Rate Law and Molecularity Law
Rate law
Rate law gives the actual rate of reaction. In case of a complex reaction means which processed in two or more steps and the Rate-law corresponds to the slowest step of the reaction.
We see that for complex reactions law of mass action and rate law give different expressions for the reaction rate.
Important
For simple reaction: the law of mass action and rate law generally give the same expression for the reaction rate.
The slowest step in complex reactions gives the actual rate of reaction.
Rate law expression is useful in finding the order of reaction “rate law is the mathematical expression which denotes the experimentally observed rate of reactions in terms of the molar concentration of the reactant species at a given temperature.
Molecularity of reactions
Molecularity of a reaction is defined as the number of reactant species which collide simultaneously to brings about a chemical reaction.
Characteristics of molecularity
Molecularity of pertains to a single step reaction or a single step of a complex reaction which involves a number of steps for its completion. Molecularity of a reaction as a whole has no meaning.
Molecularity is always a whole number and is never infraction or zero.
The molecularity of a single step reaction or a step of a complex reaction does not exceed three.
Reactions with Molecularity equal to three are also rare. The simultaneous collision of the molecules means that the third molecule must collide the other two molecules at the same time when they are in the process of collision. The chances for the occurrence of such collisions are very rare. It has been found that there are certain reactions which involve a large number of species but still, they are very fast.
Such a reaction cannot take place in one step and are said to proceed through a sequence of two or more consecutive steps and thus called complex reactions.
Order of reaction
It is defined as the sum of the exponents to which the molar concentration terms in the rate law are raised to express the observed rate of a reaction.
Consider a single step reaction : mA+ nBžproduct
For this, the order of reaction = m+n
Comparison of molecularity and order of reaction
Molecularity
Its value does not exceed three.
Depends upon the conditions of a reaction
Molecularity is the number of molecules of the reactant species taking part in a single step chemical reaction.
It is always a whole number.
Molecularity pertains to one step reaction which involves several steps for its completion.
It can be obtained from a single balanced chemical equation.
It does not change with the change in temp. and pressure.
It also does not exceed three.
It also depends upon conditions of a reaction.
Order of reaction is the sum of exponents to which molar concentrations of the reactants are raised in the rate law equation of a chemical reaction.
Order of reaction pertains to reactions as a whole irrespective of the intermediate steps involved for its completion.
It cannot be obtained from a single balanced chemical equation and can be determined only experimentally.
It changes with the change in temperature and pressure.
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