, $\Delta H$ (negative, since energy is released overall). The diagram also implicitly defines a reverse activation energy -- the height of the same …

Trace the profile reactants -> activated-complex peak -> products, labelling $E_a$ as the rise to the peak and $\Delta H$ …

Do not confuse the height of the peak above the reactants ($E_a$, always positive, needed even for an exothermic reaction) with the net drop from reactants to products …