Reactions of higher order (>3) are very rare due to very less chances of many molecules to undergo effective collisions.

Explanation:

Introduction:
Higher order reactions, also known as bimolecular or termolecular reactions, involve the simultaneous collision of more than two reactant molecules. These reactions are relatively rare compared to first and second order reactions, which involve the collision of two molecules.

Reasons for the rarity of higher order reactions:

1. Low probability of simultaneous collision:
In higher order reactions, all the reacting species must collide simultaneously for the reaction to occur. This simultaneous collision can be quite rare, especially when more than three molecules are involved. As the number of molecules increases, the probability of all of them colliding at the same time decreases significantly. Therefore, the likelihood of a higher order reaction taking place becomes very low.

2. Shifting of equilibrium towards reactants:
In a chemical reaction, the system tends to shift towards the side with fewer molecules in order to achieve a lower energy state. In higher order reactions, the number of reactant molecules is higher than the number of product molecules. As a result, the equilibrium position of the reaction tends to favor the reactants. This makes the forward reaction less favorable, further reducing the likelihood of a higher order reaction occurring.

3. Loss of active species on collision:
In higher order reactions, multiple collisions are required for the reaction to proceed. However, each collision between reactant molecules has a certain probability of resulting in the loss of an active species. This can happen due to factors such as energy dissipation, change in molecular orientation, or formation of weakly bound complexes. As the number of collisions increases, the probability of losing active species also increases. This reduces the overall efficiency of the reaction, making higher order reactions less favorable.

4. Increase in entropy and activation energy:
As the number of molecules involved in a reaction increases, the entropy of the system also increases. This increase in entropy makes the reaction less favorable, as it goes against the natural tendency of the system to decrease its entropy. Additionally, higher order reactions generally have higher activation energies compared to lower order reactions. The higher activation energy makes it more difficult for the reaction to overcome the energy barrier and proceed to the products.

Conclusion:
In summary, higher order reactions are rare due to the low probability of simultaneous collision of all the reacting species, the shifting of equilibrium towards the reactants, the loss of active species on collision, and the increase in entropy and activation energy as more molecules are involved. These factors make higher order reactions less favorable and less likely to occur compared to lower order reactions.