Temperature is a direct measure of the kinetic energy of molecules; temperature also is a measure of the number of reactant molecules with enough energy to react. All reactions take place faster at higher temperature.

Side reactions may dominate the desired reaction, however. Not all reactions increase at the same rate as the temperature increases. For instance at a lower temperature, the reactants may produce 80% A and 20% B, while at a higher temperature they produce 40% A and 60% B. Rates of production of A may slow, but rates of reaction of the reactants increases. The reactants are used up more quickly in these examples.

Catalysts are a special problem because the catalysts may become inactive due to side reactions that are not present at lower temperatures. Biological enzymes frequently react with the surrounding solution to change geometry at higher temperatures. These enzyme molecules may then be inactive as catalysts. Some inorganic catalysts are "poisoned" at higher temperatures by reacting to form stable compounds that are not catalysts.

Temperature effects are treated more quantitively in the subsequent sections using the activation energy and the Arrhenius equation.

Temperature Effects