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4.5 Precipitation Reactions4.5 Precipitation Reactions
Precipitation ReactionsPrecipitation Reactions
When 2 solutions are mixed When 2 solutions are mixed and an insoluble substance and an insoluble substance formsforms
PrecipitatePrecipitate- this insoluble - this insoluble substancesubstance
To figure out what the solid is, To figure out what the solid is, we need to know what we need to know what individual species are present individual species are present in the solution after the mixture in the solution after the mixture occursoccurs
ExampleExample
1.1. Write the reactants as they exist in solution.Write the reactants as they exist in solution.
KK22CrOCrO44 (aq) + Ba(NO (aq) + Ba(NO33))22(aq) (aq)
2K2K++(aq) + CrO(aq) + CrO442-2- (aq) + Ba (aq) + Ba2+2+(aq) + 2NO(aq) + 2NO33
--(aq) (aq)
We add potassium chromate and barium nitrate together and a yellow solid forms. What is it?
ExampleExample
2.2. Find the possibilities for the Find the possibilities for the solid.solid.
Since KSince K22CrOCrO44 and Ba(NO and Ba(NO33))22 are are
the beginning solutions, they the beginning solutions, they cannot be the yellow solid. cannot be the yellow solid.
The only other options are:The only other options are:
KNOKNO33 OR OR BaCrOBaCrO44
ExampleExample
3.3. Look at the solubility rules to Look at the solubility rules to
figure out if either of these are figure out if either of these are
soluble.soluble.
Since KNOSince KNO33 is soluble, the solid must be: BaCrO is soluble, the solid must be: BaCrO44
So equation must be:So equation must be:
KK22CrOCrO44 (aq) + Ba(NO (aq) + Ba(NO33))22(aq) (aq)
KNOKNO33(aq) + BaCrO(aq) + BaCrO44(s)(s)
4.6 Describing 4.6 Describing Reactions in SolutionReactions in Solution
3 Types of Equations3 Types of Equations
MolecularMolecular - overall reaction but not - overall reaction but not the actual species in solutionthe actual species in solution
Complete IonicComplete Ionic – represents all strong – represents all strong electrolytes as ions electrolytes as ions
Net IonicNet Ionic – Only species that undergo – Only species that undergo change are included, no spectatorschange are included, no spectators
ExampleExample
Aqueous potassium chloride is Aqueous potassium chloride is
added to aqueous silver nitrate.added to aqueous silver nitrate.
Molecular Equation:Molecular Equation: Check solubility rules to figure out what symbols Check solubility rules to figure out what symbols
to use after products.to use after products.
KCl(aq) + AgNOKCl(aq) + AgNO33(aq) (aq) KNO KNO33(aq) + AgCl(s)(aq) + AgCl(s)
ExampleExample
Complete Ionic Equation:Complete Ionic Equation: Break all strong electrolytes apart Break all strong electrolytes apart
into ionsinto ions Leave solids, gases, liquids, and Leave solids, gases, liquids, and
weak electrolytes alone.weak electrolytes alone.
KK++(aq) + Cl(aq) + Cl--(aq) + Ag(aq) + Ag++(aq) + NO(aq) + NO33--(aq) (aq)
KK++(aq) + NO(aq) + NO33--(aq) + (aq) +
AgCl(s)AgCl(s)
ExampleExampleNet Ionic Equation:Net Ionic Equation:
Cancel out all Cancel out all spectator ionsspectator ions: :
species that are identical on both species that are identical on both
sides of the equationsides of the equation Must have same charges, Must have same charges, states, formulas, etc.states, formulas, etc.
KK++(aq) + Cl(aq) + Cl--(aq) + Ag(aq) + Ag++(aq) + NO(aq) + NO33--(aq) (aq)
KK++(aq) + NO(aq) + NO33--(aq) + AgCl(s)(aq) + AgCl(s)
ClCl--(aq) + Ag(aq) + Ag++(aq) (aq) AgCl(s) AgCl(s)
4.7 Stoichiometry of 4.7 Stoichiometry of Precipitation ReactionsPrecipitation Reactions
StoichiometryStoichiometry
Determine limiting reactant 1Determine limiting reactant 1stst
Always write down species Always write down species presentpresent
Use volume and molarity to find Use volume and molarity to find molesmoles
StepsSteps
1.1. Identify species presentIdentify species present
2.2. Write balanced net ionic Write balanced net ionic equationequation
3.3. Calculate moles of reactantsCalculate moles of reactants
4.4. Determine limiting reactantDetermine limiting reactant
5.5. Calculate moles of productCalculate moles of product
6.6. Convert to grams or other unitConvert to grams or other unit
ExampleExample
Sodium sulfate Sodium sulfate and lead (II) and lead (II) nitrate are mixed nitrate are mixed and a precipitate and a precipitate forms. Find the forms. Find the mass and identity mass and identity of precipitate.of precipitate.
