What Is a Second Order Reaction in Chemistry?

A hand combining chemicals from two different flasks
Chemical combination.

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A second order reaction is a type of chemical reaction that depends on the concentrations of one-second order reactant or two first-order reactants. This reaction proceeds at a rate proportional to the square of the concentration of one reactant or the product of the concentrations of two reactants. How fast the reactants are consumed is called the reaction rate. This reaction rate for a general chemical reaction

aA + bB → cC + dD

can be expressed in terms of the concentrations of the reactants by the equation:

rate = k[A]x[B]y

k is a constant
[A] and [B] are the concentrations of the reactants
x and y are the orders of the reactions determined by experimentation and not to be confused with the stoichiometric coefficients a and b.

The order of a chemical reaction is the sum of the values x and y. A second order reaction is a reaction where x + y = 2. This can happen if one reactant is consumed at a rate proportional to the square of the reactant's concentration (rate = k[A]2) or both reactants are consumed linearly over time (rate = k[A][B]). The units of the rate constant, k, of a ​second-order reaction are M-1·s-1. In general, second-order reactions take the form:

2 A → products
A + B → products.

10 Examples of Second Order Chemical Reactions

This is a list of ten-second order chemical reactions.

Note that some reactions are not balanced. This is because some reactions are intermediate reactions of other reactions. The listed reactions are all second order.

H+ + OH- → H2O
Hydrogen ions and hydroxy ions form water.

2 NO2 → 2 NO + O2
Nitrogen dioxide decomposing into nitrogen monoxide and oxygen molecule.

2 HI → I2 + H2
Hydrogen Iodide decomposing into iodine gas and hydrogen gas.

O + O3 → O2 + O2
During combustion, oxygen atoms and ozone can form oxygen molecules.

O2 + C → O + CO
Another combustion reaction, oxygen molecules react with carbon to form oxygen atoms and carbon monoxide.

O2 + CO → O + CO2
This reaction often follows the previous reaction. Oxygen molecules react with carbon monoxide to form carbon dioxide and oxygen atoms.

O + H2O → 2 OH
One common product of combustion is water. This, in turn, can react with all the loose oxygen atoms produced in the previous reactions to form hydroxides.

2 NOBr → 2 NO + Br2
In the gas phase, nitrosyl bromide decomposes into nitrogen oxide and bromine gas.

Ammonium cyanate in water isomerizes into urea.

An example of the hydrolysis of an ester in the presence of a base. In this case, ethyl acetate in the presence of sodium hydroxide.