Order

The exponents in a rate law (like the 2 and the 0 calculated in the example above) are called orders. We can say that the reaction is 2^nd order with respect to A or that it is 0^th order with respect to B. We can also say that the reaction is 2^nd order overall (adding the exponents).

So, if there was a reaction

3 D + 2 X --> Q + R    with a rate law of the form   rate=k[D]²[X]¹,

we would say that the reaction is 2nd order with respect to D, that it is 1st order with respect to X, and 3rd order overall.

The Meaning of Order

So, what does all of this mean? The order tells how much influence the concentration of a particular reactant has on the rate. When something is first order, tripling the concentration will triple the rate. If a reactant is 2^nd order, then tripling the concentration will increase the rate by a factor of 9 (because 3²=9).

Zeroth order

There is one odd case that is worth taking an extra moment to consider. Sometimes the exponent for a reactant is 0. In that case, we say that the reaction is zeroth order with respect to that reactant.

Since anything raised to the 0^th power is 1, then if a reactant is zeroth order, its concentration has no effect on the rate of a reaction.

A well known example of this would be the body’s metabolism of alcohol. Many of you will have been taught that it takes an hour for a single drink to be metabolized by (removed from) the body. Two drinks take two hours, and three drinks take three hours. In other words, no matter how much alcohol is in the body, the rate of metabolism is 1drink/hr.

How is this possible? How can some things cause dramatic changes in the rate while other reactants have no effect at all? The answer depends on mechanisms.