Most reaction do not proceed in a single step; instead, they go through a series of steps. Each of these steps is called an elementary reaction and is directly caused by the collisions of the molecules and ions in the reactants.

For example, conside the reaction of NO and Cl₂ to form NOCl at low temperature. This reaction occurs in two elementary steps: the first is fast, the second is slow.

NO(g) + Cl₂(g) < = > NOCl₂(g)    (fast)

NOCl₂(g) + NO -> 2NOCl    (slow)

---

2NO(g) + Cl₂ -> 2NOCl

While in general we cannot write down the rate expression for a reaction from looking at the balanced chemical equation, we can do this for an elementary reaction. The rate expression for an elementary reaction is just the rate constant k for the reaction multiplied by the concnetrations of the reactants raised to the power of the coefficient in the balanced chemical equation.

In our example above, we have two elementary reactions. By the above rule, the rate expression for each is

• NO + Cl₂ - > NOCl₂    rate = k₁[NO][Cl₂]
• NOCl₂(g) + NO -> 2NOCl    rate = k₂[NOCl][NO]

Example: The reaction

2NO₂ -> 2NO + O₂

Solution: Follow the above rule. The only reactant is NO₂, and since it's coefficient in the balanced reaction is 2, the rate expression must be

rate = k[NO₂]²

Information provided by: http://learn.chem.vt.edu