 Lastly, we've got catalysts. As we mentioned before, a catalyst is a substance that makes a reaction go faster without participating in the reaction itself, or rather without being consumed by the reaction. Essentially, the catalyst that you put in at the beginning of a reaction can be recovered at the end of the reaction unchanged. Many catalysts, particularly biological ones like enzymes, are highly selective in their reaction and only accelerate one of a number of different reactions that could occur. An everyday example of a catalyst that you nevertheless may not be aware of is the catalytic converter in a car. This is a device that takes the products of incomplete combustion, like carbon monoxide, and speeds up their conversion to less toxic compounds like carbon dioxide. The reaction that turns carbon monoxide into carbon dioxide has a high activation energy and is usually a slow reaction, but the catalyst in the catalytic converter, usually a metal like platinum or rhodium, makes this happen much faster, and therefore reduces the harmful emissions from the car. So how does it do it? Well, back to our Maxwell-Boltzmann distribution. In thinking of ways to speed up a reaction, we've used surface area, concentration and pressure to increase the overall rate of collisions. And we've used temperature to both increase the overall rate of collisions and also increase the proportion of successful collisions. All of these methods involve trying to get as many molecules as possible over the activation energy. But what if we could change the activation energy, make the hurdle easier to get over? Well, this is what a catalyst does. If this is the activation energy of a normal reaction, then this would be what might happen if we added a catalyst. The distribution of particle energies remains exactly the same, but now that the activation energy has been lowered, more collisions are successful. Now what I've just said gives the impression that the activation energy can somehow be dialed down for a reaction, when in fact this is impossible. Since the activation energy has to do with bond energies, and the bond energies in a molecule are pretty much fixed, you can't actually change the activation energy of the reaction process. What a catalyst does is to provide an alternative way through, a sneaky way of completing the reaction, without having to go via the pathway that involves the high activation energy. There's a nice analogy from the website chemguide.co.uk that I'll use to illustrate this. Suppose you have a mountain between two valleys, so that the only way for people to get from one valley to the other is over the mountain. Only the most active people will manage to get from one valley to the other. Now suppose a tunnel is cut through the mountain. Many more people will now manage to get from one valley to the other by this easier route. You could say that the tunnel route has a lower activation energy than going over the mountain. But you haven't lowered the mountain. The tunnel has provided an alternative route, but it hasn't lowered the original one. The original mountain is still there, and some people will still choose to climb it. In the chemistry case, if particles collide with enough energy, they can still react in exactly the same way as if the catalyst weren't there. It's simply that the majority of particles react via the easier catalyzed route. So to summarize catalysts, adding a catalyst to a reaction provides a pathway with a lower activation energy, and the effect of this is to increase the proportion of successful collisions. Remember the other way to increase the proportion of successful collisions is to increase the temperature, but often it's not convenient to increase the temperature of a reaction. It may be a better option to find a catalyst that allows you to perform the reaction faster at a lower temperature.