SEPARATION & QUALITATIVE ANALYSIS OF CATIONS & ANIONS
The goal of this experiment is to study some double replacement (metathesis) reactions and develop an
understanding of the solubility rules which are applied when predicting the outcome of double replacement
reactions.
What are double replacement reactions?
1. In double replacement reactions, there is an exchange of ions between the two reacting species. The
general form of the reaction is:
A+B– + C+D–  A+D– + C+B–
Notice the exchange of ions. That is, the positive ion, A+ in compound AB, combines with the negative ion, D–,
in compound CD combine to form the product AD. Likewise, the positive ion, C+ in compound CD, combines
with the negative ion, B–, in compound AB combine to form the product CB.
2. Double replacement reactions only occur when ions are added or removed from the solution.
Such as when . . . . . .
 An insoluble solid product, called a “precipitate” is formed (the type studied in this lab).
 A molecular species such as a liquid, H2O(l), or gas such as carbon dioxide, CO2(g), is formed.
 A liquid(l) or solid(s) reactant is consumed (used up).
Another goal of this experiment is to use each of the four methods of writing the equation for a chemical
reaction. The four types are:
1. Word equation
In word equations, the names of the reactants and products are written out. An example of a word
equation is:
sodium iodide + lead nitrate → sodium nitrate + lead iodide
2. Formula equations
Formula equations consist of formulas substituted for the names in the word equation. Once the
formulas are determined, the reaction can be balanced. Substituting formulas for names in the
reaction above:
3. Ionic equations
Using the solubility guidelines, all water soluble compounds in an aqueous solution are separated
into ions in ionic equations. If the substance is a solid, liquid, or gas, the substance is NOT separated
into aqueous ions as they do not exist with these substances.
The next step in the process is to re-write the balanced equation giving the state (solid, liquid, gas, or
aqueous) of each of the reactants and products. Since sodium iodide, NaI, lead nitrate, Pb(NO3)2, and
sodium nitrate, NaNO3, are predicted to be soluble in water, they are given the aqueous, (aq),
designation. Since lead iodide, PbI2, is predicted to be “insoluble” in water, it is given the solid, (s),
designation.
2NaI(aq) + Pb(NO3)2(aq)  2NaNO3(aq) + PbI2(s)
Now that we have a balanced formula equation we can write the ionic equation. That is, separate each water
soluble substance (aq) in the formula equation into its ions. In this example, NaI, Pb(NO3)3, and NaNO3 have the
aqueous solution designation and therefore must be written as ions.
2Na+(aq) + 2I–(aq) + Pb2+ (aq) + 2NO3–(aq)  2Na+(aq) + 2NO3–(aq) + PbI2(s)
Since there are two moles of sodium iodide, 2NaI(aq), in the balanced formula equation, we will separate the
two moles sodium iodide into 2 moles of Na+(aq) and two moles of I– (aq). Likewise, the one mole of lead
nitrate, Pb(NO3)2(aq), becomes one mole of lead ions, Pb2+(aq) and two moles of nitrate ions, NO3–(aq). In the
product, only the two moles of sodium nitrate, 2NaNO3(aq), is separated into ions as it’s the only water soluble
product.
4. Net ionic equations
The net ionic equation contains all of the particles in the ionic equation less any “spectator” ions such as the
Na+(aq) and NO3–(aq) in this example.
The net ionic equation represents the actual reaction which is taking place in your experiment. Notice that
in the ionic equation, the sodium ions and nitrate ions are identical on each side of the balanced equation.
Their presence is not important and they can be eliminated from the overall reaction. They are called
spectator ions. Spectator ions do not participate in the chemical reaction. That is, they are identical on both
sides of the equation. In the reaction above, the sodium, Na+(aq) and nitrate, NO3–(aq), ions are the same on
both sides of the equation and therefore can be eliminated.
2Na+(aq) + 2I–(aq) + Pb2+ (aq) + 2NO3–(aq)  2Na+(aq) + 2NO3–(aq) + PbI2(s)
Net ionic equation:
Pb2+ (aq) + 2I–(aq)  PbI2(s)
What does the net ionic equation tell us?
