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