Transcript Exp 9A: The Identity of an Insoluble Precipitate

Exp 2: Identification of a Compound:
Physical Properties
Introduction
• Chemistry is based on the relation between experimental
observations and theory
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Experiments lead to theories lead to other experiments
Accurate, precise and complete observations are the basis for
successful experiments
Without good observations there is no basis for developing a
coherent theory
Purpose
• Identify an unknown compound based on properties of known
compounds
• Practice proper and accurate observations and focus on the
conclusions that can be drawn from those observation
• Correct and complete observations are the only tools that you
need for this experiment
Exp 2: Identification of a Compound
Successful qualitative analysis
“. . . when you have excluded the impossible, whatever remains must
be the truth.”
(Sherlock Holmes)
Exp 2: Identification of a Compound
Experimental Concept
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Determine the identity of an unknown compound:
– observe chemical reaction of a series of solutions of known
solid compounds
– observe the reactions of an unknown compound and
compare to the known compounds
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Known compounds
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sodium chloride, NaCl
sodium carbonate, Na2CO3
magnesium sulfate, MgSO4
ammonium chloride, NH4Cl
water, H2O
Exp 2: Identification of a Compound
Experimental Concept (cont’d)
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Test compounds for
– reaction of solutions with silver nitrate reagent, AgNO3
– reaction of solutions with sodium hydroxide reagent, NaOH
– reaction of solutions with hydrochloric acid reagent, HCl
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Observe Evidence of a Reaction (chemical change)
– change of color in a solution
– change in odor of a solution
– evolution of a gas
• bubbles forming in the solution (not just a few, but a lot)
– appearance or disappearance of a precipitate (insoluble material)
• solids can be so fine that they are sometimes hard to see
• a precipitate will eventually settle at the bottom of a test tube
Formation of an Insoluble Precipitate
Reaction of lead nitrate with sodium chromate…
…yields lead chromate and sodium nitrate
Pb(NO3)2 + Na2CrO4  PbCrO4 + 2 NaNO3
Exp 2: Identification of a Compound
Preparation: Put 10 drops of each of the solutions (NaCl, Na2CO3,
MgSO4, NH4Cl and water) in separate wells of the clean, marked test
plates (spot plates). These are numbered 1-5 across the length of the
plate. Label the wells along the width with A,B and C for each test
A. Testing with Silver Nitrate
1. Add 3-5 drops of the silver nitrate (AgNO3) solution. Add 5-10
more drops to those that seem to change.
2. Record your observations including color, “thick” or “thin”
precipitate!
B. Testing with Sodium Hydroxide
1. Slowly add 5-10 drops of the sodium hydroxide, NaOH, solution.
Check for gas, odor (use proper techniques) and formation of
precipitates
2. Record your observations including color, “thick” or “thin”
precipitate!
Exp 2: Identification of a Compound
D. Testing with Hydrochloric Acid
1. Slowly add 5-10 drops of the hydrochloric acid reagent, HCl, to
each solution. Check for gas, odor (use proper techniques).
2. Observe closely; Record your observations including color, odor
if any
Testing the unknown solution:
1. Place 10 drops of the unknown solution in three wells
of column 6
2. Test with the reagents, using the same proportions as with the
known solutions. Compare results and identify your unknown.
Clean up:
1. Dispose solutions in the liquid waste container in the fume hood
2. Rinse the plates with tap water and DI water
Exp 2: Identification of an Unknown Compound
Reaction/Observation Worksheet
NaCl
Na2CO3
MgSO4
NH4Cl
H2O
Unknown
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AgNO3
test
NaOH
test
HCl
test
Key: c = color: w,y,bl,br,gr
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pc precipitate + color
cc cloudy + color
nr no reaction
*: indicate number or letter and name of the unknown compound
g gas, no odor
go gas, odor
Exp 2: Identification of a Compound
Next week
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SPRING BREAK: Study for Lecture Quiz Monday
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Tuesday June 9: Turn in Pre-lab for Exp 10: Vinegar Analysis
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Read procedure and safety precautions