Defining Elementary Reactions
An elementary reaction is a chemical reaction that occurs in a single step, exactly as written in the stoichiometric equation. Unlike overall reactions, which can involve multiple steps, an elementary reaction describes the actual molecular event that takes place. It represents the smallest division of a chemical process.
Molecularity and Reaction Order
The molecularity of an elementary reaction refers to the number of reactant molecules, atoms, or ions that collide and react in that single step. It can be unimolecular (one reactant), bimolecular (two reactants), or termolecular (three reactants). For an elementary reaction, the reaction order with respect to each reactant is equal to its stoichiometric coefficient in the balanced equation, and the overall order is the sum of these coefficients.
Example: Bimolecular Reaction
A common example is a bimolecular reaction where two molecules collide directly, such as the initial step in the decomposition of ozone: O₃ + Cl → ClO + O₂. In this specific elementary step, one ozone molecule and one chlorine atom collide. Its rate law would be Rate = k[O₃][Cl].
Importance in Reaction Mechanisms
Understanding elementary reactions is crucial for elucidating reaction mechanisms, which are the series of individual steps that constitute an overall chemical reaction. By identifying and studying elementary steps, chemists can determine the rate-determining step, predict how changes in reactant concentrations affect reaction rates, and gain deeper insights into how reactions proceed at the molecular level.