Experiment 13. Classifying Elements An Introduction To The Periodic Table. Introduction. Materials (for each lab team of 2 students)

Transcription

1 Experiment 13 Introduction Classifying Elements An Introduction To The Periodic Table Intent This experiment is intended to serve as an introduction to the periodic table and to some of the properties that serve to distinguish among the three categories of elements. Objectives 1. To test and compare some physical and chemical properties of several solid elements. 2. To classify the elements tested into groups with similar properties. 3. To identify the groups as containing metals, nonmetals, or metalloids Materials (for each lab team of 2 students) Apparatus 24-well test plate Forceps or tweezers Conductivity tester Distilled water wash bottle Safety goggles (2) Lab apron (2) Reagents 0.5 M HCl(aq), about 2 ml 0.5 M CuCl 2 (aq), about 2 ml Samples of elements a-h Preparation Hints A class of 30 students, working in pairs, will use only 25 ml of each of the solutions. The concentrations are not critical, so you may choose to dilute some stock solutions (especially for the HCl) that were prepared for other experiments. 0.5 M HCl: Stir 42 ml of concentrated acid into 958 ml of distilled water. Allow to cool before use. 0.5 M CuCl 2 : Dissolve 85 g CuCl 2 2H 2 O in 1000 ml of distilled water. If the earth science teacher will part with them, small hammers are a nice touch for this experiment. While it is easy to make a simple conductivity tester, most suppliers sell them at reasonable cost. If you plan to purchase a set (one for each two lab teams is sufficient, although one per team is ideal), consider models that have two or more LED's. Since each different color draws a different current, multiple-led models can be used in later experiments to distinguish between strong and weak electrolytes or even between solutions of very different concentration. Prelaboratory Discussion World of Chemistry 71 Copyright Houghton Mifflin Company.

2 This experiment works best as an introduction to the periodic table and classification of elements. Spend the bulk of your prelab discussion on the techniques of testing. Be certain students understand that the tests with HCl and CuCl 2 must be done on separate samples and that the conductivity of the elements Procedure Hints 1. Small, plastic "soufflé cups" make convenient holders for the samples. 2. Monitor students' work as you dispense iodine from a container that you carry. 3. Preparation of samples: a. carbon--use inexpensive mechanical pencil leads ( mm), broken into about 2 mm pieces. b. magnesium--cut pieces of ribbon about 2 mm in length. c. silicon--break up into approximately 1 mm 2 pieces. d. tin--cut 2-mm square pieces from foil, or 2-mm pieces from wire. e. sulfur--break into small lumps, about 1 mm 2. f. iron--#0000 steel wool, twist into a 1-2 mm braid, then cut into 2-3 mm lengths. Wire is neater, but not as obviously reactive in the chemical tests. Rinse the steel wool in acetone before cutting to remove oil coating, which is intended to prevent tarnishing. In a pinch, remove the wire from paper-covered twist ties. The wire is iron. g. zinc--cut small pieces from sheet, 2mm x 2mm. Mossy zinc is ok, but depending on its age, students may not see the luster. Like the steel wool, zinc is often given an oil coating to protect the finish. h. iodine--select at least one piece for each group that is large enough to test for conductivity. Disposal 1. Provide a large tray, lined with paper towels, for students to shake out the contents of the well plate. 2. Once all the solid pieces have been removed, the well plate may be washed in soapy water, rinsed with distilled water and left on paper towels to dry. Postlab Discussion Thoroughly discuss the criteria and the placement. All reasonable answers deserve credit. What the student learns and remembers from this lab will be the most important set of ideas of at least the first semester--why the periodic table looks as it does. You may have to assist the students in coming to the appropriate conclusions however, it is best to arrive at the conclusions in an interactive discussion with the class. World of Chemistry 72 Copyright Houghton Mifflin Company.

3 Name Report for Experiment 13 Section Date Classifying Elements Prelaboratory Questions 1. Consult the periodic table on the inside back cover of your text. Based on the elements you are familiar with, where are most of the metals found? Most are in the transition element region; a few are in the numbered groups. 2. Look at the numbering system for the groups as explained in Chapter 3 of your textbook. Do the A groups contain mostly metals or mostly nonmetals? Do the groups in the middle of the table, (transition elements) contain mostly metals or mostly nonmetals? The first two A groups contain metals (except for hydrogen); the rest of the A groups contain a combination of metals and nonmetals. The transition elements are all metals. 3. What basis did Mendeleev use to divide the elements into groups? The types of chemical reactions they exhibit. Data/Observations Data Table Element Malleable or Brittle? Conducts? Shiny or Dull? Color or Other Characteristics a brittle yes dull dark gray b malleable yes sort of shiny sort of silver c brittle yes (dull or shiny) dark color d malleable yes shiny silvery e brittle no dull yellow f malleable yes (dull or shiny) gray g malleable yes shiny silvery h brittle no dull black/purple World of Chemistry 73 Copyright Houghton Mifflin Company.

4 Report for Experiment 13 Name Analysis and Conclusions 1. For each of the five tests of physical and chemical properties, separate the eight elements according to their behavior on that particular test. Place the letter of each element in the space below the descriptions that fit. A. Physical Properties Malleable Shiny conduct d, f, g b, g, h b, c, d, (f?), g a, b, c, d, f, Brittle Dull Nonconducting a, c, e, h a, (c?), e, (f?, h?) e, h B. Chemical Properties reacts with HCl(aq) reacts with CuCl 2 (aq) b, f, g b, d (slight), f, (h?) unreactive with unreactive with CuCl 2 (aq): HCl(aq) a, c, d, e, h a, b, f 2. Combine your five pairs of lists from Question 1 into two groups of elements such that all the members of a given group are alike in at least most of the properties tested. You may find that you have one or two elements that don t clearly belong to either of your two categories because some properties fit one group while other properties fit the second group better. If that happens, make them a third category. World of Chemistry 74 Copyright Houghton Mifflin Company.

5 Report for Experiment 13 Name Category 1 Category 2 Category 3 (?) b, d, f, g e, h a, c 3. Describe briefly the criteria you used to make your groupings. Identify any difficulties you encountered in deciding where to place each element. Category 1: malleable, conducts, shiny, reacts with one or both solutions. Category 2: brittle, nonconducting, dull, not reactive Category 3: conducts, and C is shiny, but they don't do anything else that metals do 4. Using the definitions for metals, nonmetals and metalloids from your textbook, and your groupings, try to identify each of the elements a through h as being a metal, a nonmetal, or a metalloid. Element A B C D E F G H Classification metalloiloimetal metal metal- metal non- metal metal nonmetal World of Chemistry 75 Copyright Houghton Mifflin Company.