Writing Equations for Precipitation Reactions
Precipitation reactions can be represented using several types of chemical
equations: complete-formula equations, complete ionic equations, and
net ionic equations.
Complete-Formula Equations
barium nitrate
Ba(NO3)2
copper(II)
?
?
sulfate
CuSO4
When these compounds react
Ba(NO3)2 + CuSO4 BaSO4 +
with one another we get a double
Cu(NO3)2
displacement reaction .
Remember to write the formulas
using the rules for valences (the
Then balance!
zero sum rule OR the cross-over
rule.
 Precipitation reactions are often represented using completeformula equations.
 Their advantage is that they show the compounds that are involved.
 Their disadvantage is that they do not show the forms of the
chemicals that are actually involved in the reaction.
Complete Ionic Equations
Complete ionic equations represent soluble ionic compounds as separated
ions.
Ba(NO3)2 + CuSO4 BaSO4(s) + Cu(NO3)2
Ba2+ + 2 NO3- + Cu2+ + SO42- BaSO4(s) + Cu2+ + 2 NO3In the example shown above, barium nitrate exists in solution as separate
barium ions and nitrate ions.
Copper(II) sulfate exists in solution as separate copper(II) ions and sulfate
ions.
The barium and sulfate ions combine to form the solid barium sulfate
precipitate.
The copper(II) ions and the nitrate ions remain in solution. This
representation is called the complete ionic equation.
The copper(II) ions and the nitrate ions are not actually involved in the
reaction. They are called spectator ions.
Net Ionic Equations
If we eliminate the spectator ions from the complete ionic equation we
have what is called the net ionic equation. It shows only the ions that are
actually involved in the reaction.
Ba2+ + 2 NO3- + Cu2+ + SO42Ba2+ + SO42-
BaSO4(s) + Cu2+ + 2 NO3BaSO4(s)