Order of reaction and molecularity

Order of reaction and molecularity-

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Order of reaction –

“It is given by the number of atoms or molecules whose concentration changes during a chemical reaction.”

OR

“It is given by the number of concentration terms of atoms or molecules which determine the rate law.”

For ex.-

a) Reaction is said to be first order , if

r = dx / dt = KCA

b) Reaction is said to be second order , if

r = dx / dt = KCA² or KCA.CB

c) Reaction is said to be third  order , if

r = dx / dt = KCA³ or KCA².CB or KCA.CB²

n₁ A + n₂ B + n₃ D ——-> product

A , B  and D are reactants

dx/dt = KC_Aⁿ¹ .C_Bⁿ².C_Dⁿ³

K = constant

n = n₁ + n₂ + n₃

n is order of rxn.

” Order of reaction may also be defined as , the sum of the powers to which the concentration terms of the reactants are raised in order to express the reaction rate.”

Molecularity-

” It is equal to the sum of the number of molecules of various reactants that take part in a chemical reaction as represented by a balanced chemical equation.”

If molecularity is one then  reaction is unimolecular.

If molecularity is two & three  then  reaction is bimolecular and trimolecular or termolecular respectively.

Ex.1) Inversion of cane sugar-

C₁₂H₂₂O₁₁ + H₂O (excess) —————-> C₆H₁₂O₆ + C₆H₁₂O₆

order of rxn. = 1

molecularity = 2 (bimolecular)

Ex.2) Hydrolysis of methyl acetate-

CH₃COOCH₃ + H₂O (excess) ————> CH₃COOH + CH₃OH

order of rxn. = 1

molecularity = 2 (bimolecular)

Difference between order of reaction and molecularity-

Order of reaction –

1) “It may also be defined as , the sum of the powers to which the concentration terms of the reactants are raised in order to express the reaction rate.”

2)It may be whole number , zero or fractional value .

3) It is determined experimentally , it is determined by rate for overall reaction.

4) It can be obtained from a balanced chemical equation.

Molecularity-

1) ” It is equal to the sum of the number of molecules of various reactants that take part in a chemical reaction as represented by a balanced chemical equation.”

2) It is always a whole number.

3) It is a theoretical concept , it depends upon the rate determining step in the reaction mechanism.

4) It is obtained from a balanced chemical equation.