Chemical kinetics studies how fast reactions occur and the pathways molecules take when transforming from reactants into products. Although reactions may appear simple, the transformation at the molecular level is often a complex, multi-stage process. To understand this complexity, chemists break down the overall reaction into its most fundamental parts. The simplest unit of this molecular pathway is the elementary reaction. This foundational concept allows for a precise description of chemical change.
Defining the Elementary Reaction
An elementary reaction is a chemical process that occurs in a single, concerted step. It proceeds without any detectable intermediate species being formed or consumed during the transformation. This single step involves reactants transitioning directly to products, passing through only one high-energy arrangement of atoms called the transition state. The entire reaction event is viewed as a single, effective collision between the reacting species.
A reaction is classified as elementary only if it genuinely proceeds in this single-step manner. This means the reactants must collide with sufficient energy and the correct orientation to immediately form the products in that one collision event. If intermediates are observed or must be proposed, the reaction is not elementary.
Understanding Molecularity
Molecularity is a classification system for elementary reactions. It defines the number of reactant molecules or atoms involved in the single collision step, and this number must be an integer. The three main classifications are unimolecular, bimolecular, and termolecular (or trimolecular).
A unimolecular reaction involves one reactant molecule rearranging its atoms or dissociating to form products. Bimolecular reactions are the most common, occurring when two reactant species collide and react. The molecularity is the sum of the stoichiometric coefficients of the reactants in the elementary step.
Termolecular reactions involve the simultaneous collision of three reactant species. These reactions are statistically rare because the probability of three particles colliding at the exact same point in space and time is extremely low. Most reactions that appear termolecular are actually a sequence of more probable unimolecular and bimolecular steps.
Rate Laws and Elementary Reactions
The concept of an elementary reaction is mathematically significant because it provides the only scenario where the rate law can be directly deduced from the balanced equation. The rate law relates the reaction rate to the concentrations of the reactants. For general reactions, the exponents in the rate law (reaction orders) must be determined experimentally, and they often do not match the stoichiometric coefficients.
For an elementary reaction, the reaction order for each reactant exactly corresponds to its stoichiometric coefficient in the balanced elementary equation. This direct relationship exists because the elementary equation precisely describes the single collision event that determines the reaction rate.
For example, if the elementary reaction is \(A + B \rightarrow P\), the rate law is simply written as Rate \(= k[A][B]\). If the elementary reaction is \(2A + B \rightarrow P\), the rate law is Rate \(= k[A]^2[B]\), showing the reaction is second-order with respect to \(A\). This one-to-one correspondence between stoichiometry and reaction order is the unique property of elementary reactions. If the experimental rate law for an overall reaction does not match the stoichiometry, it is immediate evidence that the reaction is not elementary but proceeds through multiple steps.
Elementary Reactions within Complex Mechanisms
Most observable chemical changes, often called complex or multistep reactions, are composed of a sequence of two or more elementary steps. This sequence of individual steps is referred to as the reaction mechanism. Within this mechanism, chemical species called intermediates are formed in one elementary step and then immediately consumed in a subsequent step.
The overall speed of a complex reaction is governed by the slowest elementary step in the entire mechanism, which is known as the rate-determining step. This slowest step acts like a bottleneck, limiting how quickly the final products can be formed. The experimentally determined rate law for the overall complex reaction is dictated by the rate law of this single, slowest elementary step.
Understanding the individual elementary steps allows chemists to propose and validate a reaction mechanism that accurately explains the observed overall reaction rate. While the stoichiometry of the overall reaction equation rarely predicts the rate law, the stoichiometry of the individual rate-determining elementary step always does. The elementary reaction serves as the fundamental building block for modeling and predicting the kinetics of all chemical reactions.