Question

Explain order and molecularity of a reaction giving example.

Hint:

Order is derived from rate law; molecularity refers to the mechanism of a single step. Order can be zero or fractional, molecularity cannot.

Answer 1

Order of a reaction:
- The order is the sum of the powers of concentration terms in the experimentally determined rate law.
- It is determined experimentally, not from the balanced equation.
Example: For hydrolysis of an ester: \[ \mathrm{CH_3COOC_2H_5 + H_2O \;\longrightarrow\; CH_3COOH + C_2H_5OH} \] Rate law: \(r = k[\mathrm{CH_3COOC_2H_5}]^1[\mathrm{H_2O}]^0 = k[\mathrm{CH_3COOC_2H_5}]\).
Thus, the reaction is first order.
Molecularity of a reaction:
- It is the number of molecules or species taking part in an elementary step of the reaction mechanism.
- Molecularity is always a whole number (1, 2, 3 …).
Example: Unimolecular decomposition: \[ \mathrm{N_2O_5 \;\longrightarrow\; 2NO_2 + \tfrac{1}{2}O_2} \] Here, molecularity = 1.
\[ \boxed{\text{Order is experimental and may be fractional; molecularity is theoretical and always an integer.}} \]