Problem 75 - Chemistry: The Central Science

Question

(a) What is a catalyst? (b) What is the difference between a homogeneous and a heterogeneous catalyst? (c) Do catalysts affect the overall enthalpy change for a reaction, the activation energy, or both?

Step-by-Step Solution

Answer

(a) A catalyst is a substance that increases the rate of a chemical reaction by lowering the activation energy, without undergoing any permanent chemical change. (b) Homogeneous catalysts are in the same phase as the reactants, while heterogeneous catalysts are in a different phase. Homogeneous catalysts generally allow for faster reactions but are harder to separate, while heterogeneous catalysts are easier to separate and more suited for large-scale applications. (c) Catalysts only affect the activation energy of a reaction, not the overall enthalpy change, allowing the reaction to proceed faster and at lower temperatures while the overall energy difference between reactants and products remains unchanged.

Step 1: Definition of a catalyst

A catalyst is a substance that increases the rate of a chemical reaction by lowering the activation energy. Importantly, a catalyst remains chemically unchanged after the reaction and can be reused multiple times.

Step 2: Homogeneous vs. heterogeneous catalysts

The main difference between homogeneous and heterogeneous catalysts lies in their phase relative to the reactants. A homogeneous catalyst is in the same phase (solid, liquid, or gas) as the reactants, while a heterogeneous catalyst is in a different phase. Generally, homogeneous catalysts are more effective in accelerating reactions, as their particles can easily interact with the reactants. However, heterogeneous catalysts are often easier to separate from the reaction mixture and can be more practical for large-scale applications.

Step 3: Catalysts' effect on enthalpy change and activation energy

Catalysts only affect the activation energy of a reaction, not the overall enthalpy change. By lowering the activation energy, catalysts enable the reaction to proceed faster and at lower temperatures. However, the overall enthalpy change for the reaction remains unchanged, as it is determined by the difference in energy between the reactants and products, which is independent of the catalyst's presence.