ExampleExample
1.1. Identify species presentIdentify species present
Molecular Equation:Molecular Equation:
NaNa22SOSO44(aq) + Pb(NO(aq) + Pb(NO33))22(aq) (aq)
2NaNO2NaNO33(aq) + PbSO(aq) + PbSO44(s)(s)
Complete Ionic Equation:Complete Ionic Equation:
2Na2Na++(aq) + SO(aq) + SO442-2-(aq) + Pb(aq) + Pb2+2+(aq) + 2NO(aq) + 2NO33
--(aq) (aq)
2Na2Na++(aq) + 2NO(aq) + 2NO33--(aq) + PbSO(aq) + PbSO44(s)(s)
ExampleExample
2. Write balanced net ionic equation2. Write balanced net ionic equation
Spectator ions: sodium and nitrateSpectator ions: sodium and nitrate
2Na2Na++(aq) + SO(aq) + SO442-2-(aq) + Pb(aq) + Pb2+2+(aq) + 2NO(aq) + 2NO33
--(aq) (aq)
2Na2Na++(aq) + 2NO(aq) + 2NO33--(aq) + PbSO(aq) + PbSO44(s)(s)
Complete Ionic Equation:Complete Ionic Equation:
SOSO442-2-(aq) + Pb(aq) + Pb2+2+(aq) (aq) PbSO PbSO44(s)(s)
ExampleExample
3.3. Calculate moles of reactantsCalculate moles of reactants
Sodium Sulfate:Sodium Sulfate:
Lead (II) Nitrate:Lead (II) Nitrate:
24
42
24
42
4242 0500.0
1
1
1
0250.000.2 molSO
SOmolNa
molSO
SOLNa
SOmolNaSOLNa
2
23
2
23
2323 0625.0
)(1
1
)(1
)(0500.0)(25.1 molPb
NOmolPb
molPb
NOLPb
NOmolPbNOLPb
ExampleExample
4.4. Determine the limiting reactantDetermine the limiting reactant
Limiting reactant is SOLimiting reactant is SO442-2- since extra Pb since extra Pb2+2+ is available. is available.
OROR
Limiting reactant is SOLimiting reactant is SO442-2- since there is not enough SO since there is not enough SO44
2-2-
neededmolPbmolSO
molPbmolSO
2
24
22
4 0500.01
10500.0
neededmolSOmolPb
molSOmolPb
2
42
242 0625.0
1
10625.0
ExampleExample
5. Calculate moles of product5. Calculate moles of product Always start with LRAlways start with LR
6. Convert to grams6. Convert to grams
424
424 0500.0
1
10500.0 molPbSO
molSO
molPbSOmolSO
44
44 2.15
1
3.3030500.0 gPbSO
molPbSO
gPbSOmolPbSO
Example 2Example 2
Calculate the Calculate the mass of solid mass of solid NaCl that must be NaCl that must be added to 1.50 L of added to 1.50 L of a 0.100 M AgNOa 0.100 M AgNO33
solution to solution to precipitate all the precipitate all the AgAg++ ions in the ions in the form of AgCl.form of AgCl.
Example 2Example 2
Write balanced net ionic equationWrite balanced net ionic equationNaCl(aq) + AgNONaCl(aq) + AgNO33(aq) (aq) NaNO NaNO33(aq) + AgCl(s)(aq) + AgCl(s)
NaNa++(aq) + Cl(aq) + Cl--(aq) + Ag(aq) + Ag++(aq) + NO(aq) + NO33--(aq) (aq)
NaNa++(aq) + NO(aq) + NO33--(aq) + AgCl(s)(aq) + AgCl(s)
ClCl--(aq) + Ag(aq) + Ag++(aq) (aq) AgCl(s) AgCl(s)
Find moles of AgFind moles of Ag++ that need to be precipitated that need to be precipitated
molAgmolAgNO
molAg
LAgNO
molAgNOLAgNO 150.0
1
1
1
100.050.1
33
33
Example 2Example 2
Find the moles of Ag+ neededFind the moles of Ag+ needed
Find grams of NaCl needed Find grams of NaCl needed
gNaClmolNaCl
gNaCl
molCl
molNaClmolCl 77.8
1
45.58
1
1150.0
molCl
molAg
molClmolAg 150.0
1
1150.0