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 activation energy is the minimum amount of energy needed for a collision to be successful. It is different for different reactions.

Two pairs of particles move towards each other.

Colliding particles

1. Two pairs of particles move towards each other

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.

The mean rate of reaction can be calculated using either of these two equations:

mean~rate~of~reaction = \frac{quantity~of~reactant~used}{time~taken}

mean~rate~of~reaction = \frac{quantity~of~product~formed}{time~taken}

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.

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 useful 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-1 or cm3 min-1.

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

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 steeper the line, the greater the rate of reaction. Faster reactions - where the line becomes horizontal - finish sooner than slower reactions

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

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

Units for rates of reaction - Higher

The rate of a chemical reaction can also be measured in Mol s-1.