a level chem

Why when a reactant is present in vast concentration does this make its order of reaction zero?

When you have a vast excess of a reagent, its concentration doesn’t change much by the time the reaction goes to completion.

So let’s take this reaction as an example:

2NO + 2H2 —> N2 + 2H2O

The rate equation is

Rate = k

[NO2]^2

Suppose we were to choose the reaction conditions so that the initial concentration of NO is 10 mol dm^-3 and the initial concentration of H2 is 0.10 mol dm^-3.

The initial rate would be

k x 10^2 x 0.1
= 100k x 0.1
(…)

Now suppose that we allow the reaction to proceed for long enough that

is now 0.01 mol dm^-3. That would mean a decrease in concentration of both reactants by 0.09 mol dm^-3 (both reactants react in a 1:1 mole ratio) and so [NO] would be 9.91 mol dm^-3

The rate now would be

k x 9.91^2 x 0.01
= 98.2081k x 0.01
(…)

Notice how in both these example expressions we get something in the form

something close to 100 x k x

That something close to 100 x k is an “observed rate constant” and the rest of the equation becomes independent of all concentrations except the limiting reagent.

Essentially what happens is the much larger concentrations being almost constant, even at the end of the reaction, end up becoming integrated into the rate constant. We can measure an observed rate constant that the concentrations of the excess reagents become merged into, hence we say for the sake of simplicity that their reaction orders become 0.