Chemical reactions often take place in a series of steps called the reaction mechanism. In a multi-step reaction, the slowest step is called the rate-determining step. The reaction cannot go faster than the rate-determining step. If the concentration of a reactant appears in the rate equation, then the reactant or something derived from it takes part in the slow step of the reaction. Nitrogen dioxide reacts with carbon monoxide to form nitrogen monoxide and carbon dioxide.NO2 + CO → NO + CO2The result of rate experiments gives the rate equation: rate = k[NO2]2 This means that two molecules of NO2 are involved in the rate-determining (slow) step. NO3 is an intermediate as it does not appear in the overall equation. Intermediates are usually short-lived.