UNKNOWN SALTS: WEEK 1-IDENTIFICTION OF THE CATION

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1 University of Puget Sound Department of Chemistry Experiment 8 Chem 110 UNKNOWN SALTS: WEEK 1-IDENTIFICTION OF THE CATION 1. Be familiar with patterns in chemical properties in the periodic table. a. Write balanced chemical equations showing reactions between alkali metals and water and oxygen. b. State trends in reactivity of alkali metals. c. Given the formula, write the name or given the name write the formula for the following polyatomic ions: OH - PO 4 3-, MoO 4 2-, SO 4 2-, CrO 4 2-, ClO - and NO 3 -. d. Write balanced chemical equations showing precipitation reactions between alkaline earth cations and polyatomic anions. e. State trends in reactivity of alkali earth metals to form precipitates. f. Identify unknown salts using flame and solubility tests. g. Write balanced equations for reactions between halides and halogens, and predict the direction of spontaneity for them. h. Identify halides and halogens by their appearance in aqueous and organic solvents. 2. Develop formal writing skills. a. Write a properly voiced, tensed, and structured Experimental Section. INTRODUCTION Similarities between the elements chemical and physical properties were known early in the nineteenth century. In 1817 Johann W. Döbereiner showed in a number of instances that when three elements with similar chemical properties were listed in order of increasing atomic weight, the middle element had properties that were an approximate average of the other two. His groups, or triads included: Li S Cl Ca N As Na Se Br Sr P Sb K Te I Ba As Bi Each triad is a part of a single group of elements in the modern periodic chart. In 1866 John Newlands established his Law of Octaves. Being a lover of music, he theorized that when the elements were arranged in order of increasing atomic weight every eighth element had similar chemical and physical properties, just like every eighth note on the musical scale has a similar sound! For the lighter elements this periodicity is surprisingly valid when the noble gases and the transition elements are omitted. It remained until 1869 for two scientists, Dmitri Mendeléev from Russia and Lothar Meyer from Germany, to independently establish periodic charts similar to our present day arrangement of the elements. Mendeléev showed that when the elements are arranged in order of increasing atomic weight, their chemical properties recur periodically. Meyer s chart was virtually identical. In 1913 H. G. J. Moseley s study of the elements X-ray spectra refined Mendeléev s periodic chart when the elements are arranged in order of increasing atomic number, certain chemical and physical properties repeat periodically. Unknown Salts: Week 1-ID of Cation 1

2 Ordering elements according to increasing atomic number is the method on which our modern periodic chart is established. Each horizontal row of elements on a modern periodic chart is called a period and each column is called a group. All elements within a group have similar chemical and physical properties. Some of the terminologies associated with the periodic chart are: 1. Metallic elements: elements to the left of the stairstep line that runs diagonally from B to At. 2. Nonmetallic elements: elements to the right of the stairstep line. 3. Metalloids: elements that lie adjacent to the stairstep line, excluding aluminum. 4. Alkali metals: Group 1 elements. 5. Alkaline-earth metals: Group 2 elements. 6. Halogens: Group 17 elements. 7. Noble gases: Group 18 elements at the extreme right. In studying group properties, it is important to make a clear distinction between the elements themselves and their compounds. For example, Part 1 will demonstrate reactivity of some metallic elements, while Part 2 will deal with reactions of ions in solution. The elements sodium (a very reactive metal) and chlorine (a toxic, yellowgreen, diatomic gas) are very different from sodium chloride, a colorless, watersoluble ionic solid which contains the sodium ion and chloride ion and is used to enhance food flavors. In this experiment you will be encountering both elements and their ions in the exercises listed below: Week 1 1. A video will show the reactivity of the alkali metals (elements) with water and with oxygen in air. 2. (a) You will investigate the solubilities of various salts of Group 1 and 2A elements by mixing solutions containing Group 1 or Group 2 cations with solutions of selected anions from their sodium salts (phosphate, PO 3-4 ; molybdate, MoO 2-4 ; sulfate, SO 2-4 ; and chromate, CrO 2-4 ) and noting whether a precipitate forms. (b) You will investigate the relative solubilities of the alkaline earth (Group 2) hydroxides: Mg(OH) 2, Ca(OH) 2, Sr(OH) 2 and Ba(OH) 2. (Group 1 hydroxides are all very soluble.) Table 1. Solutions used in solubility test. All solutions are 0.10 M. Cation Solutions Anion Solutions Group 1 Group LiNO 3 Mg(NO 3 ) 2 NaOH Na 2 MoO 4 NaPO 4 NaNO 3 Ca(NO 3 ) 2 Na 2 SO 4 KNO 3 Sr(NO 3 ) 2 Na 2 CrO 4 Ba(NO 3 ) 2 3. You will perform flame tests on salts containing Group 1 and Group 2 metal ions. These ions often impart characteristic colors to the flame, due to the emission spectra of the excited metal atoms. Unknown Salts: Week 1-ID of Cation 2

