Factors Affecting Reaction Rate

The rate of a chemical reaction as expressed in the applicable rate law, as we have seen, involves
both a rate constant and, except for zero order reactions, the concentration of reactants. The rate of
a reaction, then, could be increased by either increasing the concentration of the reactants (which
increases the number of effective collisions between the reactant molecules) or by altering the value
of the rate constant. We have pointed out above that the rate constant ultimately depends upon the
energy difference between the reactants and the transition state. The smaller this activation energy
is (the smaller the gap between the two energy levels), the larger the rate constant, and the faster
the reaction will proceed. Two factors that most commonly affect the rate of a reaction are
temperature and the presence of a catalyst.

For nearly all reactions, the reaction rate will increase as the temperature of the system increases.
Since the temperature of a substance is generally a measure of the particles’ average kinetic energy,
increasing the temperature increases the average kinetic energy of the molecules. Consequently,
the proportion of molecules having energies greater than Ea (thus capable of undergoing reaction)
increases with higher temperature. Again, this is valid even for reactions that are exothermic
because the activated complex is at a higher potential energy than the reactants. Raising the
temperature of a system in which an exothermic reaction is occurring would shift the equilibrium in
favor of the reactants (from Le Châtelier’s principle), but the system would attain this equilibrium
faster.

A CLOSER LOOK

The rate    for most    reactions   approximately   doubles for every   10°C    increase    in  the

temperature.

Catalysts are substances that increase the reaction rate without themselves being consumed; they
do this by lowering the activation energy. Catalysts are important in biological systems and in
industrial chemistry; enzymes are biological catalysts. Catalysts accomplish this lowering of