There are 5 important factors which affect rates of reactions:
1.    Nature of the Material
2.    Reactant Concentration
3.    Temperature
4.    Action of Catalysts
5.    Surface Area

Nature of the Material includes the chemical composition of the substance and the physical state in which it exists. For example white phosphorous ignites spontaneously in air while wood requires an ignition source and water will not burn. Generally reaction occurs fastest in the gas phase, the second fastest in the liquid phase, and much more slowly in the solid phase.

Reactions in general go more quickly at higher concentrations. For example flammable materials burn much more quickly at high oxygen concentrations. Since air is 78% nitrogen and 21% oxygen by volume, nitrogen serves to lower the oxygen concentration and reduce the rate of combustion.

As a general rule the rate of the reaction increases as the temperature increases. For example food spoils when not refrigerated. As a general rule the rate of a chemical reaction doubles for every rise of 18^oF (10^oC).

Reactions also proceed more quickly in the presence of a catalyst. Catalysts speed up a reaction without being used up in the reaction. This is usually done by lowering the activation energy which is the energy required to initiate a chemical reaction. The combustion of hydrogen and oxygen will not occur without a spark unless a piece of platinum is added and then they explode violently.

Figure 5 Adding manganese dioxide to a 3% solution of hydrogen peroxide releases oxygen gas. Placing a burning wood splint into the oxygen rich atmosphere drastically increases its combustion rate.

Generally reactions occur more quickly with a greater surface area. For example a flammable liquid will burn more quickly in a container that will allow it to assume the largest surface area.

Figure 6 Lycompodium powder burns at a much faster rate if rendered as a powder in the air.