Which of the following statements about catalysts is true? A) A catalyst affects the rate constant of a reaction. B) A catalyst affects the equilibrium constant of a reaction. C) A catalyst causes the reaction to proceed at a lower rate. D) A catalyst increases the activation energy of a reaction. Or E) A catalyst only affects the rate of the forward reaction, not the reverse reaction.
A catalyst is simply any substance that when applied to a reaction increases the rate of that reaction but does not undergo permanent chemical change because of it. That sounds quite complicated, but it’s really very simple. Let’s take, for instance, the Haber process, which is the industrial process for producing ammonia from nitrogen and hydrogen gases. The reaction between nitrogen and hydrogen is exothermic. So the products of the reaction are more stable than the reactants.
However, in order to form ammonia, we need to break the bonds in nitrogen and hydrogen first. And that means that there’s a quite high activation energy. This makes the reaction very slow under normal conditions. That’s why, in an industrial setting, an iron catalyst is used.
The catalyst lowers the activation energy of a reaction, helping to speed it up. However, the reaction that produces the ammonia — even under the best industrial conditions — is reversible, meaning some of the ammonia becomes hydrogen gas and nitrogen gas once more. The catalyst provides a route for the reaction to occur with the lower activation energy. But it does work in both directions, meaning that it speeds up both the forward reaction and the reverse reaction. At this point, we’re armed with enough information to tackle some of the statements.
Statement C says that catalyst causes the reaction to proceed at a lower rate. This is definitely false because it’s completely contrary to the definition of a catalyst. Interestingly, there are actually chemicals that can slow down reactions, but these are called antagonists or inhibitors. The next statement says that catalyst increases the activation energy of a reaction. But if we increase the activation energy of a reaction, we’ll actually slow it down rather than speed it up. And we know from our example that a catalyst should decrease the activation energy of a reaction. So statement D is false.
The fifth statement is that a catalyst only affects the rate of the forward reaction, not the reverse one. But we know that to be false. If you had a mountain and you simply made it shorter by chopping off the top, you’d make it easier to climb, no matter which side you climbed it from. It’s the same with reducing the activation energy for a reaction. It makes the reverse reaction easier, just like it makes the forward reaction easier.
We have just two statements left. So let’s have a look at them in detail. Statement one says that a catalyst affects the rate constant of a reaction. The rate constant is just one of the terms in the rate equation. The rate of a reaction is the rate at which reactants are consumed or products are produced. And the rate can be calculated if you know how concentrated the reactants are and you know the rate constant, which is an empirically determined value based on the reaction and the conditions. The rate equation for the Haber process looks something like this, where the rate constant and the values of 𝑥 and 𝑦 are determined experimentally.
What this means is that the rate of our reaction is proportional to the rate constant. If the rate constant for some reason is suddenly higher, the rate of the reaction should be higher. Therefore, if a catalyst affects the rate of a reaction as we know it does, it should also affect the rate constant. So it looks like statement A is true. But let’s have a look at the second statement just to be safe.
Statement B says that a catalyst affects the equilibrium constant of a reaction. An equilibrium constant is usually given a symbol capital K. And the equilibrium constant for the Haber process is equal to the concentration of ammonia squared divided by the concentration of nitrogen multiplied by the concentration of hydrogen cubed. The equilibrium constant is quite a complicated thing and where it comes from is beyond the scope of this video. But what we do know about the equilibrium constant is that it’s only affected by temperature.
The equilibrium constant arises from the balance of the forward and the reverse reactions. And if the temperature affects these reactions differently, then the equilibrium constant will change as well. But we know that a catalyst affects the forward and reverse reactions to the same degree. So a catalyst will not affect the equilibrium constant of a reaction. So statement B is false, leaving us with statement A. A catalyst affects the rate constant of a reaction.
The reason I hesitated to say this statement was fundamentally true is that catalysts don’t just speed up a reaction. They provide it with a different way to happen. That can mean that the rate equation, even the concentration terms change. However, fundamentally, a catalyst will change the rate of a reaction. And that almost, almost certainly means that the rate constant will change as well, meaning that out of the five statements, the only one that’s true is that a catalyst affects the rate constant of a reaction.