In a reaction , it is not always clear how to measure the amount of reactant that is being used up or the amount of product being formed. Chemists , in order to measure the change in the concentration of a substance measure the change in the physical properties of the chemical system. This change reflects the change occurring in the concentration of a reactant or product.
Examine the following video clip of two different chemical reactions.
  1. What physical property could the experimenter use to measure the rates of this reaction?
  2. Chemical reactions involve the formation of new substance and this often involves changes in state or increased or decreased number of molecules. What are some of the other physical properties that might change as a result of such changes in a chemical system?
Rates can be measured using any number of methods or techniques. The choice of technique depends on the reaction that is being studied and the equipment available. Some of the following are commonly used.
  1. Mass of a solid. Measuring increases or decreases in the mass of a solid.
  2. Changes in pH or acidity. If an acid or base is involved the acidity or alkalinity is proportional to the amount of an acid or base that is present . This can be measured using a pH meter or pH paper.
  3. Changes in conductivity. Conductivity of a solution is proportional to the concentration of ions.
  4. Monitoring pressure. When reactions involve gases , the pressure of the system often changes as the reaction proceeds. ( NOTE: The total number of gas molecules in the reactant and products must be different for pressure to change )
  5. Monitoring colour. Colour change can be used to measure the progress of a reaction. The absorption of light by a chemical compound is directly proportional to it's concentration. A spectrophotometer can be used to measure this absorption.
  6. Volume of a Gas. If a gas is produced as a product the volume of the gas can be measured over time.
Activity on experimentally measuring rates
i) Assignment : In the following assignment are more examples of reactions . Using the information above to determine how the rates of each reaction could be measured experimentally..
Assignment : How to Measure Rate
ii) Lab : Measuring rates of Reaction
testtube
Labs on Measuring Rates
click on the test tube to measure the rate of some simple reactions