Finally, the fourth thing that can affect our rate, which can either make it fast or slow, we're going to say a catalyst. A catalyst can increase the rate of the reaction by decreasing the energy of activation. Now here we're going to have is our reactant line, and over here we're going to have our product line, and basically we say that this is called an energy diagram. Basically, our reactants are starting off here at around energy of 50 kilojoules. The reactants then have to travel up here to the top at this very point up here. This is called our transition state. And just realize a transition state is a hybrid. The transition state looks a little bit like the product, a little bit like the reactant. Now, when you're at the transition state, you have two possibilities. You could either slide back down to become a reactant again, or if you have enough energy you can tip over and slide down to become a product. We're going to say the distance from the top of the hill all the way down to the bottom of the hill where the reactant line is, that is our energy of activation or Ea, the amount of energy it takes to climb up to the top. And we're going to say the way a catalyst works is the catalyst changes the reaction pathway. It actually makes the hill shorter. You still end with the same energy for your products, but you don't have to travel up as high to get to the transition state. So it lowers the energy of the transition state, so you get to the top of the hill faster, and that way you can slide down the hill faster to become a product. That's how a catalyst will work. So, just remember the four factors that influence the rate of a reaction. If you can manipulate these guys in a certain way, you can make your reaction faster, so increasing concentration, increasing surface area, increasing temperature, or adding a catalyst, all will make your rate faster by doing each of these different factors. So just remember the four factors involved in speeding up a chemical reaction.