Learning objectives

Understand what is meant by and define the rate of a chemical reaction

Describe experimental methods for measuring the rates of various types of chemical reactions

Analyse numerical and graphical data from rate experiments

Rate of reaction is the speed at which reactants are used up or products are formed.

How to measure the rate of reaction practically

Some experimental methods can be used to measure the reaction rate:

  • measuring volume of a released gas at constant pressure and temperature.
  • measuring pressure of a released gas at constant volume and temperature.
  • measuring the change in mass if a gas released by the reaction.
  • measuring color change using colorimeter.
  • measuring the change in pH for neutralization reactions.
  • measuring the change in conductivity in aqueous solution.

Example:

For the reaction between calcium carbonate and hydrochloric acid:

CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)

The rate of this reaction can be measured by:

1. measuring the rate at which CO2 is produced

2. measuring of the rate at which the mass decreases.

Measurement of average rate at which CO2 is produced

The volume of CO2 can be measured by either as a graduated gas cylinder or by a gas syringe as in the following figures:

     OR   

AVERAGE RATE

In general, average rate of a reaction is defined as change in the amount of a reactant or a product in a given interval of time.

In this CO2 case:

INITIAL RATE

The initial rate of a reaction is the instantaneous rate at the start of the reaction (i.e., when t = 0).

It is calculated by taking slope of the amount of a reactant or a product versus time graph.

The rate of a reaction is the greatest at the beginning and it slows down by time.

Reaction rates are most often considered in terms of changing concentrations. So,

Rate of reaction is the change in concentration of reactants or products per unit time.

Units for rate of reaction are therefore mol dm−3 s−1, mol dm−3 min−1, etc.

Collision Theory
Factors that affect Reaction Rate