Question

(i) Write any two differences between order and molecularity. 
(ii) What do you mean by pseudo order reaction?

• A.
• B.
• C.
• D.

Hint:

Molecularity is only defined for elementary (single-step) reactions, whereas order applies to both elementary and complex reactions.

Answer 1

The Correct Option is A

Concept:
Order and molecularity describe different aspects of a reaction's mechanism. Pseudo-order reactions occur when the concentration of one reactant remains effectively constant.

Step 1: Differences between Order and Molecularity.

Order of Reaction:
• Sum of powers of concentration in the rate law.
• Determined experimentally.
• Can be zero or fractional.

Molecularity:
• Number of reacting species involved in an elementary step.
• A theoretical concept.
• Always a natural whole number (1, 2, 3).

Step 2: Pseudo First Order Reaction.
Consider the hydrolysis of cane sugar:

C12H22O11 + H2O → C6H12O6 + C6H12O6 (in presence of H+)

Although the molecularity is 2, water is present in large excess, so its concentration remains almost constant. Therefore, the rate depends only on the concentration of sugar.

Rate = k [C12H22O11]

Such reactions are called pseudo first order reactions.