Precipitation Reactions
A. Definitions
1. When two solutions are mixed and a solid forms
2. Precipitate = solid that forms from a precipitation reaction
3. K2CrO4(aq) + Ba(NO3)2(aq)2K+(aq) + CrO42-(aq) + Ba2+(aq) +2NO3-(aq)
a. K2CrO4 and Ba(NO3)2 are both soluble (all dissolve in water)
b. A yellow precipitate forms when these solutions are mixed
+
=
Precipitate
Spectator Ions
c. K2CrO4(aq) + Ba(NO3)2(aq) -----> BaCrO4(s) + 2KNO3(aq)
4. AgNO3(aq) + KCl(aq) ------> AgCl(s) + KNO3(aq)
B. Solubility Rules
Examples: predict what will happen when you mix:
a. KNO3(aq) + BaCl2(aq) ------>
b. Na2SO4(aq) + Pb(NO3)2(aq) ------>
c. 3KOH(aq) + Fe(NO3)2(aq) ------>
C. Reactions that give off gases
1. Sometimes the product of a reaction is not a solid, but a gas
2. We can still observe that something happened: bubbles form
Compound that
reacts with acid
Equation for Formation of the gas
Gas produced
Sulfides
2H+ + S2-  H2S
H2S
Carbonates
2H+ + CO32-  H2CO3  H2O + CO2
CO2
Bicarbonates
H+ + HCO3-  H2CO3  H2O + CO2
CO2
Sulfites
2H+ + SO32-  H2O + SO2
SO2
Bisulfites
H+ + HSO3-  H2O + SO2
SO2
Cyanides
H+ + CN-  HCN
HCN
Compound that
reacts with base
Ammonium salts
Equation for Formation of the gas
NH4+ + OH-  NH3 + H2O
Gas produced
NH3
D. Describing Reactions in Solution
1. Molecular Equation shows what compounds the ions came from
a. Does not give clear picture of what happens in solution
b. K2CrO4(aq) + Ba(NO3)2(aq) -----> BaCrO4(s) + 2KNO3(aq)
2. Complete Ionic Equation represents the form of the ions in solution
a. All strong electrolytes are represented as their ions
2K+(aq) + CrO42-(aq) + Ba2+(aq) +2NO3-(aq) ----> BaCrO4(s) + 2K+(aq) + 2NO3-(aq)
3. Net Ionic Equation shows only the ions participating in the reaction
a. The K+ and NO3- ions occur on both sides of the complete ionic eqn.
b. These spectator ions can be cancelled out of each side (algebra)
c. Ba2+(aq) + CrO42-(aq) -------> BaCrO4(s)
4. Example
a. 3KOH(aq) + Fe(NO3)3(aq) -----> Fe(OH)3(s) + 3KNO3(aq)
b. 3K+(aq) + 3OH-(aq) + Fe3+(aq) + 3NO3-(aq) ----> Fe(OH)3(s) + 3K+(aq) + 3NO3-(aq)
c. Fe3+(aq) + 3OH-(aq) -------> Fe(OH)3(s)
Example of What We Will Do Today (with different compounds)
1. Given MgCl2, HCl, Pb(NO3)2, K2CO3, and NaOH
1. D or E must be HCl and K2CO3 or vice versa
2. D must be HCl: bubbles, 1 precip, 2 N.R.
3. E must be K2CO3
4. MgCl2 reacts with HCl and K2CO3 to give N.R
And white precip. Only A does this. A = MgCl2
5. MgCl2 has 3 white precipitates
6. C = Pb(NO3)2 has 4 white precip.
7. B = NaOH gives two precip.’s and 2 NR
White ppt
MgCl2
HCl
Pb(NO3)2
K2CO3
HCl
N.O.R.
X
X
X
Pb(NO3)2
White ppt
White ppt
X
X
K2CO3
White ppt
Bubbles
White ppt
X
NaOH
White ppt
N.O.R.
White ppt
N.O.R.
Incident: NaOH Clothing Damage
Incident: Acid Splash on Latex Gloves