What are reaction rates

Mathematically, the average reaction rate can be expressed as:

$$\text{Rate} = \frac{\Delta[\text{concentration}]}{\Delta t}$$

where $\Delta[\text{concentration}]$ is the change in concentration (usually in molarity, M) and $\Delta t$ is the change in time (usually in seconds, s). Therefore, common units for reaction rate are $\text{M/s}$ or $\text{mol L}^{-1}\text{s}^{-1}$.