The Mystery of the Thirteen Test Tubes

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THE MYSTERY OF THE THIRTEEN TEST TUBES

Purpose This experiment is designed to give students who do not perform the standard qualitative analysis scheme a chance to investigate some simple descriptive chemistry. It avoids the toxicity concerns of mercury and cadmium, and can be performed in two laboratory periods. Materials A uniquely-labeled set of thirteen unknowns in test tubes contain each of the following solutions: 1M H2 SO4 6M NH3 1M Ba(NO3)2 1M Cu(NO3)2 1M Na2 S

1M K2CrO4 1M NiSO4

1M AgNO3 0.1M SnCl2

1M Fe(NO3)3 1M KCl

1M K2C 2O4 1M KSCN

Apparatus Test tubes, stirring rods, Bunsen burner, litmus paper or pH paper and nichrome wire for flame tests. Safety Precautions You cannot tell a harmful chemical from a harmless one just by looking. Follow all normal laboratory practices during this experiment. Avoid skin contact with the chemicals. Wear your safety glasses at all times. Hazardous Waste Disposal All the solutions may be washed, using water, into the Waste Bottles located in the fumehoods. Introduction Thirteen test tubes with thirteen different solutions have been filled and labeled with a secret code. The actual identity of each solution is a mystery. You are the chemical detective. You must correctly identify each solution before time runs out (at the end of 2 laboratory periods). Now since the Chemical Abstracts Services (CAS) has registered over ten million compounds, this would seem to be an impossible task. In this case however, you're

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in luck! All the other compounds have an alibi for the time in question and we know for certain what the thirteen solutions are.

The Suspects H2SO4 KSCN Cu(NO3)2

SnCl2 Ba(NO3)2 NiSO4

NH3 Fe(NO3)3 AgNO3

KCl K2 C 2 O 4

K2CrO4 Na2 S

All you have to do is match these identities to the code on each test tube. This kind of process is called qualitative analysis. This means identifying what is present. The second semester of General Chemistry is entitled Qualitative Analysis at many colleges because of the focus on this topic in the second semester. In the last 10 years, however, there has been a shift in emphasis away from areas of descriptive chemistry, i.e., a physical description of what compounds look like. The other part of Analytical Chemistry is quantitative analysis. After the identity of a compound is established, the question is how much? Many of your other experiments were intended to address this question. You will recall that the answer was often a "number". You are fortunate to have a limited list of suspects. It would be a much harder job to identify one out of ten million. On the other hand, your investigation will be severely limited. No fancy electronic equipment, and you may not question any other chemicals or have them assist in your investigation. This means that all your chemical tests will have to make use of these chemicals themselves. Once you identify one chemical, you may be able to use it to identify others. You may submit each to a lie detector test in the form of litmus paper or a flame test. You must be very careful to avoid contaminating one solution with another. Don't forget to clean and dry the stirring rod between solutions. Contamination of solutions is a gross violation of their chemical rights and shall result in dismissed cases (and possibly no credit). One set will be provided for each group of detectives. If it is spilled, lost, dropped or stolen NO MORE will be given. Don't bother consulting your neighbor; every set is different. Do your own work.

The Mystery of the Thirteen Test Tubes

Before you begin, do some homework. Find out what you can about your suspects. Make a plan for how you will proceed. Your textbook, The CRC Handbook of Chemistry and Physics, and other standard reference books might be helpful. The following may also be useful: H2SO4

Strong acid, most powerful industrial chemical in the world, may produce insoluble sulfates if metaled with.

NH3

Ammonia, alias ammonium hydroxide (NH4OH): has done important work in homes, last known employment as fertilizer, can turn ugly on any nosy detective.

K2CrO4

Best known for its bright disposition, potassium is almost inert, but the chromate may drop out if faced with silver, lead, or barium. Remains bright even when it lays low.

KCl

Nothing but a common salt, almost impossible to recognize in a crowd, but can be distinguished from other "common salt" by its pale violet response to any "trial by fire".

Fe(NO3)3

Ferric nitrate: ferric, alias "iron three", is more reactive than younger brother ferrous; may be recognized by color if not confused with other species, can be definitively identified by "bloody" encounter with greatest rival thiocyanate.

K 2 C2 O 4

Potassium oxalate: actions not well known but is of moderate toxicity, handle with care, believed to have had a "falling out" with barium.

Na2S

Sodium sulfide, alias "Le Pew": a real loner, possible messy confrontations with copper, ferric, nickel or tin; tends to linger on the skin if touched (not recommended).

