AP Chemistry Lab
—
Precipitation Reactions and Solubility Rules
Prelab:
1. To be done on a separate piece of paper and turned
in before lab starts!
Write balanced molecular equations for each of the
reactions you will be performing
in Parts I and II. (There should be a total of 42 equations
.) You will be mixing each
compound with each other compound, but not with itsel
f. It does not matter what
order in which you react the compounds. (For exam
ple. silver nitrate ± barium
chloride will give the same results as barium chloride +
silver nitrate!)
2. Prepare two data tables in your lab notebook one
for Part I and one for Part II. Use a
full page for each table! For Part I. label 7 columns
(1-7 to correspond to the top of
the 48-well reaction plate you will use) with the corre
ct formulas of each of the seven
solutions for Part I, starting with silver nitrate. Labe
l 6 rows (A-F to correspond to the
side of the 48-well reaction plate you will use) with the
correct formulas of six
solutions for Part I, starting with barium chloride since
you will not mix silver nitrate
with itself. For the second data table, repeat with the
seven solutions for Part II. Each
square in each data table must be large enough to
record the observations and
also to write the formulas of the two possible prod
ucts formed!
—
Introduction:
Discover some general rules of solubility for ionic subs
tances. Conduct 42
chemical reactions, determine the solubility of the prod
ucts. analyze the patterns, and
formulate the rules.
Background:
An ionic salt is a compound composed of two parts
caflons (positively-charged
ions) and anions (negatively-charged ions). When an ionic
salt is dissolved in water, the
salt crystal dissociates or separates into its correspon
ding cations and anions. For
example, potassium iodide (KI) dissociates into potassium
cations (K) and iodide anions
(F) according to equation 1:
—
KI (s)
K (aq)
+
F (aq)
Equation 1
Similarly, the ionic salt lead nitrate 3
[Pb(N0 dissociates into lead cations (Pb
j
2
)
) and
2
nitrate anions (N0
) according to equation 2:
3
3
Pb(
2
)
N0 (s)
2 (aq)
Pbt
±
2 N0
3 (aq)
Equation 2
When two ionic salts are mixed together in water. two
new combinations of cations and
anions are possible. In some cases, the cation from one
salt and the anion from the other
salt may combine to form an insoluble solid product. whic
h is called a precipitate. For
example. if the solutions of potassium iodide and lead nitrate are mixed together. a
solid
precipitate of lead iodide (Pb1
) forms, as shown in equation 3:
2
2Kb(aq) + 21 2
(aq)+
(
iq
Pb
) + 2N0 (aq)
Pb
(
2
s)2K
1 (aq) +2N0
3 (aq)
Equation 3
Notice that the potassium cations (K) and the nitrate anions (N0
) remain dissolved in
3
solution. These ions are referred to as spectator ions. Spectator ions do not partici
pated
in the overall reaction (hence the term spectators) and are often omitted from the net ionic
equation. A net ionic equation is one that includes only the ions participating in the
reaction. Thus, equation 3 can be reduced to equation 4:
2 (aq)
Pb
+
2F (aq)
2 (s)
Pb1
-‘
Equation 4
The example described above shows that a precipitate of Pb1
2 forms when a solution of
KI is mixed with a solution of 3
Pb(N0 However, when two salt solutions are mixed
.
2
)
together and no visible precipitate forms (indicating that no reaction occurs), it can
be
concluded that all combinations of the cations and anions from the two salt solutions
are
soluble in water.
It must be noted that every salt has some degree of solubility. Some salts dissolve
readily in water while others will not. Even if a salt is considered insoluble (formi
ng a
solid precipitate). there is still some small amount of that salt dissolved in the water.
(Hence the double arrow” in the equations above. A more correct representation would
have a top arrow to the right which is longer than a lower arrow to the left.) Tables
of
solubilities are available to predict whether or not a precipitate will form when two salts
are mixed together. However, observing precipitation reactions in a laboratory setting
and becoming familiar with the general rules of solubility can be extremely valuable.
In this laboratory experiment, 14 salt solutions will be combined, two at a time
seven in Part I and seven in part II totaling 42 chemical reactions. Observations will
be
recorded as to which combinations form a precipitate and which do not undergo a
reaction. From these results, a list of soluble substances and a list of insoluble substan
ces
will be generated. This list will be analyzed and some general rules of solubility will
then be developed.
