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E2 Precipitation and Water Purity

“E2 is a onesession lab!”

“Will we dothe entireexperiment?”

Experiment 2

Pre-lab correction Question 3 refers to Part 2A (not Part 2)

Goals for Experiment 2

One session three hour lab Complete Parts 1 and 2A and 2B (all). Complete part 3 OR 4 as assigned. Omit part 5.

Discussion: first hour of next session Prepare discussion abstract and presentation. Complete team report.

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Precipitation

A solid comes out of solution

Salt solutions before mixing After mixing

Ionic compounds with cations (+) and anions (-).

“I’m a cation.Note my eyes!”

Background: SALTS

Background: Water and Salt Solubility

In the solid, the salt ions arefixed in a rigid lattice.

The simplest ratio of theions in the solid is representedby the formula of the salt.

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Cr+3 O-2

Formula ?

= Cr2O3

Salt Formulas

Background: Water and Salt Solubility

In solution the ions are mixedwith water molecules and free tomove about in solution.

Salt Solubility in Water

Water (H2O) is polar.

DEMO

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Background: Water and Salt Solubility

Polar water molecules reduce the effective charges of the ionsin the solid and thus salts dissociate and dissolve.

NaCl(s) = Na+(aq) + Cl-(aq)BaCl2(s) = Ba2+(aq) + 2 Cl-(aq)

Salt Solubility

DEMO

• Aqueous salt solutions are electrolytes.

1. The positive ion of a dissolved salt combines withthe negative ion from a different dissolved salt.

2. The recombined ions may stay in solution or come out of solution in the form of a solid called a “precipitate”.

Hg2+(aq) + 2Cl-(aq) + K+(aq) + I-(aq)

HgI2

KCl

Precipitation reactions

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Part 1. What is the precipitate?

HgCl2 (aq) + KI (aq) → __?__ (s) clear and colorless salt solutions

__? ppt.

__?__ ↓

DEMO

REFERENCE BLANK TEST

Test mixture designed to identify reactants(forming the precipitate) and spectators (non-reactants).

Test Mixture Omit a species (ion) from the reaction mixture. Substitute a known SPECTATOR ion for the

omitted ion such as Na+ or K+ or NO3- Compare the products of the test and original

reaction mixture

Reference Blank Tests� Reaction: HgCl2 (aq) + KI (aq) → ? Clear and colorless

Conclusion?

Reference blank test:Hg(NO3)2 + KI → ___________Clear and colorless

DEMO

______________

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Reference Blank Test Design

�HgCl2 (aq) + KI (aq) → ? colorless solutions

Valid Reference Blank Test produces the identical products or noreaction.

Reference Blank Test Design

�HgCl2 (aq) + KI (aq) →

Invalid test:CuCl2 (aq) + KI (aq) →

?

DEMO

BaCl2 (aq) + AgF (aq) → white (s)?

� 3. Ba(NO3)2 (aq) + AgF (aq) → no reaction

Test Observations:1. FeCl3 (aq) + AgF (aq) → green (s)

1. _____________________2. _____________________3. _____________________

Q. What do you know about the white (s)?

2. NaCl (aq) + AgF (aq) → white (s)

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Part 2A. Precipitation Studies

Discussion questions 1-3, p.67 Is the solubility of a metal ion predictable from theposition of its element in the Periodic Table? Is precipitate color predictable from the position ofthe metal ion’s element in the Periodic Table? Is there a relationship between ion charge andsolubility? Compare the solubility data of multiplycharged ions and….

SO42-S2-C2O42-I- CrO42-Cl-Cations

Part 2A. Table, page 44; Cation assignments, page 216

Hg2+

Nitrate salts Sodium Salts

Part 2B. Can I identify it?

Identify an unknown metal ion based onprecipitation observations from part 2A:

Caution: Save your acetate sheet results from Part2A for use in Part 2B

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Part 3. Concentration and Precipitation.

Laboratory Combine assigned ion combinations at different

concentrations Observe and record the relative amounts of

precipitate formed at different concentrations.

Discussion (Question 4, page 67)Examine the relationship between concentration andamount of precipitate. Organize the class data…

Concentration and Salt Precipitation

Saturated solution = contains the maximum amount ofsalt that can be dissolved in a given solution volume.

Concentration and Salt Precipitation

DEMOSupersaturationandprecipitation.

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Concentration and Precipitation

• Precipitation (ppt.) is dependent on concentrationand occurs only if the solubility limit of the salt isexceeded.

DEMO 0.10 M KI(aq) + 0.10 M HgCl2 (aq) →

10-3 M KI(aq) + 10- 4 M HgCl2 (aq) →

________

________

• Precipitation reactions are equilibrium systems andthus there are always reactants and products presentwhen precipitation occurs

Concentration and Precipitation

DEMO:1.Remove the ppt. byfiltration2. Check for presence ofreactant ions in the filtrate.

Precipitation reactions are equilibrium systems When precipitation occurs, some reactant ions

remain in solution: Hg2+ and 2I- → HgI2(s)X

Concentration and Precipitation

“In reality, it is understood there isboth a forward and reverse arrow!”

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Part 4. Solvent Pollution and Precipitation

Laboratory Combine assigned salts in different solvents (water,

acetone, hexane) Compare salt solubility and amount of precipitate

formed by the salt mixtures in the different solvents.

Discussion Question 5, page 67What is the relationship, if any, between salt solubility,precipitation, and solvent polarity?

Solvent Polarity

Water (H2O) is very polar. O / \ H H

Hexane (C6H14) is nonpolar

Acetone (CH3COCH3) is moderately polar. O

C

/ \ CH3 CH3

CH3CH2CH2CH2CH2CH3

Solvent Polarity

Polar and non-polar solvents are immiscible

Salts will not dissociate in non-polar solvents.

DEMO

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Salt solubility and solvent polarity

Acetone is less polar than water.Acetone is added to a saturatedsolution of CuSO4(aq).

DEMO

Q1. Will the solubility of the CuSO4 Increase? Decrease? Remain the same?Q2. What will you observe?

Solvent Polarity and Solubility

The less polar the polar solvent environment, the__________ the solubility of the salt.

B = Acetone is added to A = CuSO4(aq) After mixing ____________

Any Questions?Contact nkerner@umich.edu