Order of Reaction. Reaction order and velocity law

The order of a chemical reaction is the mathematical relationship that exists between the rate of development or speed of the reaction and the concentration in quantity of matter in the reactants.

For example, consider the non-elementary reaction covered in the text Velocity law for non-elementary reactions:

CO + NO₂ → CO₂ + NO

The law of speed of this reaction is given by:

v = k. [AT THE₂]²

We have that, in this case, the global order of this reaction will be equal to 2, the order of the reaction in relation to the NO reagent₂ it will also be equal to 2 and, with respect to reagent CO, it will be equal to zero.

We reach this conclusion because the order of the global reaction is the sum of the exponents of the reactant concentrations in the law. of the speed and order of the reaction in relation to a given reactant is the exponent of its concentration in the law of velocity.

For example, consider that we have the following generic reaction:

aA + bB → cC + dD

Since the law of speed of this reaction is:

v = k. [THE]^α. [B]^β

We say that this generic reaction is:

• α order with respect to A;
• Order β with respect to B;

α + β = global order of reaction

See two more examples:

1) Elementary reaction: 2 CO_(g) + O_2(g)→ 2 CO_2(g)

Since it is an elementary reaction, the exponent in the law of velocity is equal to the coefficient of the reactant: v = k. [CO]². [O₂]¹

So, we have that this reaction is of order 2 in relation to the CO, of order 1 in relation to the O₂ and its global order is 3 (2 + 1 = 3).

2) Non-elemental reaction: 2 hours_2(g) + 2 NO_(g)→ 1 N_2(g) + H₂O_(g)

The steps of this reaction are given by:

Stage 1 (slow): 1 hour_2(g) + 2 NO_(g) → 1 N₂O_g) + 2 H₂O₍₁₎
Step 2 (quick): 1 N₂O_g) + 2 H₂O₍₁₎ → 1 N_2(g) + 2 H₂O₍₁₎

Global equation:2 hours_2(g) + 2 NO_(g) → 1 N_2(g) + 2 H₂O₍₁₎

The exponents in the velocity law are given by the coefficients of the reagents in the slow stage: v = k[H₂]¹. [AT THE]².

This reaction is from 1ª order in relation to the H₂, in 2ª order in relation to NO and of 3ª order in relation to the global reaction (sum of exponents: 1+2 = 3).

One NoteImportantly, there are some reactions that take place on the surface of metals that are zero order, because the rate does not depend on the concentration of the reactants.

By Jennifer Fogaça
Graduated in Chemistry