Question

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

Short answer

A unimolecular step is a single-step chemical reaction involving one reactant molecule in the rate-determining step, such as isomerization or dissociation. A bimolecular step involves the collision of two reactant molecules in the rate-determining step, resulting in an exchange of atoms between them. Termolecular steps are infrequent because they require the simultaneous collision of three reactant molecules with appropriate orientation and energy, which has a significantly lower probability compared to unimolecular or bimolecular steps. Additionally, the reaction is more likely to proceed through simpler steps.

Step-by-step solution

Define Unimolecular Step

A unimolecular step is a single-step chemical reaction in which only one reactant molecule is involved in the rate-determining step. These reactions typically involve isomerization, dissociation, or a rearrangement of atoms within a molecule. An example of a unimolecular reaction is the isomerization of cis-butene to trans-butene.

Define Bimolecular Step

A bimolecular step is a single-step chemical reaction in which the rate-determining step involves the collision of two reactant molecules. These reactions typically involve an exchange of atoms between the two reactants, resulting in two new products. An example of a bimolecular reaction is the reaction between hydrogen and iodine molecules to form hydrogen iodide (\(H_2 + I_2 \rightarrow 2HI\)).

Explain Infrequency of Termolecular Steps

Termolecular steps are infrequently seen in chemical reactions because they require the simultaneous collision of three reactant molecules with the appropriate orientation and energy. The probability of such collisions occurring is significantly lower than for unimolecular or bimolecular steps, making termolecular steps much less frequent in most chemical reactions. In addition, due to the increased complexity of the termolecular step, it is more likely that the reaction proceeds through a series of unimolecular and bimolecular steps instead.

Key concepts

Unimolecular Reactions

Unimolecular reactions are fascinating events in the world of chemistry, characterized by the transformation of a single molecule. This type of reaction occurs when a compound undergoes a rearrangement within itself. Imagine just one molecule shuffling its atoms around or breaking down into smaller pieces. This can happen due to internal molecular motions. A common scenario is isomerization, where one isomer transforms into another, but without any outside help.

Unimolecular reactions often occur when a molecule absorbs energy and then uses that energy to rearrange its structure. The energy could come from light, heat, or even a catalytic surface. An everyday example is when a compound changes its structure, say from "cis" to "trans" without adding anything else into the mix.

• Simple: Only one molecule changing on its own.
• Common types: Isomerization, decomposition, or structural rearrangements.
• Activation energy needed: Energy intake to trigger the change.

Bimolecular Reactions

Bimolecular reactions bring two reactant molecules together to interact, often through the sharing or swapping of atoms. These reactions are like social events, where two parties meet and exchange valuable parts, resulting in different and often new molecules. This type of reaction is more common than termolecular reactions because it involves just two reactants, increasing the likelihood of collision and interaction.

When two molecules collide with the right orientation and sufficient energy, a bimolecular reaction occurs. A classic example is the formation of water from hydrogen and oxygen molecules. This involves hydrogen molecules reacting with each other, swapping parts, and forming something entirely new.

• Two molecules meet and react.
• Collisions must have correct orientation and energy.
• Examples include many synthesis and decomposition reactions.

Termolecular Reactions

Termolecular reactions are the celestial alignments of chemical reactions, needing three molecules to collide and react with one another simultaneously. They are a rarity due to the improbability of three molecules perfectly colliding at once with the needed energy and orientation. This makes them far less common than unimolecular and bimolecular reactions.

Even though they are rare, termolecular reactions serve an educational purpose; they illustrate the necessity of perfect conditions for certain reactions to happen. Instead of relying on the simultaneous clash of three molecules, such processes are more likely to occur through sequential unimolecular or bimolecular reaction steps.

• Requires simultaneous collision of three reactant molecules.
• Less frequent due to low probability of successful collisions.
• Often broken down into simpler steps for practical occurrences.