Question

Explain why termolecular reactions are rare.

Short answer

Termolecular reactions (those involving a simultaneous collision of three molecules) are rare due to statistical improbability and higher requisite activation energy. The chances of three molecules colliding simultaneously in the correct orientation and with enough energy are significantly lower than for a bimolecular reaction.

Step-by-step solution

Understanding Termolecular Reactions

Firstly, it's important to understand what termolecular reactions are. A termolecular reaction is a collision event involving three molecules. Typically, a reaction proceeds by a combination of two molecules coming together, so a reaction involving three is understandably less common.

Computational Probability

Secondly, consider the probability. The probability of three particles colliding at the same time is less than the probability of two particles colliding. This is a result of simple statistical reasoning. It's just much less likely for three particles to be in the same place at the same time, moving in the right directions, with the right amounts of energy.

Activation Energy

Thirdly, consider the activation energy required for termolecular reactions. Termolecular reactions require a significantly higher activation energy than bimolecular reactions. This is because three molecules need to come together and collide with the appropriate orientation and energy. Thus, termolecular reactions are less likely to occur.

Key concepts

Chemical Kinetics

Chemical kinetics is the branch of physical chemistry that studies the rates of chemical processes. It examines how different experimental conditions can influence the speed of a chemical reaction and yields information about the reaction's mechanism and transition states.

For termolecular reactions, which involve three reactant molecules coming together to form products, the kinetics are more complex. The higher complexity results from the need for three separate entities to collide simultaneously in the correct orientation. The rarity of such events reflects in the overall slower reaction rates for termolecular processes compared to bimolecular or unimolecular reactions, where only two or one reactant molecules, respectively, are involved.

Reaction Mechanism

The reaction mechanism describes the step-by-step sequence of elementary reactions by which overall chemical change occurs. In the study of termolecular reactions, which are rarely observed, the suggested mechanism involves a sequence of bimolecular steps rather than a simultaneous collision of all three molecules.

This implies that termolecular reactions may actually proceed through a more probable sequential mechanism, where two molecules first react to form an intermediate, which then quickly reacts with a third molecule. This mechanism is more consistent with the observed kinetics of most chemical reactions and helps to explain why true termolecular reactions are so uncommon.

Activation Energy

Activation energy (often represented as \(E_a\)) is the minimum quantity of energy that the reacting species must possess in order to undergo a specified reaction. In termolecular reactions, the activation energy is usually higher than in bimolecular reactions, because all three reacting molecules must simultaneously have enough energy to overcome their individual activation barriers.

This synchronicity is statistically unlikely and adds to the rarity of these reactions. These reactions also require appropriate spatial orientation, making it even harder for three molecules to have successful collisions that lead to product formation.

Collision Theory

Collision theory provides an explanation of how chemical reactions occur and why reaction rates differ for different reactions. The theory states that for a reaction to proceed, reactant particles must collide, and only a certain fraction of the total collisions cause a chemical change; these are termed 'effective collisions'.

Effective collisions only happen if the particles have the requisite activation energy and the correct orientation upon impact. For termolecular reactions, the likelihood of three molecules meeting these criteria at the same time is exceptionally low, which explains their rarity. The necessity for an efficient collision in these reactions contributes to the significant difficulty in achieving termolecular reaction conditions.