A reaction mechanism is a description of the path, or sequence of steps, by which a reaction occurs.

The simplest is a One-step reaction.

example:

(above 600 K)    CO + NO₂ ----> NO + CO₂

(below 500 K, however)


Step 1. NO₂ + NO₂ ----> NO₃ + NO
Step 2. NO₃ + CO ----> NO₂ + CO₂

Together they give:
CO + NO₂ ----> NO + CO₂

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How can the rate expression be determined from the reaction mechanism?

1. For any step, the order with respect to each reactant is its coefficient in the chemical equation for that step. Considering the reaction between NO₂ and CO described above:

Step 1. NO₂ + NO₂ ----> NO₃ + NO
rate = k₁(conc NO₂)(conc NO₂ = k₁(conc NO₂)²

Step 2. NO₃ + CO ----> NO₂ + CO₂
rate = k₂(conc NO₃)(conc CO)

2. Often, one step in a mechanism is much slower than any other. In such cases, the slow step determines the rate of the overall reaction.

Step 1. in the reaction above is much slower than that of Step 2, so the rate of the overall reaction is that of the first step.
rate = k₁(conc NO₂)²

Experimentally, at low temperatures, the reaction between CO and NO₂ is 2nd order with respect to NO₂ and zero order with respect to CO.

At high temperatures, the reaction is actually single step and is first order in NO₂ and CO.

3.The final rate expression must include only those species that appear in the balanced equation for the overall reaction. Reactive intermediates must be eleminated from the rate expression.

example

Step 1. NO + Cl₂ <----> NOCl₂ (fast)
Step 2. NOCl + NO ----> 2NOCl (slow)
Together they give:

2NO + Cl₂ ----> 2NOCl

NOCl₂ is an unstable intermediate in equilibrium with NO and Cl₂.
An NOCl₂ molecule reacts with another molecule of NO in a slow rate-determining step.

rate = k(conc NOCl₂)(conc NO)