Question

A reaction occurs between reactants X and Z. Doubling the concentration of X while the concentration of Z is constant doubles the rate. Tripling the concentration of Z while the concentration of X is constant does not affect the rate. 

What is the rate law for this reaction?

Answer 1

Answer: The rate law is,
`rate=k[X]` .

Solution:- Let's say the order of reaction with respect to Z and X is n and m respectively.

So, the rate law in general could be written as:

`rate=k[Z]^n[X]^m`

where k is the rate constant.

If we take a look at the given information then, Doubling the concentration of X while concentration of Z is constant doubles the rate. It means the reaction is first order with respect to X and hence the value of m is 1.

Tripling the concentration of Z while the concentration of X is constant does not affect the rate. Here the rate is independent on the concentration of Z. It indicates the reaction is zero order with respect to Z and so the value of n is 0.

So, the rate law will be,
`rate=k[X]` .

Answer 2

The reaction rate is independent of the concentration of reactant Z from the statement of the problem. So, the concentration of reactant Z would not be in the rate law.  It is only dependent on the concentration of reactant X. The rate law would most likely be r = k[X].