—
—
Materials Needed:
Chemicals — Part I
Silver nitrate solution, 0.1 M
Barium chloride solution, 0.1 M
Sodium carbonate solution. 0.1 M
Ammonium sulfate solution, 0.1 M
Lead nitrate solution. 0.1 M
Calcium nitrate solution, 0.1M
Potassium phosphate solution. 0.1 M
Chemicals — Part II
Iron (III) chloride solution. 0.1 M
Sodium hydroxide solution, 0.1 M
Magnesium bromide solution, 0.1 M
Potassium carbonate solution, 0.1 M
Lead nitrate solution. 0.1 M
Sodium sulfate solution, 0.1M
Sodium chromate solution. 0.1M
Equipment
Reaction plate. 48-well
Pipet (cassette) holder, with labeled beral pipets. for Part
I
Pipet (cassette) holder, with labeled beral pipets, for Part
II
Distilled water bottle and Q-tips for cleaning well plate
Safety Precautions:
Barium chloride, lead nitrate, and sodium chromate solu
tions are toxic by
ingestion. Silver nitrate solution is toxic, corrosive, light-sens
itive, and stains skin and
clothing. Sodium hydroxide solution is corrosive and a body
tissue irritant. Avoid all
eve and body tissue contact with all solutions, Wear chem
ical splash goggles. chemicalresistant gloves, and a chemical-resistant apron.
Procedure:
Part I
—
Mixing Pairs of Solutions in Data Table 1
1. Take the pipet holder with the 7 labeled pipets for Part
I to the stock solutions
table. Fill (about half-way) the 7bpipets with the appropria
te solutions and take
them back to your lab bench.
2. Place a 48-well reaction plate on top of a black table
or black piece of paper.
3. Using your Part I Data Table as a guide. place 4 drop
s of silver nitrate solution
into well Al. Be sure to hold the pipet vertically for unifo
rmly-sized drops.
Continue adding 4 drops of silver nitrate solution to each
well down the vertical
column 1. wells Bi to Fl.
4. Fill each vertical column in the well plate as follows:
Place 4 drops of barium chloride solution into wells B2-F2
.
Place 4 drops of sodium carbonate solution into wells
C3-F3.
Place 4 drops of ammonium sulfate solution into wells D4-F4
.
Place 4 drops of lead nitrate solution into wells E5-F5.
Place 4 drops of calcium nitrate solution into well F6.
(Remember. it doesn’t matter which order you mix the chem
icals so only half of
the reactions need to be performed!)
5. Now add 4 drops of barium chloride solution to the solu
tion already in well Al.
Record your observations immediately. (Note: If a prec
ipitate appears, write
“PPT” in the correct box of Data Table I. Record the colo
r of the precipitate. If
no precipitate appears to have formed, write NR” for No
Reaction in the correct
box of Data Table I. Make sure to leave space in each box whether or not a
precipitate was formed!- to write the two possible products of the reaction.)
—
6. Continue the process of mixing solutions by adding 4 drops of each of the
solutions listed along the left side of your Data Table I to the correct well. Record
your observations immediately, as described in Step #5.
7. Once Data Table I is complete. turn your well plate upside down on several
thicknesses of paper towel and tap it on the towel to empty the wells.
Immediately flush the reaction plate with tap water. The remaining solutions may
be flushed down the drain with plenty of water. Rinse the well plate with distilled
water (use a Q-tip if this doesn’t completely clean out a well!) and tap it on a
paper towel to dry it out your plate is now ready for Part II.
—
Part II
—
Mixing Pairs of Solutions in Data Table H
8. Take the pipet holder with the 7 labeled pipets for Part II to the stock solutions
table. Fill (about half-way) the 7bpipets with the appropriate solutions and take
them back to your lab bench.
9. Place a 48-well reaction plate on top of a black table or black piece of paper.
10. Using your Part II Data Table as a guide, place 4 drops of iron (III) chloride
solution into well Al. Be sure to hold the pipet vertically for uniformly-sized
drops. Continue adding 4 drops of iron (III) chloride solution to each well down
the vertical column 1, wells Bi to Fl.
11. Fill each vertical column in the well plate as follows:
Place 4 drops of sodium hydroxide solution into wells B2-F2.
Place 4 drops of magnesium bromide solution into wells C3-F3.
Place 4 drops of potassium carbonate solution into wells D4-F4.
Place 4 drops of lead nitrate solution into wells E5-F5.
Place 4 drops of sodium sulfate solution into well F6.
(Remember, it doesn’t matter which order you mix the chemicals so only half of
the reactions need to be performed!)
12. Now add 4 drops of sodium hydroxide solution to the solution already in well Al.
Record your observations immediately. (Note: If a precipitate appears, write
“PPT” in the correct box of Data Table I. Record the color of the precipitate. If
no precipitate appears to have formed, write ‘NR” for No Reaction in the correct
box of Data Table 1. Make sure to leave space in each box whether or not a
precipitate was formed!- to write the two possible products of the reaction.)
—
13. Continue the process of mixing solutions by adding 4 drops of each of the
solutions listed along the left side of your Data Table I to the correct well. Record
our observations immediately, as described in Step #5.
14. Once Data Table I is complete, turn your well plate upside down on
several
thicknesses of paper towel and tap it on the towel to empty the wells.
