Experimental determination of reaction rates depends upon the nature of reaction under investigation. Rates, can be conveniently studied by measuring the suitable physical property of the system as a whole or one of the constituent of the reaction. The rate of a reaction may be influenced by a number of factors like concentration of the reactants, temperature of the reactants, nature of the reacting substances, presence of a catalysts and exposure to radiations.
When a chemical reaction occurs, the reactants change over to products. It is observed that with the passage of time the concentration of reactants decrease while those of products increase. Now, if we assume that other factors are constant than the rate of a chemical reaction decreases in concentration of the reactants.
Cato Gudberg and Peter Waage proposed a qualitative relationship between the rates of reactions and the concentration of the reacting species. This generalization is known as Law of Mass Action. According to this law at a given temperature, the rate of a chemical reaction is directly proportional to the product of molar concentrations of reacting species with each concentration term raised to the power equal to numerical coefficient of that species in the chemical equation. The rate of reaction when the concentration of each reactant is taken as unity is termed as its rate constant.
The value of rate constant gives an idea about the speed of a reaction. Greater is the value of rate constant, faster is the reaction. Each reaction has a definite value of rate constant at a particular temperature. The value of rate constant also depends on the temperature i.e., changes with change in temperature, but is independent of the concentration of reacting species.
The rate expressions which have been given on the basis of law of mass action are written on the basis of the balanced chemical equations. These expressions may or may not give the actual dependence of the reaction rate on concentrations of reacting species. It is because the actual relationship between the concentrations of reacting species and the reaction rate is determined experimentally and is given by the expression called rate law expression. Theoretically a chemical reaction takes place due to collisions between the particles of the reactants. The number of reacting species (atoms, ions and molecules) which must collide simultaneously in order to bring about the chemical reaction is called molecularity of the reaction. The molecularity of a reaction can be 1, 2, or 3. the decomposition of ammonium nitrate could be termed as a unimolecular reaction. The reactions involving simultaneous collision between two species is a bimolecular reaction like the dissociation of HI. In the same way, reactions involving collision of three species may be termed as trimolecular reactions. So in simple terms, chemical reaction mlecularity is simply the sum of molecules of the different reactants as represented by the balanced chemical equation in an elementary reaction.