Choose the correct statement.

Reactions with molecularity three are very rare.

The correct answer is Option (1) → Reactions with molecularity three are very rare.

Molecularity of a reaction is defined as the number of reacting species (atoms, ions, or molecules) that collide simultaneously in an elementary reaction step.

Molecularity is always a whole number (1, 2, or rarely 3) because it represents the number of particles participating in a single elementary collision.

In most chemical reactions:

• Unimolecular reactions involve one molecule
• Bimolecular reactions involve collision of two molecules
• Termolecular reactions involve simultaneous collision of three molecules

The probability that three molecules collide at the same time with proper orientation and sufficient energy is extremely low. Therefore, termolecular reactions are very rare.

Thus, the statement that reactions with molecularity three are very rare is correct.

Option-wise Explanation

Option 1: Reactions with molecularity three are very rare.

This is correct. Termolecular reactions require simultaneous collision of three molecules, which has a very low probability, making such reactions rare.

Option 2: Order of reaction cannot have a fraction value.

This is incorrect. The order of reaction can be fractional or even zero, depending on the experimentally determined rate law.

Option 3: Molecularity of a reaction is meaningful for complex reactions only.

This is incorrect. Molecularity is defined only for elementary reactions, not for complex reactions that occur in multiple steps.

Option 4: Order of a reaction is always determined theoretically.

This is incorrect. The order of reaction is determined experimentally, based on the rate law obtained from experiments.