THE CHEMISTRY OF FIVE ANIONS
The purpose of this experiment is to develop the essential skills of doing
science: the collection of data, the analysis of the results using chemical
theory, and the design of additional experiments. In keeping with this goal,
we have not provided you with a detailed experimental procedure. You will
be expected to develop your own procedures using the brief outline provided
In this experiment you will study the chemistry of five anions: nitrite, iodide,
carbonate, dihydrogen phosphate, and sulfite. These anions will be made available as 1
M aqueous solutions of the potassium or sodium salts of these anions. You will observe
the reactions of these anions with the following reagents: 3 M nitric acid, 0.25 M
copper(II) nitrate, 0.25 M calcium nitrate, 3 M sodium hydroxide, and 5 vol. % hydrogen
peroxide. The hydrogen peroxide solution is acidified with nitric acid as this reagent will
be used as an oxidizing agent. These reagents cover the full range of the chemistry
studied thus far in Chemistry 1: acid-base, oxidation-reduction, precipitation, and the
formation of complexes. You will be expected to write a balanced net-ionic equation
for each reaction that occurs.
All of the chemistry required to interpret your experimental observations has been
presented in lecture. You may wish to review your lecture notes and the textbook before
coming to the laboratory. Some subtle points which you would not be expected to know
are: l) Hydrogen peroxide can react with itself to yield water and molecular oxygen.
Therefore, a reaction of hydrogen peroxide with an anion cannot be inferred from the
observation of gas evolution alone. An anion might function only as a catalyst for the
decomposition of the hydrogen peroxide. 2) Bromide in an acidic solution might be
slowly oxidized by the oxygen in air.
As mentioned above, you will be required to 'predict' the products of
each reaction and write balanced chemical equations. Strategies for
accomplishing this have been described in your text. Sometimes,
more than one product seems possible. It may be helpful to consult
references such as The Merck Index or CRC's Handbook of
Chemistry and Physics for information on color, physical state,
solubilities, etc.
1
EXPERIMENTAL PROCEDURE
The first week you will carry out preliminary tests using the known anion
solutions. The second week you will be given an unknown mixture of any two of the
anions for analysis.
Preliminary Tests
Select a reagent and observe how the reagent reacts or does not react with each of
the 5 anions. Observe carefully and record your observations in your notebook. You
should prepare your notebook in advance in order to organize your observations better.
You do not have to use a large amount of material to acquire useful data. A
recommended procedure is to add 1 ml of each anion solution into a test tube using the
dispensers on the bottles containing the anion solutions. Pour approximately 10-20 ml of
the selected reagent into a small beaker and at your bench add the reagent dropwise to
each test tube. When adding the reagent to an anion solution, first add only one or two
drops, shake the test tube to mix the solutions, and make your first observation. Then
continue the dropwise addition until the reagent is present in excess. Again, shake the
test tube well to mix thoroughly the two solutions and make your second observation.
In the case of some combinations, there will be no perceptible chemical change.
A second test is recommended in these cases. Use the same amount of the anion solution
but add approximately 2 ml of the reagent and shake the test tube well to mix the two
solutions. Measure the temperature of the solution before and after addition of the
reagent. The standard enthalpy change for several reactions, e.g., most acid-base
reactions, is large enough to yield a measurable change in the temperature.
Repeat the above steps for the remaining reagents. As you are proceeding,
interpret the observations using the theory presented in lecture and in your text and record
your conclusions. Keep in mind that some of the reagents contain a mixture of reactive
species and more than one reaction will occur with some combinations of anion and
reagent. You may wish to repeat a particular test or devise a new test using the reagents
provided to resolve any ambiguities or to test a hypothesis.
It is highly recommended that you work on writing the net-ionic equations
for all the reactions, which you observe, before the second week of the experiment.
