Lab 6 Two Weeks Characterization of Inorganic Compounds*

Transcription

1 Lab 6 Two Weeks Characterization of Inorganic Compounds* *Taken from an Inorganic Lab course offered at Purdue University Chemical & Engineering News reported the levels of production for industrial chemicals and elements in the United States. The article from 2002 is available as a PDF file, and relevant data are summarized in Table 1. The inorganic chemicals, elements, and minerals in this list exhibit most of the common types of chemical behavior studied in an introductory inorganic chemistry course. They include acids and bases, oxidizing and reducing agents, covalent molecules, and ionic solids. This experiment requires that you utilize your chemical knowledge to design chemical tests to identify 20 of these chemicals from Table 1. It will give you experience working with common inorganic compounds and with their reactions, as well as in applying the scientific method to a research problem. Thus, the results really are less important than the experimental methods that you develop. Unidentified samples of these substances will be provided and you will identify them from their reactions with each other and with a limited number of test reagents. Several industrially important chemicals have been excluded from this lab; all active unknowns are denoted by a * in Table 1. These elements and inorganic chemicals: nitrogen, lime, ammonia, sodium hydroxide, chlorine, phosphoric acid, sodium carbonate, nitric acid, ammonium nitrate, hydrogen chloride, ammonium sulfate, potash, titanium dioxide, aluminum sulfate, sodium sulfate, and sodium sulfate; are included among the unknowns. Two other chemicals for which production data was not available, carbon dioxide and calcium chloride, as well as three chemicals of lesser industrial importance: iron(iii) chloride hexahydrate, copper(ii) nitrate hexahydrate, and cobalt(ii) sulfate heptahydrate, are included. Your reagents include the following: ph paper, phenolphthalein, concentrated HCl, solid MnCl 2, and a test solution of 0.1 M KMnO 4. The identity of two of these substances is ambiguous. Lime refers to both CaO and Ca(OH) 2. Potash refers to potassium containing bases such as KOH, K 2 O, and K 2 CO 3. We will provide Ca(OH) 2 and K 2 CO 3. BACKGROUND RESEARCH (PRELAB) Your background report is due before you begin your laboratory work and counts for 40% of your lab grade. Include a list of references. Be sure to keep a copy for yourself! Your prelab will be completed individually. For more detail of the grading for this report, see the final section of this handout. 1. Construct a table of Physical and Chemical Properties for the 20 elements and inorganic chemicals you have as unknowns. The table should include each substance s name and its chemical formula, physical state at room temperature, solubility in water, any characteristic odor or color, and acidity or basicity (i.e., whether it behaves as a strong acid, weak acid, more-or-less neutral substance, weak base, or strong base in water).

2 2. Construct a second table which shows the product of the reaction (or NR if no reaction occurs) for each of the elements and inorganic chemicals you have as unknowns with the other unknowns. [Resources: CRC Handbook of Chemistry & Physics, The Merck Index, Lange s Handbook of Chemistry, chemical catalogs, any of your textbooks, other chemistry books in the library, and references from the World Wide Web (search chemistry at Remember to cite your references. 3. Outline the following: A. a general procedure for identifying the unknowns using the properties you have listed from the literature. B. reactions between the unknowns and the test materials provided. C. reactions between unknowns. [Resources: qualitative analysis books, solubility rules, an inorganic chemist, your textbook, other course materials, and other chemistry books in the library.] LABORATORY WORK You will be given the 20 unknowns that are listed in the introduction, and assigned a partner. You and your partner should identify the unknowns using the procedure devised in your prelab; however, you may find it necessary to modify the procedure as you go along. In either case, record the procedure followed. Most unknowns will be solids, gases, or 1 M solutions; however, chlorine will be provided as a saturated solution. Caution: Handle these chemicals with respect. Each of these substances can damage your eyes, skin, clothing and books. Do not get these compounds on your neighbor, yourself or the lab fixtures. Should you have an accident, contact you lab instructor and clean up immediately. Most exposure to chemicals occurs because of careless technique in transferring materials. If you use an eyedropper to remove a liquid from a sample of unknown, rinse it immediately, do not lay it on the lab bench. If you use a spatula to remove a sample of a solid unknown, rinse it immediately do not lay it on the lab bench. When preparing solutions, always add concentrated acids or bases to water. Do not add water to concentrated acids to solid bases. Formation of solutions is exothermic and the heat generated by adding water can cause spattering. Do not mix unknowns without diluting them first! Preparation of Samples From Liquids: While stirring, carefully add each liquid unknown to water and prepare about 10 ml of a solution with a concentration of approximately 0.1 M. Use the unknown that contains chlorine as supplied. From Solids: Add samples of the solid unknowns, each with the approximate volume of a small pea, to 20 ml of water. If the solid is soluble, this will produce a solution with a concentration

