Rate Laws

We have said before that the rate of a reaction is increased by an increase in the concentration of the reactants.

This can be written mathematically in an equation called a rate law.

For a reaction such as the one below (clearly made up as a generic example)

2 A + 3 B --> C + 3 D 

the rate law has the form:

In this equation:

• k is a constant (called the rate constant), 
• [A] is the concentration of A, 
• [B] is the concentration of B, and 
• x and y are integers, called order. 

It would be convenient if x and y matched the coefficients in the reaction, and they often do, but not always.

The only way to determine the values of x, y (and k for that matter) are through experiment.

To understand what this equation says, and what it can do for us we need to know three things.

1. How to calculate x,and y,
2. how to calculate k, and 
3. what those values mean. 

We’ll take those on one at a time.