Thermodynamics tells us whether a reaction can occur: kinetics tells us when it will occur. The higher concentrations mean more frequent collisions; the higher temperature provides more energy per collision and also more collisions per second. However, not all reaction rates increase with increasing temperature and concentration.Once we have determined the effects of many variables on the rate, the next step is to explain the effects in terms of what the molecules are doing. The theoretical explanation is called a mechanism of the reaction. The mechanism is distinct from the stoichiometric reaction. The stoichiometry tells us the number of moles of reactants that produce the number of moles of products. For a chemical reaction, there is only one stoichiometric reaction: however, many mechanisms are consistent with the rate data. Each mechanism is a hypothesis to explain the kinetic data; the mechanism is a guide to design further experiments to better understand the results.