Example of a complex reaction: Reaction of hydrogen and nitric

1. 
   2 NO(g) ⇌ N₂O₂(g) (fast equilibrium)
   

2. 
   N₂O₂ + H₂ → N₂O + H₂O (slow)
   
3. 
   N₂O + H₂ → N₂ + H₂O (fast)
   

Reactions 1 and 3 are very rapid compared to the second, so the slow reaction 2 is the rate determining step. This is a bimolecular elementary reaction whose rate is given by the second order equation:

where k₂ is the rate constant for the second step.
However N₂O₂ is an unstable intermediate whose concentration is determined by the fact that the first step is in equilibrium, so that [N₂O₂] = K₁[NO]² , where K₁ is the equilibrium constant of the first step. Substitution of this equation in the previous equation leads to a rate equation expressed in terms of the original reactants
This agrees with the form of the observed rate equation if it is assumed that k = k₂K₁. In practice the rate equation is used to suggest possible mechanisms which predict a rate equation in agreement with experiment.
The second molecule of H₂ does not appear in the rate equation because it reacts in the third step, which is a rapid step after the rate-determining step, so that it does not affect the overall reaction rate.
Temperature dependence
Main article: Arrhenius equation

Each reaction rate coefficient k has a temperature dependency, which is usually given by the Arrhenius equation:

The pressure dependence of the rate constant is associated with the activation

volume. For the reaction proceeding through an activation-state complex:

A + B ⇌ |A⋯B|^‡ → P

the activation volume, V^‡, is:
where V̄denotes the partial molar volumes of the reactants and products and ‡ indicates the activation-state complex.
For the above reaction, one can expect the change of the reaction rate constant (based either on mole-fraction or on molar-concentration) with pressure at constant temperature to be:
In practice, the matter can be complicated because the partial molar volumes and the activation volume can themselves be a function of pressure.
Reactions can increase or decrease their rates with pressure, depending on the value of V^‡. As an example of the possible magnitude of the pressure effect, some organic reactions were shown to double the reaction rate when the pressure was increased from atmospheric (0.1 MPa) to 50 MPa (which gives V^‡ = −0.025 L/mol).[10]
A chemical equation

A chemical equation is the symbolic representation of a chemical reaction in the form of symbols and formulae, wherein the reactant entities are given on the left-hand side and the product entities on the right-hand side.^[1] The coefficients next to the symbols and formulae of entities are the absolute values of the stoichiometric numbers. The first chemical equation was diagrammed by Jean Beguin in 1615.^[2]

2

This equation would be read as "two HCl plus two Na yields two NaCl and H two." But, for equations involving complex chemicals, rather than reading the letter and its subscript, the chemical formulas are read using IUPAC nomenclature. Using IUPAC nomenclature, this equation would be read as "hydrochloric acid plus sodium yields sodium chloride andhydrogen gas."

• 
  "=" symbol is used to denote a stoichiometric relation.
  
• 
  "→" symbol is used to denote a net forward reaction.
  

• 
  "" symbol is used to denote a reaction in both directions.
  
• 
  "" symbol is used to denote an equilibrium.
  

The physical state of chemicals is also very commonly stated in parentheses after the chemical symbol, especially for ionic reactions. When stating physical state,

19. 
    denotes a solid, (l) denotes a liquid, (g) denotes a gas and (aq) denotes an aqueous solution.
    

If the reaction requires energy, it is indicated above the arrow. A capital Greek letter delta () is put on the reaction arrow to show that energy in the form of heat is