3 4. You will apply the chemical knowledge gained in Parts 2 and 3 above to identify the cation present in an unknown salt. The cation will be one of the group 1 or group 2 cations; Li +, Na +, K +, Mg 2+, Ca 2+, Sr 2+, Ba 2+. Week 2 4. You will carry out reactions which produce the halogen elements, Cl 2, Br 2 and I 2, respectively, and you will learn to recognize them by the characteristic colors they display in hexane solution. The colors of the halogens are then used as a basis for tests to determine if either bromide (Br - ) or iodide (I - ) ions are present in a salt. 5. You will apply the chemical knowledge gained in Part 5 above to identify the anion present in an unknown salt. The anion will be one of the Group 17 anions; Cl -, Br - and I -. You will then report the identity of the Unknown compound. WEEK 1 - EXPERIMENTAL PROCEDURE BE SURE TO RECORD ALL OBSERVATIONS, AS YOU MAKE THEM, IN YOUR NOTEBOOK. NOTE: Unless there are special disposal instructions, a reagent that is a liquid may be disposed in the sink flushing with water and dry solids in the wastebasket. For the second week there will be a labeled container in the hood area for the disposal of the hexane-containing solutions. PART 1. REACTIVITY OF GROUP 1 ELEMENTS IN AIR (I.E., WITH OXYGEN) AND WATER The alkali metals (Group 1) are very reactive and some of the reactions in this section are rather hazardous, so you will watch them on video. 1. Prepare a clean dry vial with a label with you name, The TA will collect the vial to dispense the Unknown Salt for Part Watch the video and record observations of the relative reactivity of the Group 1 elements. 3. Note and record the similarities within the group, and also the trends in reactivity as one moves from top to bottom of this group. Record the chemical reactions of the Group 1 elements with oxygen (in air) and with water. PART 2. SOLUBILITY OF SELECTED COMPOUNDS OF GROUP 1 AND 2 In this section you will be adding various anionic reagents (OH -, PO 4 3-, MoO 4 2-, SO 4 2-, CrO 4 2- ) to solutions containing Group 1 or 2 cations (Li +, Na +, K +, Mg 2+, Ca 2+, Sr 2+, Ba 2+ ) and looking for any evidence that a solid precipitate may have formed. A precipitate may be indicated by an immediate dense cloudiness (i.e. suspended solid particles) or it may be as subtle as a faint cloud which only develops after several seconds. If a precipitate is seen when, say, OH - ions (in the form of aqueous NaOH) are added to a solution of M + ions (in the form of aqueous MNO 3 ), we may conclude that the hydroxide MOH is insoluble. If no precipitate is seen, it can be deduced that MOH is soluble. Moreover, one can deduce relative solubilities by determining the amount of NaOH added before a precipitate is seen. The less NaOH that is needed, the less soluble is MOH. Unknown Salts: Week 1-ID of Cation 3

