Problem 11 - Chemistry An Atoms First Approach

Question

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

Step-by-Step Solution

Answer

Unimolecular steps involve a single molecule undergoing a change or reaction and follow first-order kinetics, with a general form of \(A \rightarrow P\). Bimolecular steps involve two molecules interacting or colliding to form a product or undergo a change and follow second-order kinetics, with a general form of \(A + B \rightarrow P\). Termolecular steps, which involve the simultaneous collision of three molecules, are infrequently seen in chemical reactions due to the low probability of simultaneous collisions, high activation energies, and the formation of complex and unstable intermediates.

Step 1: Definition of Unimolecular Steps

Unimolecular steps involve a single molecule undergoing a change or a reaction. These steps can be described by first-order kinetics, as the rate of the reaction depends on the concentration of the single reacting molecule. The general form of a unimolecular step is \(A \rightarrow P\), where A represents the initial molecule and P represents the product(s).

Step 2: Definition of Bimolecular Steps

Bimolecular steps involve two molecules interacting or colliding with each other to form a product or undergo a change. These steps can be described by second-order kinetics, as the rate of the reaction depends on the concentration of both reacting molecules. The general form of a bimolecular step is \(A + B \rightarrow P\), where A and B are the initial molecules and P represents the product(s).

Step 3: Infrequent Observation of Termolecular Steps

Termolecular steps involve the simultaneous collision of three molecules to form a product or undergo a change. These steps are infrequently seen in chemical reactions for several reasons. First, the probability of three molecules colliding at the same time and the proper orientation is very low. Second, termolecular steps usually have very high activation energies, making these reactions difficult to achieve under normal conditions. Finally, termolecular steps often involve complex and unstable intermediates, which may break down or react in alternate pathways before forming the desired product. Due to these factors, termolecular steps are less common compared to unimolecular and bimolecular steps in chemical reactions.