Particle Size If you put a bundle of stick on a campfire, they burn quickly. A log, however, burns more slowly. Small pieces of wood have more surface area than a log, and surface area plays an important role in deter mining the rate of reaction.The same is true for other chemical reactions. The total surface area of a solid or liquid reactant affcts the reaction rate. The smaller the particle size, the larger is the surface area for a given mass of particles. An increase in surface area increases the amount of the reactant exposed for reaction, Which further increases the collision frequency and the reaction rate. One way to increasebthe surface area of solid reaction is to dissolve them. In a solution, particles are separated and more accessible to other reactants. You can also increase the surface area of a solid by grinding it into a fine powder. Small dustlike particles, however, can be dangerous when suspended in air. As coal miners know. large chunks of coal pose little danger, but coal dust mixed with air is an explosive hazard because of its large surface area. The same is ture of dust in a flour mill or a grain elevator as the dramatic photograph in Figure 18.7 shows. Catalysts Increasing the temperature is not always the best way to increase the rate of a reaction. A catalyst is often better. Recall that a catalyst is a substance that increases the rate of a reaction with out being used up during the reaction. Catalysts permit reactions to proceed along a lower energy path, as Figure 18.8 shows.Notice that the activation energy barrier for the catalyzed reaction is lower than that of the uncatalyed reaction. With a lower activation-energy barrier, more reaction have the energy to form products within a given time. For instance, the rate of the combination reaction of hydrogen and oxygen at room temperature is negligible, bet with a trace of finely divided platinum(Pt) as a catalyst, the reaction is rapid.
Because catalyst is not consumed during reaction, it does not appear as a reactant or product in the chemical equation. Instead, the catalyst is often written above the yield arrow, as in the equation above.