Question

In a reaction between HCl(aq) and Mg (s), does doubling the concentration of HCL double the reaction rate? By intuition I wouldn't think so, but I can't think of why not.

We did an experiment in class where I recorded that at 0.5M the rate was 2.9e-5 g/s, at 1.0M it was 1.5e-4 g/s, at 3.0M it was 8.6e-4 g/s, and at 6.0M it was 2e-3 g/s. The ratio between 6.0M and 3.0M concentrations are *almost* double, but I'm not sure if that's due to measurement errors or not.

Would someone please explain? Thanks.

Answer 1

Actually, I'd probably expect the result you find, which is that the reaction rate is proportional to the concentration of acid. (In chemical kinetics language, you would say "the reaction rate is first order in the concentration of acid.") In general, the dependence of reaction rate on reactant concentrations is complicated, and depends greatly on what the kinetic mechanism of the reaction is. For example, in reaction in which the rate-limiting step is a collision between two molecules of A, the rate will usually be proportional to [A]^2.

However, in this case, it would seem very likely that the rate-limiting step of the reaction is the collision of an H3O+ cation with the metal surface. The rate of that step would indeed be proportional to the concentration of acid.

Answer 2

Ah, I see now - so although the reaction rate is not always directly proportional to the reactant concentration, it would be in this case. Thanks! Great answer!