Temperature

Suppose you are frying an egg in a frying pan. What happens if you increase the heat under the pan? The hotter the pan, the faster the egg will cook. Generally, an increase in temperature will increase the reaction rate, while a decrease in temperature will decrease the reaction rate. For instance, you store milk in a refrigerator to slow down the reactions that cause the milk to spoil. These reactions don't stop completely. Even milk stored in a refrigerator will eventually spoil. But the rate of spoiling decreases if the milk is kept cold.

Increasing the temperature of a substance causes its particles to move faster, on average. Particles that move faster are both more likely to collide and more likely to react. If the number of collisions that produce reactions increases, then the reaction rate increases.

Surface Area

Grain may not strike you as a dangerous material, but it can be explosive under the right conditions. The cause of the fire in Figure 21 was a combustion reaction between grain dust (suspended in the air) and oxygen. The rate of combustion was very rapid due to the small particle size of the grain dust.

The smaller the particle size of a given mass, the larger is its surface area. Imagine using a newspaper to cover the floor of a room. If you keep all the sections folded together, you can only cover a small area. However, if you separate the newspaper into pages and lay them out like tiles, you can cover a much larger area with the same mass of paper.

An increase in surface area increases the exposure of reactants to one another. The greater this exposure, the more collisions there are that involve reacting particles. With more collisions, more particles will react. This is why increasing the surface area of a reactant tends to increase the reaction rate.

Figure 21 This grain elevator in Potlatch, Idaho, exploded when grain dust reacted with oxygen in the air.

Applying Concepts How does surface area affect reaction rates?