Question

In the proposed example for the isomerization of n-pentane to i-pentane, which assumption is made about the rate-limiting step for deriving a rate law?

a) Assume the surface reaction is the rate-limiting step and derive a rate law.
b) Assume there is more than one rate-limiting step and derive a rate law.
c) The rate law cannot be derived based on the given information.
d) The rate law for isomerization doesn't depend on rate-limiting steps.

Answer 1

To derive a rate law for the isomerization of n-pentane to i-pentane, it is assumed that there is one rate-limiting step which determines the rate law for the overall reaction.

Step-by-step explanation:

When deriving a rate law for the isomerization of n-pentane to i-pentane, the assumption typically made about the rate-limiting step is that there is one step in the mechanism that is significantly slower than the others. This slowest step limits the rate at which the overall reaction occurs, hence referred to as the rate-limiting step. In the proposed example, the rate law can be derived by assuming the surface reaction is the rate-limiting step. Once identified, the rate law for the overall reaction would be the same as the rate law for this individual step. However, if the rate-limiting step involves an intermediate species, we might need to use equilibrium expressions from earlier steps to express the intermediate concentrations in terms of the reactants' concentrations to obtain a usable rate law.