Video: Applying Knowledge of Catalyst Energetics

For statements I and II, state for each if they are true or false. I) The addition of a catalyst increases the ΔH for a reaction. II) A catalyst provides an alternate reaction pathway with a lower activation energy. If both are true state, state if II is a correct explanation for I.

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Video Transcript

For statements I and II, state for each if they are true or false. I) The addition of a catalyst increases the ΔH for a reaction. II) A catalyst provides an alternate reaction pathway with a lower activation energy. If both are true state, state if II is a correct explanation for I.

For this question, we’re going to have to go back to the basic definition of a catalyst. A catalyst is any substance that accelerates a chemical reaction without undergoing permanent chemical change. For instance, in the Haber process, fine iron particles are used to catalyze the reaction between nitrogen and hydrogen gases, producing ammonia.

Statement I says that adding a catalyst to a reaction will increase the enthalpy change for that reaction. The enthalpy of the system is actually quite a complicated property. But we can treat it as the internal energy, from bonds and so forth, plus the energy needed to establish its pressure and volume.

The enthalpy of the system is sometimes simplified to mean heat content because that’s how it behaves. When we have increases and decreases in enthalpy, often we see that as transfers of heat. The key feature of enthalpy is that as we form bonds, we’re going to reduce the internal energy because that difference in energy will go to the surroundings.

Let’s have a look at the reaction profile for the Haber process. Our reactants are nitrogen and hydrogen gases and our product is ammonia. Our change in enthalpy is the difference in enthalpy between the products and the reactants. The forward reaction in the Haber process is exothermic. So, the change in enthalpy is negative because the enthalpy of this system decreases.

The rate of the reaction is dictated by the activation energy, which comes down to the energy we need to put in to break up the bonds in nitrogen and hydrogen. That will allow the new bonds in ammonia to form. The catalyst increases the rate of a reaction, and it must do this by reducing the activation energy. The lower the activation energy, the faster the reaction.

What we can see from the diagram is that the catalyst has not affected the enthalpy of either the reactants or the products. Therefore, the addition of a catalyst to a reaction has no effect on the change in enthalpy. Adding a catalyst doesn’t change the strength of the bonds in the reactants or the products. It only makes it easier for one to transform into the other. So, statement I is actually false. The addition of a catalyst has no effect on the change in enthalpy for a reaction.

Meanwhile, statement II says that a catalyst provides an alternate reaction pathway with a lower activation energy. We’ve already seen that adding a catalyst lowers the activation energy. Let’s have a look at how it might do that. For instance, in the Haber process, the interaction between, say, nitrogen and the iron might weaken the nitrogen bond in such a way it doesn’t have to break completely in order to interact with the hydrogen.

The details of what goes on with catalysts can be very, very complex. But it’s always true that the way a catalyst interacts with the reactants is going to be different to how the reactants themselves will interact. Catalysts aren’t magic, so they have to interact in some way.

So, the way a reaction occurs without a catalyst is going to be different to the way reaction occurs with a catalyst. So, statement II is true. Only one of the statements is true. So, we don’t have to address the last part of the question. II cannot be a correct explanation for I because I is false.

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