The rates at which reactants are consumed and products are formed during chemical reactions vary greatly. We can identify five factors that affect the rates of chemical reactions. These are: (i) chemical nature of the reacting substances (ii) surface area of the reactants (iii) temperature of the reactants (iv) concentration of the reactants, and (v) presence of a catalyst.

(i) Nature of the reactants

The rate of a reaction depends on the nature of the participating substances. Reactions that appear similar may have different rates under the same conditions, depending on the identity of the reactants. For example, when small pieces of the metals iron and sodium are exposed to air, the sodium reacts completely with air overnight, whereas the iron is barely affected. The active metals calcium and sodium both react with water to form hydrogen gas and a base. Yet calcium reacts at a moderate rate, whereas sodium reacts more vigorously.

(ii) Surface area of reactants

The area of contact between the phases determines the rate of reaction in heterogenous reaction. If a reaction involves a solid with a gas or liquid, the surface area of the solid affects the reaction rate. Because the reaction occurs at the surface of the solid, the rate increases with increasing surface area. When the size of particles is minimized, contact between the atoms, ions, or molecules in the solid state maximizes with those in a different phase.

(iii) Temperature of reactants

Temperature usually has a major effect on the rate of a reaction. Chemical reactions typically occur faster at higher temperatures. Food can spoil quickly when left on the kitchen counter. However, the lower temperature inside of a refrigerator slows that process so that the same food remains fresh for days. We use a burner or a hot plate in the laboratory to increase the speed of reactions that proceed slowly at ordinary temperatures. In many cases, an increase in temperature of only 10°C will approximately double the rate of a reaction in a homogeneous system.

(iv) Concentration of reactants

When the concentration of one or more reactants increases, rate of reaction increases. This is because increasing the concentration produces more contacts between the reacting particles, which results in increasing the rate of reaction. In the case of reactions that involve gaseous reactants, an increase in pressure increases the concentration of the gases which leads to an increase in the rate of reaction.
For example, in a polluted atmosphere where the concentration of sulfur dioxide is high, calcium carbonate deteriorates more rapidly than in less polluted air. Similarly, phosphorus burns much more rapidly in an atmosphere of pure oxygen than in air, which is only about 20% oxygen.

(v) Presence of a catalyst

Some chemical reactions proceed quite slowly. Sometimes their reaction rates can be increased dramatically by the presence of a catalyst. A catalyst is a substance that changes the rate of a chemical reaction without itself being permanently consumed. The action of a catalyst is called catalysis. The effect of catalysts on the rate of chemical reaction is illustrated.

Figure 5. Comparsion of activation energies of a catalysed and an uncatalysed reaction.

A catalyst provides an alternative energy pathway or reaction mechanism in which the potential-energy barrier between reactants and products is lowered.

Because the catalyst is not consumed by the reaction, it does not appear in the balanced chemical equation (although its presence may be indicated by writing its formula over the arrow). Catalysts may participate in one step along a reaction pathway and be regenerated in a later step.