Molecularity of chemical reaction & how it differs from Order

Molecularity of a reaction:

The total number of molecules taking part in the rate determining step as represented by the simple chemical equation, called Molecularity of the reaction. For examples, hydrolysis of ethyl acetate as represented by the equation-

 

CH3COOC2H5   +   H2O ———–> CH3COOH  +  C2H5OH

Ethyl acetate                                     Acetic acid     Ethyl alcohol

The molecularity of this reaction is two i.e. bimolecular.

Difference between molecularity and order of reaction: Some of the important points of distinction of molecularity and order of reaction are given below:

Molecularity of reactionOrder of reaction
1)    Molecularity is the number of reacting species undergoing simultaneous collision in the reaction.

2)    It is theoretical concept.

3)    Molecularity can have only integral value. It cannot be zero,

4)    Molecularity does not change with change in temperature, pressure.

5)    It does not tell us anything about reaction mechanism of reaction.

 

1)    Order is the sum of the power of concentration terms in the rate law expression.

2)    It is determine experimentally.

3)    Order of a reaction can have zero and fractional values.

4)    It changes with change in temperature and pressure.

5)    It tells us about reaction mechanism of reaction.

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