1

E1 Precipitation and Water Purity

“E1 is a onesession lab!”

“Will we dothe entireexperiment?”

Experiment 1 Pre-lab Report

Pre-lab Report (page 34) Due at the start of Lab (8am or 11am or 2pm)

Goals for Experiment 1

One session three hour lab Complete Parts 1 and 2A and 2B (all). Complete part 3 OR 4 as assigned*. Complete team report and turn in at the end

of lab or by the grace period deadlineindicated in the lab manual, p. 232.

* Team assignments are on p. 227

Discussion: first hour of next session Prepare discussion abstract and presentationduring lab or before next session.

Ionic compounds with cations (+ ions) andanions (- ions) in fixed positions in acrystalline solid.

“I’m a cation.Note my eyes!”

Background: Salts

Background: Salt Formulas

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.

See Figure 3, p. 223 of themanual for common ion chargesand periodic table position

Cr+3 O-2

Formula ?

= Cr2O3

Salt Formulas

2

Background: Water and Salt Solubility

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

In water solution the salt ionsare dissociated and free to moveabout.

Salt Solubility in Water

Water (H2O) is polar.

DEMO

Salt Solubility in Water

Polar water molecules reduce the effective charges of the ionsin the solid and thus salt ions dissociate and the salt dissolves.

NaCl(s) = Na+(aq) + Cl-(aq)

Salt Solubility

DEMO

• Aqueous salt solutions are electrolytes dueto the fact that the dissolved salt ions aremobile and carry a charge.

http://preparatorychemistry.com/Bishop Elem Prop frames.htm

Precipitation

A solid comes out of solution

Solutions before mixing After mixing

+

DEMO

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

HgI2

KCl

Precipitation reactions

3

Part 1. What is the precipitate?

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

__? ppt.

__?__ ↓

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 originalreaction mixture

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

Conclusion?

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

DEMO

Cl- is a spectator ion.

Sameproperties

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) →

• The test produces a different reaction with different productsand is therefore invalid.

?

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. Nothing.2. Ba2+ is a spectator.3. Cl- is a reactant.

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

2. NaCl (aq) + AgF (aq) → white (s)

4

Course theme

“There are structure andproperty and periodic tablerelationships”

Useful web sites are: http://www.merlot.org http://www/davah.com/periodic/

http://www.webelements.com

Part 2A. Precipitation Studies

Discussion questions 1 - 3, p. 48 Is precipitate color predictable from the position ofthe cation’s element in the Periodic Table? Is the solubility of a cation predictable from theposition of its element in the Periodic Table? Is there a relationship between cation charge andsolubility? Compare ….

SO42-S2-C2O4

2-I- CrO42-Cl-Cations

Part 2A. Table, page 23; Team cation assignments, page 227

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

Hypothesis Formation

Formulate your hypothesis and predictions based onprovided models (solubility)

Example: Predict the solubility of NaF (s) versus FeF3(s)

Figure: Water Molecules oriented about Ions

Hypothesis Formation

Formulate your hypothesis and predictions based onprovided models and periodic table info.

Example: Solubility of MgF2 (s) versus BaF2(s)?

* p.255, lab manual

5

Discussion PreparationManipulate the class data.You will NOT get points for just

reproducing the class data.1A VIIIA

1H

1s1 IIA IIIA IVA VA VIA VIIA

2He1s2

3Li

2s1

4Be

2s2

5B

2s22p1

6C

2s22p2

7N

2s22p3

8O

2s22p4

9F

2s22p5

1 0Ne

2s22p6

1 1Na

3s1

1 2Mg

3s2 IIIB IVB VB VIB VIIB VIIIB ! VIIIB IB IIB

1 3Al

3s23p1

1 4Si

3s23p2

1 5P

3s23p3

1 6S

3s23p4

1 7Cl

3s23p5

1 8Ar

3s23p6

1 9K

4s1

2 0Ca

4s2

2 1Sc

3d14s2

2 2Ti

3d24s2

2 3V

3d34s2

2 4Cr

3d54s1

2 5Mn

3d54s2

2 6Fe

3d64s2

2 7Co

3d74s2

2 8Ni

3d84s2

2 9Cu

3d1 04s1

3 0Zn

3d1 04s2

3 1Ga

4s24p1

3 2Ge

4s24p2

3 3As

4s24p3

3 4Se

4s24p4

3 5Br

4s24p5

3 6Kr

4s24p6

3 7Rb

5s1

3 8Sr

5s2

3 9Y

4d15s2

4 0Zr

4d25s2

4 1Nb

4d35s2

4 2Mo

4d55s1

4 3Tc

4d55s2

4 4Ru

4d75s1

4 5Rh

4d85s1

4 6Pd

4d10

4 7Ag

4d1 05s1

4 8Cd

4d1 05s2

4 9In

5s25p1

5 0Sn

5s25p2

5 1Sb

5s25p3

5 2Te

5s25p4

5 3I

5s25p5

5 4Xe

5s25p6

5 5Cs

6s1

5 6Ba

6s2

5 7

La*

5d16s2

7 2Hf

5d26s2

7 3Ta

5d36s2

7 4W

5d46s2

7 5Re

5d56s2

7 6Os

5d66s2

7 7Ir

5d76s2

7 8Pt

5d96s1

7 9Au

5d1 06s1

8 0Hg

5d1 06s2

8 1Tl

6s26p1

8 2Pb

6s26p2

8 3Bi

6s26p3

8 4Po

6s26p4

8 5At

6s26p5

8 6Rn

6s26p6

8 7Fr

7s1

8 8Ra

7s2

8 9Ac#

6d17s2

1 0 4 +

6d27s2

1 0 5 +

6d37s2

1 0 6 +

6d47s2

1 0 7 +

6d57s2

1 0 8 +

6d67s2

1 0 9 +

6d77s2

+ Element synthesized,

but no official name assigned

Periodic Trends

• Look for patterns within families• Look for patterns across families (pre-transition, transition,post-transition)

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….

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.

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) →

ppt.

NO ppt.

6

• Precipitation reactions are equilibrium systems andthus there are always unprecipitated reactant ionspresent when precipitation occurs

Concentration and Precipitation

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

DEMO

Part 4. Solvent Pollution and Precipitation

Laboratory Compare salt solubility and amount of precipitate

formed by the salt mixtures in the different solvents(water, acetone, and hexane).

Discussion Question 5, page 48What is the relationship, if any, between salt solubility,precipitation, and solvent polarity?

Part 4 Solvents

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

Salt solubility and solvent polarity

Acetone is less polar than water.Acetone will be 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, thelower the solubility of the salt.

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