3 of approximately 0.1 M. If you find at some later point you need a more precise concentration, weigh the solid and use a graduated cylinder as a pseudo-volumetric flask. From Gases: Prepare approximately 0.1 M aqueous solutions of the gaseous unknowns by slowly bubbling the gas into 20 ml of water for 60 seconds. Specific Chemical Tests Oxidant: To test for oxidizing properties ad 4 drops of a 0.1 M unknown solution to 6 drops of MnCl 2 test solution (a freshly prepared, saturated solution of MnCl 2 in 6 drops of concentrated 3 ] 6 HCl). Formation of a green to brown color upon warming is due to the formation of [MnCl and indicates the presence of NO 3 or Cl 2. By the way, is Mn(II) an oxidizing or reducing agent here? Reductant: To test for reducing properties, add 1 drop of the KMnO 4 test solution to 1 ml of 0.1 M unknown solution. Heat in a boiling water bath for a couple of minutes. Formation of brown MnO 2 (s) or colorless Mn 2+ (aq) indicates the presence of Cl or NH 3. By the way, is Mn(VII) an oxidizing or reducing agent here? Test for ammonium ion: To test for ammonium ion, add 4 6 M NaOH solution dropwise to 1 ml of 0.1 M unknown solution until strongly basic. Moisten a piece of red litmus paper and place it over the mouth of the test tube. You might need to place the test tube in a hot water bath for a couple of minutes. If the ammonium ion is present, the NH 3 gas released will turn the litmus blue. Note: make sure a color change is due to contact with gaseous NH 3, not with the basic solution. Checking reactivity: When checking the reactivity of one unknown with another, place 5 drops of one 0.1 M unknown solution in a test tube and add the other 0.1 M unknown solution one drop at a time until a total of 10 drops have been added. Mix the contents and examine the behavior after each drop is added. Do not use undiluted liquid unknowns for these tests. LABORATORY NOTEBOOK In your laboratory notebook, keep a record of what you did, how you did it, what you observed, whom you worked with (if you had a lab partner), and the date of the work. You may also add comments and speculations about the problem under investigation. Your laboratory notebook must be neatly written and sufficiently complete that another student in your class (1) could understand what you did from reading your notebook, (2) could reproduce your experiments, and (3) could determine if the results and observations from the repeated experiments matched yours. Copies of your notebook pages are due at the end of the lab period. As mentioned in the introduction, the primary goal of this experiment is achieving good experimental design LABORATORY REPORT Write a detailed report of your analysis and identification of each unknown. You may not cite the procedure in the lab manual for this report; a detailed Procedure section is required. This report should be sufficiently complete so that another student can come to the same conclusions about the identity of each unknown from your data. As with any research report, your lab report must describe your results sufficiently that the reader does not need to refer to your notebook. The grading scheme is a little different for this report. Your prelab (as described in this handout) is worth 4 points. Lab technique is worth 2 points. Identifying the unknowns correctly is worth 2 points. Discussion of the logic used to identify your unknowns is worth 2 points. So it is very

4 important that you identify the observations which lead to your conclusions. Finally, write balanced chemical equations for each reaction that occurred.

5 Table 1. Annual U.S. Production of Inorganic Chemicals and Minerals kilotons unless otherwise noted Production Annualized Change * Aluminum sulfate 1,185 1,091 1, % 0.2% * Ammonia 17,169 16,806 13, * Ammonium nitrate 7,819 7,498 7, * Ammonium sulfate 2,243 2,868 2, Bromine * Calcium Chloride NA * Carbon Dioxide NA * Chlorine 11,572 13,131 12, * Cobalt(II) sulfate heptahydrate NA * Copper(II) nitrate hexahydrate NA * Hydrochloric acid 3,301 4,718 4, Hydrogen * Iron(III) chloride hexahydrate NA * Lime 17,270 21,609 20, Lithium * Nitric acid 7,927 8,479 7, * Nitrogen gas Oxygen Phosphate rock 53,016 42,557 37, * Phosphoric acid 12,109 13,143 11, * Potash 1,928 1,433 1, * Sodium carbonate 10,093 11,246 11, Sodium chlorate Sodium chloride 40,021 50,274 49, * Sodium hydroxide 11,713 11,518 10, * Sodium sulfate Sulfur 11,928 11,367 10, Sulfuric acid 43,466 44,032 40, * Titanium dioxide 1,095 1,547 1, Source: C&E News, June 2002 * Denotes an unknown for this lab