Equations representing reactions can be written in a variety of forms. The various ways in which the reaction that occurs on mixing aquous sodium hydroxide with hydrochloric acid can be represented are shown below.
A word equation uses names of reactants and products to describe the overall change.
sodium hydroxide + hydrochloric acid → sodium chloride + water
A balanced overall equation represents reactants and products using formulae. Balancing using coefficients ensures that there are the same number of each type of atom on both sides of the equation.
NaOH + HCl → NaCl + H2O
An balanced overall equation with states clarifies the physical form of each species.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O
An balanced ionic equation shows soluble species as hydrated ions.
Na+(aq) + OH–(aq) + H3O+(aq) + Cl–(aq) → 2H2O(l) + Na+(aq) + Cl–(aq)
A balanced net ionic equation focuses on the chemical change as species in the reaction mixture that are unchanged as a consequence of reaction (spectator ions) are not shown.
The spectators in the reaction above are Na+(aq) and Cl–(aq).
The net equation is: H3O+(aq) + OH–(aq) → 2H2O(l)
Two additional examples of reactions and their corresponding net ionic equations are given below.
Iron(II) nitrate reacts with aqueous chlorine to give a mixture of iron(III) nitrate and iron(III) chloride.
2Fe2+(aq) + Cl2(aq)→ 2Fe3+(aq) + 2Cl–(aq)
The four nitrate ions are unchanged and do not appear in the net ionic equation.
Sodium bromide reacts with aqueous chlorine to give bromine and sodium chloride.
2Br–(aq) + Cl2(aq) → Br2(aq) + 2Cl–(aq)
The two sodium ions are unchanged and do not appear in the net ionic equation.