VS
We learned how to calculate the value of x and y (the exponents) and learned what they mean. For example, if a reaction has a rate law of RATE = k[A] 0 [B] 1 [C] 2 then the reaction is:
 Zero order with respect to reactant A

First order with respect to reactant B

Second order with respect to reactant C
However, it can also be useful to know the
. To find this, you simply add the exponents together. So, for this above example, the overall order of the reaction would be 3 rd order overall (0+1+2=3).
Knowing the overall order of the reaction can give us some more conceptual information as well as some more calculation opportunities.
Zero order reactions are reactions that have exponents that total zero (very uncommon). For this type of reaction, the rate of the reaction does not depend on the concentration of the reactants at all. Most of these types of reactions need a catalyst to occur.
First order reactions are reactions that have exponents that total one. As a first order reaction proceeds, the concentration and the rate of the reaction both decrease.
We have a special equation we can use to determine the concentration of a reactant at any time during the reaction (but we can only use this for FIRST ORDER REACTIONS): ln[A]
– ln[A]
= kt
Example: A certain first order reaction has a rate constant of 4.5 x 10 -3 s -1 . How much of a 0.050M sample will have reacted after 75.0 seconds?

Second order reactions are reactions that have exponents that total two. For example, you could have a rate law like either of these:
Rate = k[A] 2 or Rate = k[A][B]
As long as the rate law is like the first example, we have an equation to determine the concentration of the reactant at any time during the reactions (but we can only use this one for SECOND ORDER REACTIONS):
- = kt
Example: A second order reaction has a rate constant of 3.6 x 10-2 1/M·s. How much of a 0.050 M sample will have reacted after 135 seconds?