Which of the following statements is false about catalysts? A) They decrease the equilibrium constant of a system. B) They react chemically during the course of a reaction. C) They may be gases, liquids, or solids. D) They do not change ΔH of a reaction. Or E) They act by increasing the rate of both the forward and the reverse reactions.
A catalyst is a substance that increases the rate of a reaction, without undergoing permanent chemical change. Catalysts are specific to a given reaction. A substance that’s a catalyst for one reaction may not catalyze a different reaction. So, for instance, in the Haber process, where ammonia is produced from nitrogen and hydrogen gases, iron is used as the catalyst. But in one of the steps of the contact process, the catalyst is an oxide of vanadium. If we swap these two substances round, there’s no guarantee they would catalyze the other process. This is because catalysts work in specific ways for specific reactions.
Let’s take, for example, the surface of a theoretical solid catalyst. And let’s imagine we’re trying to decompose a mystery gas. In this scenario, I’m imagining that the gas has a relatively low energy. And the product, our separate gas particles, have relatively high energy. However, what I’m going to show you works no matter what the energy difference is. If we try and go directly and, for instance, just use heat to make the reaction happen, there’ll be a relatively large activation energy. This is the activation energy for the forward reaction, the difference in energy between the reactants and the top of the plot.
This energy would go into breaking bonds and disrupting the structure of our starting material. But at the same time, the reverse reaction may be happening. And we can see the activation energy of the reverse reaction as the difference from the top of the plot to the energy of the products. Now, let’s imagine that we introduce the catalyst. The gas particle interacts with a catalyst surface. This interaction might affect how the internals of that gas particle are arranged and affect the strength of various bonds. The catalyst introduces an alternate way for the reaction to happen with a lower activation energy.
Now that we’ve had a quick recap of what a catalyst is, let’s have a look at the statements. The equilibrium constant and equilibrium is often given the symbol K. And it’s the balance point between the forward and reverse reactions. For a simple equilibrium where A, B, C, and D are gases or solutes, the expression for the equilibrium constant looks like this. So we have a term representing the products in the numerator and a term representing the reactants in the denominator.
If a catalyst decreased to the equilibrium constant, that suggests that it’s influencing the concentration of the reactants and increasing the size of the reactant in turn. But we know that a catalyst will actually facilitate both the forward and the reverse reactions. And an equilibrium constant is only significantly influenced by temperature and the nature of the initial equilibrium. So statement A is false.
But just to be safe, let’s have a look at the other four. The second statement suggests that catalysts react chemically during the course of a reaction. It’s a common misconception that catalysts don’t react. In fact, it’s absolutely necessary that catalysts react in some way. They react with the reactants. They affect how the reactants can interact with one another. And then, they return to their starting state. The products are produced. But the catalyst hasn’t been chemically changed. So the second statement is true. And it’s an incorrect answer.
The third statement is that catalysts may be gases, liquids, or solids. It may come as a surprise. But yes, catalysts can be gases. For instance, CFCs, chlorofluorocarbons, can be released into the atmosphere. And these can generate radicals that catalyze the destruction of ozone. A radical can go through many regeneration cycles, destroying lots of ozone. And there are many examples of liquid and solid catalysts. So this statement is true and also not a correct answer.
Fourthly, we have the suggestion that catalysts do not change ΔH of a reaction. ΔH is the symbol for the enthalpy change. And it indicates the change in energy stored in our system. If we look back at our reaction progress diagram, we can see that the change in enthalpy for the reaction is the difference in energy between the products and the reactants. The catalyst doesn’t interact with the reactants or the products once the reaction is complete. So it can’t affect their energies.
So we can move on to the last statement. This suggests that catalysts act by increasing the rate of both the forward and reverse reactions. If we look back at the reaction progress diagram again, we can see that adding a catalyst introduces a way for both the forward and reverse reactions to happen with a lower activation energy. This means that they both happen more quickly. That’s not to say that they happen at the same rate. They’ll only happen at the same rate once the system reaches equilibrium. A catalyst increases the rate at which a system achieves equilibrium. But it doesn’t change the position of it.
So of the five statements we’ve been given, the only one that is false regarding catalysts is that they decrease the equilibrium constant of a system.