reaction coordinate diagram

A reaction coordinate diagram is the elevation profile of a hiking trail, but for a chemical reaction. The horizontal axis is the progress of the reaction from start to finish (the 'reaction coordinate'), and the vertical axis is energy. As you read left to right, you watch the energy of the molecules rise and fall as bonds break and form, just like watching a trail go uphill and down.

Reactants sit at a starting altitude on the left and products at a finishing altitude on the right. To get between them the molecules must climb over at least one hill; the top of each hill is a transition state, the highest, least stable point in a step. The height you must climb from the reactant valley to that peak is the activation energy, which sets how fast the step goes. If the path dips into a valley partway along, that valley is a reactive intermediate, a real but short-lived species that sits between two transition states. A multistep reaction therefore has several humps, one per elementary step.

These diagrams pack an enormous amount of insight into one picture. Whether the products end lower than the reactants tells you if the reaction is exothermic (releases energy, products downhill) or endothermic (absorbs energy, products uphill), which is about thermodynamics. The height of the biggest hill tells you about speed, which is kinetics. The tallest peak marks the rate-determining step. A catalyst works by carving a lower-altitude path over the mountains, never by changing the starting or ending elevations.