Question

Define what is meant by unimolecular and bimolecular steps. Why are termolecular steps infrequently seen in chemical reactions?

Short answer

Unimolecular and bimolecular steps are elementary reactions involving one or two reactant molecules, respectively. In a unimolecular step, a single reactant undergoes a change to produce products, while in a bimolecular step, two reactant molecules collide and react to form products. Termolecular steps, which involve three reactant molecules colliding, are infrequent in chemical reactions due to the low probability of simultaneous collisions with the correct orientation and sufficient energy, as well as their higher activation energy barriers.

Step-by-step solution

Define Unimolecular and Bimolecular Steps

Unimolecular and bimolecular steps describe different types of elementary reaction rates based on the number of reactant molecules involved in the process. A unimolecular step is an elementary reaction in which a single reactant molecule is involved. It undergoes a change to produce one or more products. The rate of a unimolecular reaction depends solely on the concentration of that single reactant. An example of a unimolecular reaction is the isomerization of cyclopropane to propene. A bimolecular step is an elementary reaction in which two reactant molecules are involved. These two molecules collide with each other and, providing they have sufficient energy and orientation, they undergo a change to produce one or more products. The rate of a bimolecular reaction depends on the concentration of each of the two reacting species. An example of a bimolecular reaction is the reaction between hydrogen and iodine molecules to produce hydrogen iodide.

Termolecular Steps and Their Rare Occurrence

Termolecular steps are elementary reactions that involve three reactant molecules colliding with one another and directly participating in the reaction to form products. These reactions are infrequent in comparison to unimolecular and bimolecular steps because the probability of three molecules colliding simultaneously with the correct orientation and sufficient energy is very low. Moreover, termolecular steps generally have higher activation energy barriers than their unimolecular and bimolecular counterparts. The combined effect of lower probability of three-body collisions and increased activation energy requirements result in a significantly lower reaction rate, making termolecular steps very rare in chemical reactions.