When solutions of sodium iodide, NaI(aq), and lead nitrate, Pb(NO3)2(aq), are mixed, an insoluble product
(called a “precipitate”) of lead iodide, PbI2(s), is formed. The sodium and nitrate ions remain is solution as they
are soluble in water.
Pre-Lab Assignment:
1. For each test to be performed (26 total):
(a) Assuming that each reaction is a double replacement reaction, write the names and formulas of the
products and record in the data table.
(b) Using the solubility guidelines, predict if the reactants and products are soluble or insoluble, and record
that information in the space provided in the data table.
MATERIALS:
A one foot square section of plastic wrap, paper towels, stirring rod, dropping bottles containing 0.1M solutions
of the following:
Na2CO3
NaI
Sr(NO3)2
CuSO4
Pb(NO3)2
CaCl2
ZnC2H3O2
KBr
PROCEDURE:
You and your partner will do half of the reactions (13 tests) and then pool your results with the other group
working at your table.
1. Spread a piece of plastic wrap over white paper on the lab bench. Smooth out any wrinkles.
2. For each trial: Obtain the dropper bottle of the “main” soluble compound you will be testing and place one
drop of the solution on the plastic wrap for each test to be conducted with that solution. Be sure to leave
enough space between the test spots so that they do not mix with other tests. Record the names and
formulas of all the compounds to be tested with the main solution in the data table. Holding the dropper tip
a few inches above the main drop, add one drop of each of these solutions to the main solution and observe.
*DO NOT touch the dropper tips to drops of other solutions as this will contaminate the solutions and give
inaccurate results!*
If the solution turns cloudy, a precipitate or nearly insoluble product has been produced by that
combination of materials. Record your observations by writing "ppt" to denote a precipitate and "NR" if
there is no reaction observed (both products are soluble in water) in the "reaction" column on your data
sheet. (Even though a color change with no precipitate is considered to be "no reaction or NR" in this
lab, it doesn’t mean that there was no reaction…it just means that the products were soluble…)
3. When you are finished with your tests, collaborate with the other group working at your table and obtain the
data for the solutions you did not mix together.
4. Clean up: place another piece of paper towel on the plastic wrap to absorb the solutions. (Don’t touch it,
just let it soak up.) Then roll everything up in the plastic wrap and place in the trash. Thoroughly wash
your hands as heavy metals used in this experiment are toxic.
Group 1 Tests:
Group 2 Tests:
Strontium nitrate with
Calcium chloride
Sodium carbonate
Copper sulfate
Zinc acetate
Sodium iodide
Potassium bromide
Calcium chloride with
Sodium carbonate
Copper sulfate
Zinc acetate
Sodium iodide
Lead nitrate
Potassium bromide
Sodium carbonate with
Copper sulfate
Zinc acetate
Lead nitrate
Potassium bromide
Copper sulfate with
Zinc acetate
Sodium iodide
Lead nitrate
Potassium bromide
Zinc acetate with
Sodium iodide
Lead nitrate
Potassium bromide
Sodium iodide with
Lead nitrate
Potassium bromide
Lead nitrate with
Potassium bromide
QUESTIONS:
1. For only the tests in which a reaction occurred (i.e. a precipitate formed):
(a) Write a complete word equation for the double replacement reaction studied. Be certain to note whether
the material is dissolved in water with the (aq) for aqueous solution or if the product is considered
insoluble using (s) for solid notation.
(b) Write a complete, balanced formula equation for each of the reactions studied.
(c) Give the ionic equation for each of the reactions studied.
(d) Eliminate the spectator ions from the ionic equation and write the net ionic equation for each reaction
studied.
2. Were there any discrepancies between the solubility guidelines and what you observed in this lab? If so,
explain possible reasons for this.
3. Some of the solutions were not tested with one another. For example, sodium carbonate was not tested with
sodium iodide, nor was strontium nitrate tested with lead nitrate. Explain why you did not have to test
these solutions with one another.
Reactant #1
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