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. 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. An upturned measuring cylinder can also be used but this is less accurate. The units for rate are usually cm3/s or cm3/min.

Measuring colour change or formation of a precipitate - Higher

A few reactions produce a colour change, for example due to the production of iodine. A colorimeter measures colour change.

The formation of a precipitate causes the reacting mixture to change from transparent to opaque. The rate of reaction can be measured by timing how long it takes for a cross (drawn on a white tile) to disappear.

The result of adding dilute acid to a transparent sodium thiosulfate solution is a more opaque solution.

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 horizontal line shows that no more product is being made - the reaction has finished
  • rate of reaction does not affect the mass of product formed

The gradient or steepness 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