## Reaction rates

The reaction rate is the rate at which a reactant is consumed or a product is produced. We can determine it by measuring changes in colour, conductivity, pressure, volume, pH, mass, density, or concentration of a reactant or product over a series of time intervals. We usually use concentration:

r = (Delta c)/(Delta t).

reaction rate (r)
the rate at which the reaction progresses, measured in moles per litre-second (mol/L·s); it is negative when the substance is being consumed and positive when the substance is being produced
concentration change (Delta c)
the change in the concentration of the substance in question, measured in moles per litre (mol/L)
time interval (Delta t)
the time that passes during the concentration change, measured in seconds (s)

Given the rate for one reactant or product in an equation, we can use stoichiometry to determine the rates for the other reactants and products.

There are five main factors that affect the rate of a reaction:

• chemical nature of reactants
• concentration of reactants
• temperature of reactants
• surface area (for heterogeneous reactions)
• presence of a catalyst

A catalyst is a substance that increases the reaction rate by providing an alternative reaction pathway requiring a lower activation energy, without being consumed itself. If it is in the same phase as all reactants, it is called a homogeneous catalyst; otherwise, it is heterogeneous.