Sec. 4.2 Precipitation Reactions
1


A precipitation reaction occurs when two ionic
compounds react together to form a soluble and an
insoluble product
Precipitation rxns are also called double replacement
reactions
◦ AC + BD  AD + BC where one product is a ppt.

Products can be predicted using Solubility Rules
2

See Table 4.1 Solubility Guidelines For
common Ionic Compounds in Water
3

Example: Aqueous solutions of silver nitrate
and sodium chloride are mixed together
 AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq)
 AgCl (s) is the ppt since “all chlorides are soluble
except Ag+, Hg22+, and Pb2+”
4

Example: Aqueous solutions of potassium
hydroxide and iron (III) iodide are mixed.
 3KOH (aq) + FeI3 (aq)  3KI (aq) + Fe(OH)3 (s)
 Fe(OH)3 (s) is the ppt since “all hydroxides are
insoluble except Ca2+, Sr2+, and Ba2+
5

Ionic Equations– chemical equations that show
the soluble reactants/products as ions
◦ Ex: AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq)
Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq) 
AgCl(s) + Na+(aq) + NO3- (aq)
Ex: 3KOH (aq) + FeI3 (aq)  3KI (aq) + Fe(OH)3 (s)
3K+(aq) + 3OH-(aq) + Fe3+(aq) + 3I-(aq) 
3K+(aq) +3I- (aq) + Fe(OH)3 (s)
6


Net Ionic Equations (NIE’s) – chemical equations
that show only the ions responsible for the
precipitate
The soluble ions are called spectator ions
◦ AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq)
◦ Ag+ (aq) + NO3- (aq) + Na+ (aq) + Cl- (aq) 
AgCl (s) + Na+ (aq) + NO3- (aq)
◦ NIE: Ag+ (aq) + Cl- (aq)  AgCl (s)
7
•
•
Ex: 3KOH (aq) + FeI3 (aq)  3KI (aq) + Fe(OH)3 (s)
3K+(aq) + 3OH-(aq) + Fe3+(aq) + 3I-(aq) 
3K+(aq) + 3I- (aq) + Fe(OH)3 (s)
• 3K+(aq) + 3OH-(aq) + Fe3+(aq) + 3I-(aq) 
3K+(aq) +3I- (aq) + Fe(OH)3 (s)
•
NIE: 3OH-(aq) + Fe3+(aq)  Fe(OH)3 (s)
8

You try it:
◦ Write the net ionic equation for the following reaction:
 Aqueous solutions of manganese (IV) acetate and lithium
phosphate are reacted together
 3Mn(C2H3O2)4 (aq) + 4Li3PO4 (aq) 
Mn3(PO4)4 (s) + 12LiC2H3O2 (aq)
 3Mn4+ (aq) + 4C2H3O2- (aq) + 12Li+ (aq) + 4PO43- (aq)
 Mn3(PO4)4 (s) + 12Li+ (aq) + C2H3O2- (aq)
 NIE: 3Mn4+ (aq) + 4PO43- (aq)  Mn3(PO4)4 (s)
9

Guidelines to writing net ionic equations (NIE’s)
Identify the reaction type based on the given reactants
Write the balanced molecular equation for the reaction
Rewrite the equation to show the ions that form in
solution when each soluble strong electrolyte breaks up
into its component ions
Soluble strong electrolytes: soluble ionic compounds, strong
acids, strong bases
Leave elements, molecular compounds, weak and nonelectrolytes, solids, liquids, and gases in their original
form (no ions)
Soluble weak electrolytes: soluble molecular compounds,
weak acids, weak bases
Identify and cancel spectator ions that occur on both
sides of the equation
10