The Chemistry of Five Anions

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.

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.

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 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.

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 noreaction 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