How do catalysts affect the rate expression?

Catalysts affect the rate expression by altering the rate constant, but not the order of reaction.

Catalysts are substances that increase the rate of a chemical reaction by providing an alternative reaction pathway with a lower activation energy. They achieve this without being consumed in the reaction, meaning they can be used repeatedly. In the context of the rate expression, which is typically given as rate = k[A]^m[B]^n, where k is the rate constant, [A] and [B] are the concentrations of the reactants, and m and n are the orders of reaction, catalysts primarily affect the value of k.

The rate constant, k, is a measure of the speed of a reaction. It is temperature-dependent, and generally increases with increasing temperature. This is because at higher temperatures, more molecules have the kinetic energy required to overcome the activation energy of the reaction. However, when a catalyst is introduced, it lowers the activation energy, meaning that more molecules can react at a given temperature. This effectively increases the value of k, leading to a faster reaction rate. To understand more about how temperature and molecular collisions affect reaction rates, see Basics of Collision Theory.

However, it's important to note that while catalysts change the rate constant, they do not change the order of reaction. The order of reaction with respect to a particular reactant is determined by the mechanism of the reaction, specifically the number of molecules of that reactant involved in the rate-determining step. This is not affected by the presence of a catalyst. Therefore, while the rate expression for a catalysed reaction may have a different value of k, it will have the same form as the rate expression for the uncatalysed reaction.

In summary, catalysts increase the rate of a reaction by providing an alternative reaction pathway with a lower activation energy, which increases the rate constant in the rate expression. However, they do not change the order of reaction. For further detail on how energy profiles are altered by catalysts, see Energy Profiles. Additionally, the mathematical formulation of rate expressions and their dependence on reaction conditions can be explored further in Rate Equations.