The rate of a chemical reaction is the speed of that chemical reaction. The rate of a reaction can be affected by four main factors:
  • Concentration
  • Surface Area
  • Temperature
  • Presence of a Catalyst
For each of these factors, if you increase them, the speed of the reaction will increase as well. You saw this in the reaction rates lab, where you went from station to station and timed the different reactions that happened there.

Concentration

Concentration, as discussed here, is the amount of something dissolved in something else. According to collision theory, a reaction can only happen if two particles collide. If you have two particles you want to react and they are mixed with 500 billion water molecules, the chances that they will find each other and react is very small. That means that the reaction will take a while to react. But if you have a couple billion particles that you want to react, the chance that they will find each other among 500 billion water molecules is much greater, and that means that the reaction will happen very quickly. It's the same if you have one blue ball in a basket of 100 red balls versus one blue ball in a basket of 20 red balls. The first one might take a lot longer to find the blue ball in than the second one.

Surface Area

This factor is most important when you have solids involved. The surface area of a 3-dimensional object is like the perimeter of a 2-dimensional object. It is a measure of how much you have exposed to the outside. In a reaction, only the outside particles can react, so more surface area means a faster reaction. So a large cube of something should react faster than a small partcle, right? The cube has more surface area after all, but what's important isn't exactly surface area so much as the ratio of surface area to volume. A large cube has a bad ratio while a small particle has a good one. This means that the particle will react faster and be used up long before the cube is all used up. Another way to think about it is like this:
Imagine you have a cube of salt that is 100 x 100 x 100 atoms. That means there are a total of 1 million atoms in that cube and it has a surface area of about 600 atoms. That's a pretty bad ratio. Compare that single cube to a 10 x 10 x 10 cube. There are only 1 thousand atoms in that smaller cube, but if you have a thousand of those then you have 1 million atoms (1,000 x 1,000 = 1,000,000). Each small cube has a surface area of about 60 atoms. That's a better ratio for a single cube, but since you have a thousand cubes, you can add the surface areas of all the cubes up to get the total. That gives you 60 x 1,000 = 60,000, that's 60 thousand atoms that can react. That's a lot more than the 600 of the big cube, which means that if you were going to dissolve them, the smaller ones would react faster than the big one.

Temperature

Temperature is a number that tells you how hot or cold something is. It is also a number that tells you the average kinetic energy of the particles in something. According to collision theory, particles need to collide in order to react, and they need to have a certain amount of energy when they do that. If they have more energy, they will be more likely to react than if they have less energy. Temperature itself doesn't tell you how much energy is in a substance, but it tells you the average kinetic energy of the particles. If you increase that number, then more particles will have enough energy to react. You can increase the temperature by adding heat to the system. Or, if you want to slow down a reaction, you can take energy away from a system by cooling it down.

Catalyst

A catalyst is something that increases the reaction rate without being used up in the process. In a normal, uncatalyzed reaction, the only things that can effect the reactions are the reactants, and in some cases the products as well. When the reactants come together and react, though, they are used up and a chemical change takes place. A catalyst, however, helps the reactants along without being used up in the process. Because of this, a catalyst can participate in millions of reactions and more. A catalyst works by lowering the activation energy for a reaction. Remember the activation energy is the energy needed to get started. If that energy is lower than normal, reactions can happen much faster.