Energy Diagrams

The vertical axis is potential energy; the horizontal axis is just reaction progress, not time. There's no clock here because the diagram isn't tracking how fast anything happens. It's tracking how the energy stored in bonds changes as reactants turn into products. Reactants start at one energy level, climb to a peak, then settle at the product energy level.

For an exothermic reaction, draw reactants on the left at some energy, a hump above them (the transition state), then a curve dropping down to products sitting in a well below the reactants. The vertical drop from reactants to products is delta H, and because products end up lower, delta H is negative: energy left the system, mostly as heat. Flip the reaction and you flip the diagram left to right, which flips the sign of delta H too, since reactants and products just swap places.

The height of the hump above the reactants is the activation energy, and it's a completely separate number from delta H. A reaction can have a huge, favorable drop in energy (very negative delta H) and still be slow, because the barrier up to the transition state is tall. Delta H tells you about stability, how far downhill the products are. Activation energy tells you about rate, how hard it is to get over the hump in the first place. This is exactly the handoff into kinetics: the same diagram, now read for its peak instead of its endpoints, becomes the reaction coordinate diagram used to talk about rate and catalysts.