Name: ________________________________________ Hour: _____ Date: ________________
Qualitative Analysis of Net Ionic Reactions
37
Qualitative analysis was introduced in Experiment 18 as the process of separating and identifying
chemical elements. In developing a process of analysis, the chemist looks for unique properties
which can be used to identify and separate one element from another. The analytical process
considered here takes place in aqueous solutions and includes the use of many basic chemical
principles such as acid-base, solubility, equilibrium, and complex ion formation reactions.
To describe the chemical changes in analysis, it is customary for chemists to use net ionic
equations. In net ionic equations only those ions taking part in the reaction are written. Other ions
present in the solution but not involved in the reaction are known as spectator ions and are not
included in the equation. For example, a reaction between an aqueous acid, HCl, and an
aqueous base, NaOH, would have the ionic equation
H3O+ + Cl- + Na+ + OH-  H2O(l) + Na+ + ClThe only chemical change occurring is the formation of H2O from H3O+ and OH-. Sodium and
chloride ions are spectator ions and may be dropped from the equation. The remaining ions form
the net ionic equation
H3O+ + OH-  2H2O(l)
Similarly, a solubility reaction between aqueous solutions of CaCl 2 and K2CO3 would have the
equation
Ca2+ + 2Cl- 2 2K+ + CO32-  CaCO3(c) + 2K+ + 2ClThe net ionic equation would be
Ca2+ + CO32-  CaCO3(c)
It is important to note, that substances occurring in a reaction in molecular form are written as
molecules. For example, in the formation of a complex ion, a high concentration of aqueous
ammonia will react with Cu2+ to form tetraammine copper (II) ions, Cu(NH3)42+. The net ionic
equation would be
4NH3 + Cu2+  Cu(NH3)42+
In this experiment, you will conduct a series of tests on solutions containing the metal ions Pb 2+,
Ag+, and Hg22+ in an attempt to isolate chemical characteristics which can be used to identify the
metals.
Objectives
In this experiment you will:
 Conduct a series of chemical tests on solutions containing Pb2+, Ag+, and Hg22+ ions;
 Write net ionic equations for each reaction;
 Describe and explain any equilibrium change; and,
 Propose an analysis scheme for the three metals.
Equipment
Three to six test tubes (13 X 100 mm)
A dropper pipet
Procedure
CAUTION: Ammonia and lead and mercury compounds are poisonous. AgNO3, HCl, NH3, and HNO3 cause
severe burns. Avoid skin contact with these compounds!
1.
Obtain three test tubes. Place 3 mL
portions of 0.1 M solutions of Pb(NO3)2 in
one test tube, AgNO3 in a second test tube,
and Hg2(NO3)2 in the third test tube. Label
the test tubes for the solution in each.
6.
Let the two test tubes containing the
Hg22+ and the Ag precipitates cool.
Decant the liquid, taking care to retain
most of the precipitate. (Centrifuge
before decanting if possible.)
2.
Add about 1 mL of 6 M HCl to each test
tube and observe the results. Record your
observations for the reaction in each test
tube.
7.
Add about 3 mL of 6 M NH3(aq) solution to
each test tube. Agitate the test tubes.
Note the results.
3.
Let the precipitates settle. (Use a centrifuge
if one is available.) Decant the liquid from
all three test tubes. Retain the precipitates.
The decanted liquid may be discarded.
8.
To the test tube in which the precipitate
dissolved, add about 4 mL of 6 M HNO3.
Record your observations.
4.
Add about 2 mL of distilled water to each
test tube. Place the test tubes in hot water
bath. Are any of the precipitates soluble in
hot water?
5.
Using your dropper pipet, remove a few
drops of the clear solution from the test
tube containing the precipitate which is
most soluble in water. Transfer the drops of
solution to a clean test tube. Add a few
drops of 0.1 M K2CrO4 to the solution.
Record your observations.
Analysis
1. Propose an analytical scheme for separating and identifying Pb2+, Ag+, and Hg22+.
2. Write balanced net ionic equations for all chemical reactions which occurred in this
experiment.
Conclusions
1.
Upon what chemical characteristic is the separation of the Pb2+ ion from the Hg22+ and
Ag+ ions based?
2.
Upon what chemical property is the separation of Hg22+ and Ag+ ions based?
3.
Would NH3(aq) be classified as an acid or a base? Explain.