Which one of the following statements about catalysts is false? A) Catalysts increase the rate of a reaction. B) After catalysts react, they are regenerated. C) Catalysts are unreactive. Or D) catalysts do not appear in the overall reaction equation.
Before we look at the statements again, let’s have a quick recap of what a catalyst is. The simple definition of a catalyst is a substance that increases the rate of a reaction without undergoing a permanent chemical change. Statement A says the catalysts increase the rate of a reaction. Since this conforms with the definition, we know this is true. Since we’re looking for a false statement about catalysts, this cannot be a correct answer.
When the definition says that the substance increases the rate of a reaction, it means a specific reaction. A catalyst for one reaction may not be a catalyst for another. For example, let’s have a look at one of the steps in the contact process, the production of sulfuric acid.
One of the vital steps is the conversion of sulphur dioxide to sulphur trioxide by reaction with oxygen. This reaction is exothermic, but on its own it has a relatively high activation energy. The higher the activation energy of a reaction, the slower the reaction proceeds.
To make the process economical in industry, a catalyst is added to accelerate the rate of the reaction. The catalyst in this case is usually vanadium pentoxide V₂O₅. The introduction of the catalyst reduces the activation energy of the process, allowing the reaction to occur faster.
The first step of this process is the interaction of reactants with the catalyst producing an intermediate. In the second step, this intermediate breaks down releasing the product sulphur trioxide and the original catalyst. When we add these two reactions together, because the catalyst is regenerated in its original form, it cancels from both sides giving us the overall reaction equation SO₂ plus half O₂ react to form SO₃. This demonstrates that after catalysts react, they are regenerated. Since the second statement is true, it cannot be a correct answer.
What about catalysts being unreactive? Well, the definition of a catalyst requires there be no overall change, not that the catalyst does not participate in any reactions. In this step of the contact process, the catalyst is consumed in the first step and regenerated in the second. If a catalyst did not interact with the reactants in some way, then it would not be able to influence the rate of a reaction.
So, a catalyst must be able to react. So, the statement that catalysts are unreactive is false. Since we’re looking for false statements about catalysts, this is our correct answer. But let’s have a look at the last statement just to be safe.
The final statement says that catalysts do not appear in the overall reaction equation. That’s equation number three. As we can see, the catalyst is not present as a reactant or as a product. Since the statement is true, it can’t be a correct answer.
As a side note, you may sometimes see catalysts written above the reaction arrow. Those indicate conditions and not reactants or products, so the fourth statement is still true. So, we’ve demonstrated that of the four statements given the one about catalysts that is false is that catalysts are unreactive.