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Writing Equations: Precipitation Reactions

Key Concepts

In a precipitation reaction a product of the reaction is only slightly soluble, or insoluble. This product is formed as a solid, also known as a precipitate.
Solubility Rules can be used to determine if a product is insoluble (forms a precipitate)
Ions in solution that are not used to form the precipitate are called spectator ions
It is important to include the states of matter in the chemical equation:
(s) for solid, the precipitate
(g) for gas
(l) for liquid
(aq) for substances in aqueous solution

Equations written to represent precipitation reactions can be written in one of three ways:

Molecular Equations
All reactants and products are written as if they are molecules
Ionic Equations
All reactants and products that are soluble are written as ions, only the precipitate is written as if it were a molecule
Net Ionic Equations
Only the reactants and product taking part in the reaction are written in the equation, the reactants as ions, the product as a molecule.
Spectator ions are not included in the equation

Example

Consider the reaction between solutions of sodium chloride, NaCl(aq), and silver nitrate, AgNO₃(aq).

The possible products of the reaction are sodium nitrate, NaNO₃, and silver chloride, AgCl.

From the solubility rules we find that sodium nitrate, NaNO₃, is soluble since all Group I ions form soluble salts and also all nitrates are soluble. Silver chloride, AgCl, is insoluble since all chlorides are soluble EXCEPT those of silver, lead (II), mercury (I), copper (II) and thallium.

Writing the precipitation reaction equations

Molecular Equation
All species in the reaction are written as if they are molecules, species in solution must include the (aq), the precipitate must include the (s)
That is: NaCl(aq), AgNO₃(aq), NaNO₃(aq), AgCl(s)
NaCl(aq) + AgNO₃(aq) -----> NaNO₃(aq) + AgCl(s)
Ionic Equation
All species in solution are written as ions, the precipitate is written as if a molecule.
That is; REACTANTS:Na⁺(aq), Cl^-(aq), Ag⁺(aq), NO₃^-(aq)
PRODUCTS: Na⁺(aq), NO₃^-(aq), AgCl(s)
Na⁺(aq) + Cl^-(aq) + Ag⁺(aq) + NO₃^-(aq) ------> Na⁺(aq) + NO₃^-(aq) + AgCl(s)
Net Ionic Equation
Written as for Ionic Equation except that spectator ions are not included in the equation:
That is; Na⁺(aq), NO₃^-(aq) are not included.
Only the species involved in producing the precipitate are included in the equation
That is; Ag⁺(aq), Cl^-(aq), AgCl(s) are included in the equation
Ag⁺(aq) + Cl^-(aq) ------> AgCl(s)