Cu(NO3)2

Cupric nitrate: first name officially changed to "copper(II)"; leading chemical citizen, many business ventures include electrical wire manufacturing and production of alloys, notably brass; in solution easily recognized by "melancholy" disposition. Once suspected of conspiring with ammonia to impersonate ink.

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NiSO4

Nickel(II) sulfate: once very valuable, now net worth greatly reduced; "Nick" is easily recognized by his "envious" nature.

SnCl2

Stannous chloride, a.k.a. tin(II) chloride: a hard worker, known since ancient times, currently employed in food packaging industry, recyclable; fluoride form prevents tooth decay; unfortunate confrontation with "Le Pew" (Note: the preparation of aqueous solutions often uses large amounts of HCl which may make the solution appear more acidic than normal.)

KSCN

Potassium thiocyanate: poisonous little creature, approach with caution, long-standing "blood" feud with the iron brothers.

Ba(NO3)2

Barium nitrate: little known on this one, chance encounters with sulfuric acid have often "precipitate" pale consequences.

AgNO3

Silver(I) nitrate: usually pure but can have a "dark side" at times, has been known to be fatal if taken in by a careless detective.

An anonymous tipster has left this message: "The nitrates are a red herring." Procedure In preparation for this experiment, you should prepare a neat, organized data sheet or table to record your observations on. Bring this table and any notes, books, or lists that you think you might need to lab. Round up your container of suspects. Record the code on the bottles for your set on your report sheet. Take care of your suspects; you cannot get replacements. You may (and should) perform litmus tests. Use a nichrome wire to perform flame tests on the suspects. Ask for help if you are uncertain how to perform or interpret these tests. Take care, however, that you do not wastefully exhaust all of your solution. Record any useful observations on your report sheet. Collect solids and dispose in the trash.

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References Nitz, Otto W., Ondrus, Martin G., and Melton, Tammy.

Laboratory Manual for

Introductory Chemistry, fifth edition: Wm. C. Brown Publishers: Dubuque, 1993, 311 – 315.

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Mystery of the Thirteen Test Tubes REPORT SHEET: Partner Credit:

Name: _______________________ Y

(Circle if yes)

Partner's Name: ________________ Lab Section: _______________

Code identifying your set ____________________ Report the formula of the solution in each bottle and also give the correct chemical name of each reagent. 1.

___________________________

8. ___________________________

2.

___________________________

9. ___________________________

3.

___________________________

10. ___________________________

4.

___________________________

11. ___________________________

5.

___________________________

12. ___________________________

6.

___________________________

13. ___________________________

7.

___________________________

Report one balanced reaction equation for each of the thirteen compounds identified in this experiment. The reaction equations reported should be ones that helped you to separate the compounds from one another (i.e. reactions that showed a positive result). Also indicate the colour of any precipitate formed in the reactions or the colour of resulting solutions. Note: No two reaction equations maybe the same. Also include neat, organized tables of observations for all reactions and tests performed.

The Mystery of the Thirteen Test Tubes

The Mystery of the Thirteen Test Tubes Pre-lab Questions: 1.

Be able to name or write the chemical formula for any of the thirteen compounds.

2.

List two of the chemical solutions that have a smell.

3.

Name the two chemical solutions that would turn blue litmus red.

4.

Name the chemical solution that will turn red litmus blue.

5.

Name one major use of the element Cu.

6.

What is the difference between ferrous ion and ferric ion?

7.

Which one of the chemicals may be used in the fertilizer business?

8.

Which chemical solution has had a falling out with barium?

9.

By just looking which of the thirteen chemicals are safe to handle?

10.

Define qualitative analysis.

11.

Define quantitative analysis.

12.

What is involved in a "flame test"?

13.

What is one of the major problems you should try to avoid?

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EQUILIBRIUM CONSTANTS AT 25°C Dissociation Constants for Weak Acids & Bases –