4 Each of the metal ions studied below has a characteristic pattern of solubilities with the various reagents. Once you establish what these patterns are, you will be able to test an unknown salt in the same way (Part 5), and get clues as to the metal ion present by comparing the solubility data for the unknown with data for the known metal ions. You and you partner will each do the following tests. After completion of the tests you will need to compare your results with your partner to be sure you each have consistent results. If there are any questionable results then redo those tests so your results agree. 2(a) Phosphates, molybdates, sulfates and chromates of Group 1 and 2 Before proceeding, wash out the 24-well reaction plate scrupulously with soap and water and rinse with de-ionized water. 1. Place a 24-well plate on a black background oriented with four columns of 6 rows. 2. To the first row add 10 drops of 0.1 M LiNO 3 solution to 4 wells. 3. Add 10 drops of 0.1 M NaNO 3 solution to the wells in the second row. 4. Continue in like fashion with the remaining I cation, KNO 3, in Table To the first column add 10 drops of 0.1 M PO 3-4 (0.10 M Na 3 PO 4 ) solution. 6. Continue with all the anions in Table 1. EXCEPT use 20 drops of the 2- MoO Stir the solutions by moving the plate in small circles. 8. Neatly record your observations in a table in your laboratory notebook. 9. Repeat the procedure using the group II cations. Q.1. Was a trend observed in the results? If so, what is it? Q.2. What does your data suggest for alkali metal (Group 1) solubilities? Propose a general rule-of-thumb for alkali metal salt solubilities. Q.3. What does your data suggest for alkaline earth metal (Group 2) solubilities? Propose a general rule-of-thumb for alkaline earth metal salt solubilities. 2(b) Hydroxides of magnesium, calcium, strontium and barium Before proceeding, wash out the 24-well reaction plate scrupulously with soap and water and rinse with de-ionized water. 1. Prepare the reaction plate as above with one column of the group 2 cations. 2. Count drops of the OH- solution needed to produce precipitate. Do not exceed Neatly record your observations in a table in your laboratory notebook. 4. The second column can be used to test your unknown in Part 5. Q.4. Describe the relative solubilities of the hydroxides of the Group 2 cations. Propose a general rule-of-thumb for alkali metal salt solubilities. Q.5. Propose a general rule-of-thumb for nitrate solubilities. Unknown Salts: Week 1-ID of Cation 4

5 PART 3. FLAME TESTS FOR GROUP 1 AND 2 METALS In the Atomic Spectra lab you saw how different elements (e.g. mercury, hydrogen, helium, sodium) gave different emission spectra, and that the wavelengths of lines in a spectrum could be used to identify the element responsible for producing the spectrum. Without actually determining the detailed spectrum of an element, it is often possible to identify it simply by the color it imparts to a flame. Although such flame tests may seem crude and imprecise in comparison with true spectroscopy, they are quick, easy, and can be very useful. In this exercise you will observe the colors in a Bunsen flame when different Group 1 and 2 metal salts are introduced into it. The colors are usually due to emission by excited metal atoms (not ions or compounds) which are present. Later, in Part 4, you will do flame tests on an unknown salt, and can refer to the results obtained here for clues as to the identity of the metal cation in the unknown. The procedure is quite simple. You will be provided with some wooden splints that have been soaked in DI water that can be held in a flame. Perform flame tests using the samples of the solid salts supplied of lithium, sodium, potassium, magnesium, calcium, strontium, and barium. Observe and record the colors of the different samples. (3) Flame Test Procedure 1. Soak a wooden toothpick (or splint) in DI water. The water serves two purposes, (i) it prevents the wood from igniting too quickly and (ii) the salts will adhere to the wood more easily when it is damp. 2. Dip the toothpick into the provided salt. A small quantity of the salt should stick to the toothpick. To avoid contamination of the salt, use a fresh toothpick for each test. 3. Place the toothpick into the flame. Note the color produced and record in your table. Do not hold it in the flame too long or the wood will ignite. 4. Be sure the toothpick is extinguished and dispose of the used toothpick in the provided waste container. PART 4.IDENTIFICATION OF THE CATION OF AN UNKNOWN SOLID BY ITS CHEMICAL PROPERTIES 1. In a clean dry vial, obtain an unknown solid from the CA or instructor. This salt will contain a Group 17 halide ion (Cl -, Br -, or I - ) in combination with one of the Group 1 or Group 2 cations (Li +, Na +, K +, Mg 2+, Ca 2+, Sr 2+, or Ba 2+ ). SAVE ~1/2 of your unknown salt for the anion analysis next week. 2. Dissolve about half of your sample in DI water using a concentration of ~1.0 g of the unknown/10 ml of water. Reserve the remainder to use in the flame test and testing for the anion. 3. Use the solubility tests (see Part 2) in conjunction with flame tests (Part 3) to establish what cation is present in your unknown. (In assessing the solubility results, consider that your unknown salt solution is likely to have a different concentration from the metal ion solutions used in Part 2.) Understand that the deductive reasoning process involved in identifying the presence or absence of a given ion in the unknown solid may be based on two types Unknown Salts: Week 1-ID of Cation 5

