Question

How does the presence of a catalyst affect the enthalpy of a reaction? Group of answer choices 1. It depends on whether you are talking about the forward or the reverse reaction. 2. A catalyst decreases the enthalpy change of a reaction. 3. A catalyst does not affect the enthalpy change of a reaction. 4. A catalyst increases the enthalpy change of a reaction.

Answer 1

Option 3. The catalyst does not affect the enthalpy change (
`\Delta H_\text{rxn}`) of a reaction.

Step-by-step explanation:

As its name suggests, the enthalpy change of a reaction (
`\Delta H_\text{rxn}`) is the difference between the enthalpy of the products and the reactants.

On the other hand, a catalyst speeds up a reaction because it provides an alternative reaction pathway from the reactants to the products.

In effect, a catalyst reduces the activation energy of the reaction in both directions. The reactants and products of the reaction won't change. As a result, the difference in their enthalpies won't change, either. That's the same as saying that the enthalpy change
`\Delta H_\text{rxn}` of the reaction would stay the same.

Refer to an energy profile diagram. Enthalpy change of the reaction
`\Delta H_\text{rxn}` measures the difference between the two horizontal sections. Indeed, the catalyst lowered the height of the peak. However, that did not change the height of each horizontal section or the difference between them. Hence, the enthalpy change of the reaction stayed the same.