Rate of reaction

The rate of a reaction is a measure of how quickly a reactant is used up, or a product is formed.

Collision theory

For a chemical reaction to happen:

  • reactant particles must collide with each other
  • the particles must have enough energy for them to react

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.

For a reaction to be successful, the collision needs to take place with a minimum amount of energy, known as the activation energy. This is different for different reactions.

Two pairs of particles move towards each other

1. Two pairs of particles move towards each other

Measuring rates of reaction

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.

Question

Rusting is a slow reaction. Give four examples of a very fast reaction.

Combustion, explosions, neutralisation reactions and precipitation reactions are very fast reactions.

Measuring mass

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.

Measuring volume

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.

Methods to measure volume of gas collected.Two ways to measure the volume of a gas produced in a reaction

Formation of a precipitate

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 ).

Graphs

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.

Graph of total mass of product against time from start of reaction. A line labelled 'fast reaction' rises sharply from zero before gradually levelling off. A line labelled 'slow' reaction rises less sharply but eventually levels off at the same height as the fast reaction line.

The gradient of the line is equal to the rate of reaction:

  • the steeper the line, the greater the rate of reaction
  • fast reactions finish sooner (when the line becomes horizontal) than slow reactions