## 4.2: Rate lAws

The rate law of a reaction is an equation that calculates the speed of a reaction (usually in M/s, but any amount/time is acceptable).

The general form of a rate law is:

The general form of a rate law is:

Rate = k[A]

^{m}[B]^{n}Where

It is important to note that whether a reactant appears in the rate law, and the value of its order,

If we consider on reactions with one reactant, then there are three most common rate laws

**k**is the**rate constant**--a constant value for a given reaction at a given temperature- [A] and [B] are the concentrations of reactants (there may be only one concentration here, or many). In rare cases non-reactants may appear in the rate law.
**m**and**n**are the**order of the reaction**in A and B. Typically these are zero, one, or two, but they may have other values, including negative numbers or non-integers. There are rare.

It is important to note that whether a reactant appears in the rate law, and the value of its order,

__is a completely experimental result.__You cannot predict it based on the reaction equation.If we consider on reactions with one reactant, then there are three most common rate laws

0th Order: Rate = k

1st Order: Rate = k[A]

2nd Order: Rate = k[A]

1st Order: Rate = k[A]

2nd Order: Rate = k[A]

^{2}**Method of Initial Rates**

An easy way to distinguish between rate laws--just double the concentration and see what happens to the rate at the start of the reaction. If the rate doesn't change, it was zeroth order. If the rate doubles, it was first order. If the rate quadruples, it was second order.

You can do this even for reactions that have more than one reactant; you just have to only change one at a time.