StellarA reaction energy profile is a graph that shows how energy changes during a reaction. The y-axis represents energy, and the x-axis represents reaction progress. The high point on the graph is the transition state, and the energy difference between the reactants and that high point is the activation energy. This is the minimum energy needed for the reaction to occur.
The graph can also show whether the reaction is overall exothermic or endothermic. If the products are lower in energy than the reactants, the reaction releases energy and is exothermic. If the products are higher in energy, the reaction absorbs energy and is endothermic. The activation energy matters for rate because a larger activation barrier means fewer particles can successfully react at a given temperature.
Example 1: If the products lie below the reactants on an energy profile, the reaction is exothermic.
Example 2: If a reaction has a very large activation energy hump, the reaction will usually be slower because fewer particles can overcome that barrier.