Immediately flush the reaction plate with tap water. The remaining solutio
ns may
he tiushed down the drain with plenty of water. Rinse the well plate with
distilled
water (use a Q-tip if this doesn’t completely clean out a well!) and
tap it on a
paper towel to dry it out.
Analysis:
Part I
—
Making a List of Soluble, Insoluble, and Unknown Substances
I. In each box where a reaction was performed in Data Tables I and II. write
the
formulas for the two possible double replacement products. This should
be done
even for those combinations that produced no reaction.
2. Use a full page in your notebook to make the following table:
Categories of Substances
Soluble Substances
[ Insoluble Substances
] Unknown Substances
Use this table to make a list of all the substances that you know to be soluble in
water. Generate this list of soluble substances in two ways.
a. Write down each solution that you used in this experiment from the materials list.
(Since these are all solutions with no precipitates. you can assume they are all
soluble.)
h. Look at your recorded observations in the data tables. For each reaction that did
not produce a precipitate (NR), four substances are now known to be soluble the
two starting substances and the two possible products. For example. when mixing
two soluble substances such as sodium carbonate and ammonium sulfate, no
reaction occurs. This tells you that sodium sulfate and ammonium carbonate are
also soluble.
—
3. Generate a list of all substances that you know to be insoluble. Look at your data
tables. Examine each of the reactions that did produce a precipitate. Each pair of
solutions mixed together can produce 2 new possible substances, as discussed in
the background section. Make the assumption that if a precipitate is formed when
mixing two solutions, the precipitate is formed by only 1 of the 2 new substances.
Compare each of the new substances to the list of soluble substances. If one of
the new substances appears on the soluble list, the other must be the precipitate
and can be add to the insoluble column. If neither one of the new substances
appears on the soluble list, then place both substances in the unknown column.
Write the two substances together as a pair. This way. later you will know that if
one substance is found to he soluble, the other must be insoluble.
4.
Look at your lists of soluble and insoluble substances. Cross out any duplicate
entries, using a different colored pen. Leave the list of unknown substances
unchanged.
5. In ‘our notebook. make a separate table like the one above. This time categorize
and alphabetized column 1 (soluble substances) and then column 2 (insoluble
substances) by the anionic part (second work!) of the salt. For example.
potassium chloride would be listed as ‘chloride, potassium” and would be
grouped with all the other chlorides in that column. Leave column 3 (unknown
substances) as is, with the salts written in pairs. Do not mix up the pairs!
6. Examine the pairs of substances in the unknown column of your table.
Remember, if one substance is found to be soluble, the other must be insoluble.
Look for any substances that can now be definitely determine to be soluble or
insoluble. Move these into the appropriate column of your table, and crossing out
the old entry, using your different colored pen.
7. Look for any common relationships among the substances in the unknown
column and that of the soluble or insoluble substances. The key objective at this
point is to remove all of the substances from the unknown column and place
them in either the soluble or the insoluble column using logic and the process of
elimination. There will be a point where no more substances can be moved from
the unknown column by simply looking at laboratory data. At this point,
some
inferences and generalizations will need to he made. For example,
are most (or
all) of the nitrate compounds in the same column? How about the ammo
nium or
potassium compounds? Use the following categories to help with this
step:
carbonates: chlorides and bromides: chromates; hydroxides; nitrates;
phosphates;
sulfates: ammonium. potassium and sodium compounds. As you remove
a
substance. cross out the old entry and place the substance in its new colum
n using
your different colored pen. Under your tinal table. for each substa
nce that you
remove from the unknown column by generalization/inference, list
the
reason that you used to move the substance.
8. After completely eliminating the unknown column, examine the
substances in the
soluble and insoluble columns. Each column should be organized by
group (i.e..
carbonates, chlorides, etc.) if you completed step #5 correctly. In your
notebook.
make a final table like the one below with the solubility of each group,
including any exceptions that you observe. Be sure that your final table
matches your experimental data and not what you “think” you should
get!
-
-
General Solubilitv Rules
Substances
Carbonates
Soluble or Insoluble
Exceptions
Chlorides and Bromides
Chromates
Hydroxides
Nitrates
Phosphates
Sulfates
Ammonium, Potassium,
and Sodium Compounds
9. Write balanced net ionic equations for each metathesis (doubl
e replacement)
reaction that produced a precipitate in this experiment. Be sure to include
states
of matter (aq. s. 1. g).
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—
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____
____
____
____
____
____
_
/arne:
Categories oi Substances
:5oluble Substances
insoluble Substances
Unknown Substances
____
____
____
____
____
____
____
1
a
me.
Categories of Substances
•1
Soluble Substances
Insoluble Substances
Unknown Substances
________________
___________
£‘Ialne.
General Solubility Rules
3ubstances
Soluble
Carbonates
Chlorides and Bromides
I
Chromates
iydroxides
I
I
I
I
,
“itrates
Phosphates
uIfates
!
Xmmonium,
2odium
Potassium
arid
or
Insoluble
Exceptions