For a chemical reaction to happen:
A collision that produces a reaction is called a successful collision. The frequency of successful collisions is affected by factors such as concentration, temperature and surface area of the reactants.
There are different ways to determine the rate of a reaction. The method chosen usually depends on the reactants and products involved, and how easy it is to measure changes in them.
In addition, how long a reaction is observed for depends on the rate of reaction. Reactions can vary from being almost instantaneous to taking years to complete. In the lab, reactions are usually followed over a few seconds or minutes.
Rusting is a slow reaction. Give four examples of a very fast reaction.
Combustion, explosions, neutralisation reactions and precipitation reactions are very fast reactions.
The change in mass of a reactant or product can be followed during a reaction. This method is useful when carbon dioxide is a product which leaves the reaction container. It is not suitable for hydrogen and other gases with a small relative formula mass, Mr. It is also not suitable if there are no gaseous products that leave the reaction vessel and so there is no loss in mass. The units for rate are usually g/s or g/min.
The change in volume of a reactant or product can be followed during a reaction. This method is used when a gas leaves the reaction container. The volume of a gas is measured using a gas syringe, or an upside down burette or measuring cylinder. The units for rate are usually cm3/s or cm3/min.
If a precipitate forms during a reaction, the solution will change from being transparent to being opaque. The length of time it takes for the solution to become so cloudy that it is no longer possible to see through it can be measured. This method only works if the formation of the precipitate is slow. The units for rate are usually /s (s-1 ) or /min (min-1 ).
The rate of reaction can be analysed by plotting a graph of mass or volume of product formed against time. The graph shows this for two reactions.
The gradient of the line is equal to the rate of reaction: