What do the peaks on the graphs in figure 7-3 represent

The given graphs are Reaction Coordinate Diagrams which depict the conversion of reactants into products.

Diagram 1: Represents an exothermic reaction. These reactions proceed with the release of heat energy as a result the energy of the products is lower than the reactants.

Diagram 2: Represents an endothermic reaction. In contrast, these reactions proceed with the absorption of heat energy as a result the energy of the products is higher than the reactants.

In both diagrams, there is a peak or a hump which corresponds to the activation energy for the reaction. This is the minimum amount of energy required by the reactants as they collide to form the products.

a:

by the combination of an unlimited number of elements