2
Investigation of an Unknown
The final step in the experiment will provide a test of the thoroughness of your
observations. On the basis of your observations and your interpretation of the
observations, devise a method to analyze an unknown mixture that will contain two of the
anions. You will be given enough unknown solution to run several separate tests. The
presence of an anion should be supported by positive evidence. You might surmise that
an anion is present by process of elimination of other ions, but you should confirm its
presence with a positive test and not rely on negative evidence alone. We suggest that
you get together in small groups before the second week of the experiment to discuss your
methods of analysis of an unknown solution. (It is not acceptable to simply try every
reagent to "see what happens." By careful design, you should be able to identify the two
anions in your unknown mixture with only two or three reagents. The results from your
first test can guide your choice of reagent for the second test.) Write down your
procedure, which can be in the form of a flow chart or a series of written steps, on a
separate page of your notebook. Your procedure should include not only which reagent
you would add first, etc., but also what possible results are expected and what each result
tells you about the presence or absence of a particular ion. The yellow sheet from your
notebook with your procedure will be collected at the beginning of the lab period. During
the class discussion period before the beginning of the lab, groups will be called at
random to describe their procedures to the rest of the class so that the procedures can be
critiqued by the class. After the class discussion, if you have handed in a yellow sheet
with your procedure, you will be issued a test tube containing an unknown mixture of any
two of the anions. Use the five reagents and the procedure that you have developed to
analyze your unknown mixture. Before analyzing your unknown solution, we suggest
that you test your method on known mixtures that you can prepare from the solutions of
the five anions.
Report
As usual, hand in the yellow copies of the entries in your laboratory notebook at
the end of each period; however, in this experiment a greater portion of the grade will be
based on the organization and the thoroughness of the observations and comments in your
notebook. Do not record any observations on scraps of paper with the intent of
recording them later in an edited fashion in the notebook. (The laboratory instructor
reserves the right to confiscate these scraps of paper.)
For your net-ionic equations you will be expected to omit all spectator ions and to
3
label each species with the proper notation to indicate the state of the species, i.e., gas =
(g); liquid = (1), solid (precipitate) = (s), and aqueous = (aq). Include only the
predominant species for a given compound, for example, the weak acid, HA, should be
written as HA (aq) and not be shown ionizing.
The following report is due one week after the completion of the experiment at the
beginning of the lab period.
4
Name_________________________Lab Section_______________
Date Report Submitted ____________
THE CHEMISTRY OF FIVE ANIONS
A)
Net-Ionic Chemical Equations for Reactions Observed in Preliminary Tests.
For each anion neatly write balanced, net-ionic chemical equations for all the
reactions which you observed in your preliminary tests. Clearly label the type of
chemical reaction (acid-base, oxidation-reduction, precipitation, or complex
formation) in each case. If you observed a temperature change indicate this by
including "heat” on the appropriate side of the equation. If no reaction occurred
indicate this by writing NR in place of an equation. If excess reagent causes an
additional reaction a separate equation should be given.
1) nitrite
a) nitric acid
b) copper(II) nitrate
c) calcium nitrate
d) sodium hydroxide
e) hydrogen peroxide in nitric acid
2) iodide
a) nitric acid
b) copper(II) nitrate
c) calcium nitrate
d) sodium hydroxide
e) hydrogen peroxide in nitric acid
5
6
3) carbonate
a) nitric acid
b) copper(II) nitrate
c) calcium nitrate
d) sodium hydroxide
e) hydrogen peroxide in nitric acid
4) dihydrogen phosphate
a) nitric acid
b) copper(II) nitrate
c) calcium nitrate
d) sodium hydroxide
e) hydrogen peroxide in nitric acid
5) sulfite
a) nitric acid
b) copper(II) nitrate
c) calcium nitrate
d) sodium hydroxide
e) hydrogen peroxide in nitric acid
7
8
Name__________________________________Sample No.___________
B)
Investigation of an Unknown. Your deduction of the anions present in your
solution depends on your observations and conclusions based on your observations.
Succinctly describe your observations and state your conclusions . If you have more
than four observations and conclusions, put them on the back of this sheet.
l) Reagent and Observation 1
Conclusion 1
2) Reagent and Observation 2
Conclusion 2
3) Reagent and Observation 3
Conclusion 3
4) Reagent and Observation 4
Conclusion 4
C)
Anions present in the unknown ________________________
9
10