6 of test results. Positive tests give a color change, a reaction, a precipitate etc., whereas negative tests signify that nothing happened. Do not overlook the significance of a NEGATIVE TEST RESULT. A negative test result often will tell you what is NOT present. IN YOUR LAB NOTEBOOK CLEARLY INDICATE THE IDENTITY OF THE CATION OF YOUR UNKNOWN. NOTE: The remaining half of your unknown solid will be used next week to conduct test(s) based on the Part 5 experiments to identify the anion. (If the anion is not bromide or iodide, you can be sure it is chloride.) Unknown Salts: Week 1-ID of Cation 6

7 WHAT TO DO BRING to lab From Exp. 7-Models Molecular Bonding and Covalent Compounds Nothing - it was turned in during lab. For Exp. 8 Unknown Salts-Week 1 BRING TO LAB Print a copy of this experiment, read it and bring it to your laboratory class. Bring your Lab Notebook with the following PreLab assignment completed. You will not be allowed to do the experiment without the prelab assignment completed. Your CA will check the copy pages that contain the PreLab Assignment at the beginning of the lab. PreLab Assignment 1. Set up your lab notebook appropriately for this experiment including title bar(s), a purpose, the Table of Reagents (see #2 below) and a Data Table (see #4 below). (For the proper entries for the Table of Reagents refer to the Guidelines for Tables.) 2. Table of Reagents Prepare a Table of Reagents in your notebook with the following substances: (a) Ba(NO 3 ) 2 ; (b) NaClO (bleach), (c) Na 2 MoO 4, (d) Na 2 SO 4, (e) Na 2 CrO 4, (f) H 2 SO 4, (g) H 2 O 2, (h) hexane. These are a representative group of compounds used as well as some of the more hazardous compounds you will encounter during this experiment. Be sure to indicate any special waste disposal requirements. (NOTE: There will be a labeled container in the hood area for the disposal of the hexane-containing solutions.) 3. Prepare a page with the heading Observations of the Reactivity of Group 1 Elements to record observations and chemical equations for Part Prepare a separate page titled Questions Week 1 and record your responses to the questions in Part 2b. 5. Prepare a table for recording the solubility and flame tests results in Parts 2 and 3 (See Table 1 below). This table must be drawn landscape on one full page in your notebook. Unknown Salts: Week 1-ID of Cation 7

8 Table 1. Solubility Results (sample) Cation Anion Phosphate (PO 4 3- ) Molybdate (MoO 4 2- ) Sulfate (SO 4 2- ) Chromate (CrO 4 2- ) # of drops for ppt with Hydroxide (OH - ) Results of Flame Test Li + sol 1 Na + K + Mg 2+ Ca 2+ Sr 2+ Ba 2+ Unknown 1 sol = soluble; insol = insoluble; sl = slightly soluble In Lab: Do Parts 1 through 3 and answer questions. Make sure to carefully record your observations in your lab notebook. Complete Part 4. Carefully consider your results in Parts 1-3 and devise a plan to identify the cation part of your unknown salt. In your notebook, neatly write out the plan to identify the cation (Li +, Na +, K +, Mg 2+, Ca 2+, Sr 2+, or Ba 2+ ) in an unknown solid sample. Be specific as to the tests you will do and the expected results and conclusions. A flow chart may be helpful to organize your plan as a specific procedure you can follow. SAVE ~1/2 OF YOUR UNKNOWN SALT FOR NEXT WEEK. To be turned in the next laboratory time: a) a printed copy of a formal Experimental section for the procedure from Parts 2-4. A template for the experimental section will be supplied and you will need to complete the document by adding an appropriate final paragraph, carefully following the instructions and style of the Guidelines for Writing the Experimental Section of a Report. b) a copy of the Experimental section uploaded to the Turnitin.com website. See the c110 Lab Information Moodle site for the login instructions and appropriate passwords. Any additional information will be given in the lecture class during the week. sol sol Unknown Salts: Week 1-ID of Cation 8