Mixing of two solutions of soluble ionic solids, may result in a chemical reaction referred to as a precipitation. This is because, on mixing, a precipitate (insoluble solid) is formed.
The precipitate formula can be predicted from the solubility rules and the formulae of the ionic solids mixed.
Consider mixing solutions of two soluble ionic solids AX and BY.
What are the possible precipitates?
Precipitation occurs if one of AY or BX has the cation and the anion of an insoluble solid.
Writing the nett ionic equation for the precipitation reaction
Product:
Precipitate (PbCl2 for example).
Reactants:
Ions giving rise to the precipitate (Pb 2+ and Cl–)
Net ionic equation:
Pb 2+(aq) + 2Cl–(aq)
PbCl2(s)
The net ionic equation shows only the chemical change (no spectator ions!!).
The precipitate formula can be predicted from the solubility rules and the formulae of the ionic solids mixed.
Consider mixing solutions of two soluble ionic solids AX and BY.
AX exists as A+ and X– ions in solution.
BY exists as B+ and Y– ions in solution.
The mixed solution contains two cations (A+ and B+) and two anions (X– and Y–).
BY exists as B+ and Y– ions in solution.
The mixed solution contains two cations (A+ and B+) and two anions (X– and Y–).
What are the possible precipitates?
Precipitates are insoluble ionic compounds with no overall charge.
The two possible precipitates are AY and BX. Each has the cation of one of the soluble solids and the anion of the other.
The two possible precipitates are AY and BX. Each has the cation of one of the soluble solids and the anion of the other.
Precipitation occurs if one of AY or BX has the cation and the anion of an insoluble solid.
No precipitate forms if both possible products are soluble!
What happens to the other pair of ions?These ions are not changed and are referred to as spectator ions.
These remain in solution.
How can the two products be separated?These remain in solution.
The precipitate can be separated from the solution by filtration.
The soluble substance can be recovered by evaporation of the filtrate.
The soluble substance can be recovered by evaporation of the filtrate.
Writing the nett ionic equation for the precipitation reaction
Product:
Precipitate (PbCl2 for example).
Reactants:
Ions giving rise to the precipitate (Pb 2+ and Cl–)
Net ionic equation:
Pb 2+(aq) + 2Cl–(aq)
PbCl2(s)The net ionic equation shows only the chemical change (no spectator ions!!).