Methods of measuring reaction rates

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Since the reaction rate is the change in the amount of a product or a reactant per unit time, any property that is related to amount of product or reactant present can be used to measure the rate of reaction. Some properties that can be used to measure the reaction rate are given in the following table. Note that the sign for the changes ratio has been taken as positive in all case although that may not be the case depending on the property being measured.

Methods of Measuring Reaction Rates
Property Observation Rate
Mass Mass of product increases

Mass of reactant decreases

Temperature Exothermic reaction
  • Heat is released
  • Temperature increases

Endothermic reaction

  • Heat is absorbed
  • Temperature decreases
Colour Amount of light absorbed depends on the amount of absorbing species present. If a reactant or product absorbs light, then the color intensity will vary as the amount of this species changes DCdt.gif
Pressure If there is a change in number of moles of gas as the reaction proceeds and the reaction is run at a fixed volume then pressure will change as the reaction proceeds DPdt.gif
Conductivity If the reaction involves ions in solution then the conductivity of the solution will change as the reaction proceeds. DLdt.gif
Concentration The concentration of reactant and products changes as the reaction proceeds. So anything that measures concentration can be used. DConcdt.gif

The rate of reaction usually depends on the concentration of reactants. So the reaction rate is normally given as change in concentration over time. Then the rate will have units like M s-1. If the change in some other property over time is used to measure the rate, this property is usually converted back into concentration units. For example, if the amount of light absorbed by a solution is used measured the rate, then the absorption is usually converted into the concentration of the species which is then used to calculate the rate of the reaction.