What is the significance of a reactant being zero, first, or second order when calculating the rate of a reaction?

In simple terms, we need to know whether a reactant is first, second or zero order if we want to understand what effect changing the concentration of that reactant has on the rate of the reaction.

Zero order= change in conc. has no effect on rate

1st Order= change in conc. causes same change to rate e.g. conc. x2 = rate x2

2nd Order= change in conc. causes the same change of rate to the power 2 e.g. conc x2 = rate x4

If I was to give an example for the reaction:

2X_(g) + Y_(g) --> A_(g) + B_(g)

The effect the concentration of each reactant has on the rate of the reaction is displayed through the rate equation, which includes the reaction constant (k).

Rate=k[X]²

K is constant at a constant temperature.

We can see from the rate equation that the only reactant that has an effect on the rate of the reaction is X, as it is the only one mentioned. This means the rest of the reactants are zero order and have no effect.

What we can also derive from the rate equation is that X is second order in relation to rate, as it has a ² next to it's concentration. This means that when the concentration of X is doubled, the rate of the reaction increases by a factor of 2² = 4.

If the rate equation was rate= k[X], when the concentration of X was doubled, the rate of the reaction would be doubled (if [X] was tripled, rate would be tripled etc). This is known as a first order relationship.