This Demonstration considers the simple exchange reaction in which the fragments (possible single atoms) , , and are shown on the superimposed graphic in green, blue, and red, respectively. The progress of the reaction can be pictured as the reactants and "climbing a hill" of height and sliding down to the lower energy state of the products and . A parameter describes the extent of reaction as it varies between 0 and 1. We consider exothermal reactions in the moderate range of between -50 and -100 kJ/mol. Many other reaction enthalpy changes are as large as -1000 kJ/mol.

Many reactions can be made to run many times faster by use of an appropriate catalyst. A catalyst, represented schematically on the graphic as a black rectangle, takes part in the mechanism of the reaction but is regenerated after the products are produced. A catalytic pathway is accompanied by a much-reduced activation energy , shown by a red curve on the energy plot. Catalysis can involve complex reactions on a molecular level, with possible intermediate species of the general form , , and . Ultimately the last species dissociates to produce the final products , with the regenerated catalyst now able to take part in repetitions of the reaction.

Almost all biological processes depend on the action of enzymes, which are catalysts usually composed of protein structures. Enzymes specific to a given biochemical reaction are able to speed up reaction rates by factors of several hundred thousand.

Moving the slider for the extent of reaction between and gives an idealized picture of the progress of the reaction.