HC2H3O2 H+ + C2H3O2 HCN H+ + CN–

K 1.8 x 10–5 4.0 x 10–10



HSO4 H+ + SO 2" 4

1.3 x 10–2

H2S H+ + HS–

1.1 x 10–7

HS– H+!+ S2-

1.3 x 10–13 –

H2CO3 H+ + HCO3

4.2 x 10–7



HCO3 H+ + CO 2" 3

4.8 x 10–11



HCrO4 H+ + CrO 2" 4

3.2 x 10–7 –

Cr2O 2" 7 + H 2O ! 2HCrO4 + – NH4OH NH ! 4 + OH

2.9 x 10–2 1.8 x 10–5

! Formation Constants for Complex Ions +

Ag(NH3)2

Ag+ + 2NH3

6 x 10–8

2+ + 4NH Cu(NH3) 2+ 3 4 Cu

5 x 10–15

2+ + 4NH Zn(NH3) 2+ 3 4 Zn

3 x 10–10

2+ 2+ + 4NH Cd(NH 3) 4 Cd 3 ! Ag(CN)2 Ag+ + 2CN– ! 3" + + 4CN – Cu(CN) 4 Cu ! 2+ + 4CN – Cd(CN) 2" 4 Cd

2+ + 4CN – Zn(CN) 2" 4 Zn ! – – 2+ !CdCl3 Cd + 3Cl – + !HgCl2 (aq) HgCl + Cl

HgCl+ Hg2+ + Cl– Fe(SCN)2+ Fe3+ + SCN–

9 x 10–8 1.8 x 10–19 2 x 10–27 1.4 x 10–17 1.2 x 10–18 4 x 10–3 3.3 x 10–7 1.8 x 10–7 1 x 10–3

The Mystery of the Thirteen Test Tubes

Solubility Products AgCl Ag+ + Cl–

1.8 x 10–10

– Hg2Cl2 Hg 2+ 2 + 2Cl

1.4 x 10–18

PbCl 2 Pb2+ + 2Cl–

1.6 x 10–5

2+ + Ag2 S ! 2Ag + S

10–30

2+ Hg2S Hg 2+ 2 + S

10–33

PbS Pb2+ + S2+

10–28

HgS ! Hg2+ + S2+

1 x 10–52

CuS Cu2+ + S2+

1 x 10–37

Bi2 S 3 2Bi3+ + 3S2+

about

SnS Sn2+ + S2+ SnS2 Sn4+ + 2S2+

10–90 10–26

less than 1 x 10–45

CdS Cd2+ + S2+

1 x 10–28

NiS (freshly ppct.) Ni2+ + S2+

1 x 10–20

NiS (aged) Ni2+ + S2+

about

10–28

ZnS Zn2+ + S2+

1 x 10–20

FeS Fe2+ + S2+

6 x 10–18

MnS Mn2+ + S2+

5 x 10–15

PbSO4 Pb2+ + SO 2" 4

1.8 x 10–8

BaSO4 Ba2+ + SO 2" 4

1.1 x 10–10

2+ + SO 2" CaSO4 Ca! 4

7.1 x 10–5

PbCrO4 Pb!2+ + CrO 2" 4

10–15

2" 2+ BaCrO4 Ba ! + CrO 4

1.2 x 10–10

2" CaCrO4 Ca2+ ! + CrO 4

7 x 10–4

Ag2CrO4 2Ag!+ + CrO 2" 4 MgCO3 Mg2+! + CO 2" 3

! !

1.1 x 10–12 6.8 x 10–6

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CaCO3 Ca2+ + CO 2" 3

5.0 x 10–9

BaCO3 Ba2+ + CO 2" 3

2.6 x 10–9

2+ + 2OH– Mg(OH)2 Mg !

5.6 x 10–12

– 2+ Ca(OH)2 Ca ! + 2OH

4.7 x 10–6

Ba(OH)2 Ba2+ + 2OH–

5.0 x 10–3

Pb(OH)2 Pb2+ + 2OH–

1.4 x 10–20

Cu(OH)2 Cu2+ + 2OH–

1.6 x 10–19

Sn(OH)2 Sn2+ + 2OH–

5.4 x 10–27

Cd(OH)2 Cd2+ + 2OH–

5.3 x 10–15

Fe(OH)2 Fe2+ + 2OH–

4.9 x 10–17

Fe(OH)3 Fe3+ + 3OH–

2.6 x 10–39

Al(OH)3 Al3+ + 3OH–

5 x 10–33

Cr(OH)3 Cr3+ + 3OH–

1 x 10–31

Ni(OH)2 Ni2+ + 2OH–

5.5 x 10–16

Zn(OH)2 Zn2+ + 2OH–

5 x 10–17

Mn(OH)2 Mn2+ + 2OH–

2.1 x 10–13

Mn(OH)3 Mn3+ + 3OH–

1 x 10–36

CuI Cu+ + I–

1.3 x 10–12

PbI2 Pb2+ + 2I–

8.5 x 10–9

HgI2 Hg2+ + 2I–

2.8 x 10–29

AgI Ag+ + I–

8.5 x 10–17