Rate Laws

So, as discussed on the previous page, the rate of a reaction can't be figured out entirely from the chemical reaction. While that is true, you can figure out the relative rate of a reaction using the equation by using something known as a rate law. As discussed later, it has been experimentally found that reaction rates are dependent on quite a few factors, and one of them is the concentration of the reactants. The following equation is an example of a rate law:

The general way to write a rate law is this.

If we have a reaction of the form:

aA + bB → products

the rate law for that given reaction is:

Essentially, it's the constant k multiplied by the concentration(mol/L) of both reactants raised to the power of their stoichiometric coefficients.

The reaction order is the sum of the exponents in the rate law. For this reaction, the reaction order is a+b.

Based on the reaction order, the units of the constant k are different to make sure the rate is always of the form (change in concentration)/(change in time). Below is a table for the orders:

We can use a table like this to figure out the exponents of the rate law of the reaction. As you can see, doubling the concentration quadruples the rate, so the reaction order for CH₃CHO is 2. This type of analysis is how you find relative reaction rates through experimental initial rates.