Precipitation ReactionsA. 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

c. K2CrO4(aq) + Ba(NO3)2(aq) -----> BaCrO4(s) + 2KNO3(aq)

4. AgNO3(aq) + KCl(aq) ------> AgCl(s) + KNO3(aq)

+ =

Precipitate Spectator Ions

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

Equation for Formation of the gas Gas produced

Ammonium salts NH4+ + OH- NH3 + H2O 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 versa2. 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.RAnd white precip. Only A does this. A = MgCl2 5. MgCl2 has 3 white precipitates6. C = Pb(NO3)2 has 4 white precip.7. B = NaOH gives two precip.’s and 2 NR

MgCl2HCl 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.

White ppt

Incident: NaOH Clothing Damage

Incident: Acid Splash on Latex Gloves