Elementary Processes or Steps
An elementary process is also called an elementary step or elementary reaction. It expresses how molecules or ions actually react with each other. The equation in an elementary step represents the reaction at the molecular level, not the overall reaction. Based on numbers of molecules involved in the elementary step, there are three kinds of elementary steps: unimolecular step (or process), bimolecular process, and trimolecular process.
An elementary step is proposed to give the reaction rate expression. The rate of an elementary step is always written according to the proposed equation. This practice is very different from the derivation of rate laws for an overall reaction.
When a molecule or ion decomposes by itself, such an elementary step is called a unimolecular step (or process). A unimolecular step is always a first order reaction. The following examples are given to illustrate this point:
\(\ce{O3 \rightarrow O2 + O}\), \(Rate = k \ce{[O3]}\)
or in general
\(\ce{A \rightarrow B + C + D}\), \(Rate = k \ce{[A]}\)
\(\mathrm{A^* \rightarrow X + Y}\), \(Rate = k \mathrm{[A^*]}\)
\(\mathrm{A^*}\) represents an excited molecule.
A bimolecular process involves two reacting molecules or ions. The rates for these steps are 2nd order, and some examples are given to illustrate how you should give the rate expression. The simulation illustrates a bimolecular process.
\(\ce{NO + O3 \rightarrow NO2 +O2}\), \(Rate = k \ce{[NO] [O3]}\)
\(\ce{Cl + CH4 \rightarrow HCl + CH3}\), \(Rate = k \ce{[Cl] [CH4]}\)
\(\mathrm{Ar + O_3 \rightarrow Ar + O_3^*}\), \(Rate = k \ce{[Ar] [O3]}\)
\(\ce{A + A \rightarrow B + C}\), \(Rate = k \ce{[A]^2}\)
\(\ce{A + B \rightarrow X + Y}\), \(Rate = k \ce{[A] [B]}\)
A trimolecular process involves the collision of three molecules. For example:
\(\ce{O + O2 + N2 \rightarrow O3 + N2}\), \(Rate = k \ce{[O] [O2] [N2]}\)
\(\ce{O + NO + N2 \rightarrow NO2 + N2}\), \(Rate = k \ce{[O] [NO] [N2]}\)
The \(\ce{N2}\) molecules in the above trimolecular elementary steps are involved with energy transfer. They can not be canceled. They are written in the equation to give an expression for the Rates. In general, trimolecular steps may be,
\(\ce{A + A + A \rightarrow products}\), \(Rate = k \ce{[A]^3}\)
\(\ce{A + A + B \rightarrow products}\), \(Rate = k \ce{[A]^2 [B]}\)
\(\ce{A + B + C \rightarrow products}\), \(Rate = k \ce{[A] [B] [C]}\)
Three molecules colliding at an instant is rare, but occasionally these are some of the ways reactions take place.
Elementary processes are written to show how a chemical reaction progresses leading to an overall reaction. Such a collection is called a reaction mechanism. In a mechanism, elementary steps proceed at various speeds. The slowest step is the rate-determining step. The order for that elementary process is the order of a reaction, but the concentrations of reactants in that step must be expressed in terms of the concentrations of the reactants.