denfification of Substances by Physical Properties

Transcription

1 denfification of Subtance by Phyical Propertie To become acquainted with procedure ued in evaluating phyical propertie and the ue of thee propertie in ~dentffying ubtance. EXPERIMENT 2 OBJECTIVE balance 250-mL beaker burner and hoe dropper 50-mL Erlenmeyer flak 10-mL graduated cylinder Iarge tet tube (2) mall tet tube (6) tet-tube rack patula 10-mL pipet 5-mL pipet tubing with right-angle bend ring tand and ring utility clamp wire gauze thermometer no. 3 two-hole topper with one of the hole lit to the ide or a buret clamp two-hole topper tirring rod mall rubber band (or mall ection of ~-in. rubber tubing) 20 ml eyclohexane boiling chip 15 ml ethyl alcohol 1 g naphthalene 5 rnl toluene two unknown (one liquid, one olid) capillary tube (5) 50-mL beaker (2) mall watch gla oap olution APPARATUS AND CHEMICALS Propertie are thoe characteritic of a ubtance that enable u to identify it and to ditinguih it from other ubtance. Direct identification of ome ubtance can readily be made by imply examining them. For example, we ee color, ize, hape, and texture, and we can mell odor and dicern a variety of tate. Thu, copper can be ditinguihed from other metal on the bai of it color. Phyical propertie are thoe propertie that can be oberved without altering the compoition of the ubtance. Wherea it i difficult to aign definitive value to uch propertie a tate, color, and odor, other phyical DISCUSSION and refractive index, can be expreed quantitatively. For example, the melting point of copper i 1087, and it denity i 8.96 g/cm 3. A you probably realize, a pecific combination of propertie i unique to a given ubtance, thu making it poible to identify mot ubtance jut by careful determination of everal propertie. Thi i o important that large book 21

2 22 E periment 2 Identification of Subtance by Phyical Propertie have been compiled liting characteritic propertie of many known ubtance. Many cientit, mot notably everal German cientit during the latter part of the nineteenth century and earlier part of the twentieth, pent their entire live gathering data of thi ort. Two of the mot complete reference of thi type that are readily available today are The Chemical Rubber Company Handbook of Chemitry and Phyic and N. A. Lange Handbook of Chemit~ d. In thi experiment you will ue the following propertie to identify a ubtance whoe identity i unknown to you: olubility, denity, melting point, and boiling point. The ohlbility of a ubtance in a olvent at a pecified temperature i the maximum weight of that ubtance that diolve in a given volume (uually 100 ml or 1000 ml) of a olvent. It i tabulated in handbook in term of gram per 100 ml of olvent; the olvent i uually watch In the preceding experiment you learned that the denity of a ubtance i defined a the ma per unit volume: m Melting or freezing point correpond to the temperature at which the liquid and olid tate of a ubtance are in equilibrium. Thee term refer to the ame temperature but differ lightly in their meaning. The freezing point i the equilibrium temperature when approached from the liquid phae, that i, when olid begin to appear in the liquid. The melting point i the equilibrium temperature when approached from the olid phae, that i, when liquid begin to appear in the olid. A liquid i aid to boil when bubble of vapor form within it, rie rapidly to the urface, and burt. Any fiquid in contact with the atmophere will boil when it vapor preure i equal to atmopheric preure--that i, the liquid and gaeou tate of a ubtance are in equilibrium. Boiling point of liquid depend upon atmopheric preure. A liquid will boil at a higher temperature at a higher preure or at a lower temperature at a lower preure. The temperature at which a liquid boil at 760 mm Hg i called the normal boiling point. To account for thee preure effect on boiling point, people have tudied and tabulated data for boiling point veru preure for a large number of compound. From thee data, nomograph have been contructed. A homograph i a et of cale for connected variable (ee Figure 2.5 for an example); thee cale are o placed that a traight line connecting the known value on ome cale will provide the unknown value at the traight line interection with other cale. A nomograph allow you to find the correction neceary to convert the normal boiling point of a ubtance to it boiling point at any preure of interet. PROCEDURE A. Solubility CAUTION: Cyclohexane i highly flammable and mut be kept away from open flame. Qualitatively determine the olubility of naphthalene (mothball) in three olvent: water, cyclohexane, and ethyl alcohol. Determine the olubility by adding a few crytal of napthalene to 2 to 3 ml (it i not neceary to meaure either the olute weight or olvent volume) of each of thee three olvent in eparate, clean, d~y tet tube. Make an attempt to keep the amount of napthalene and olvent the ame in each cae. Place a cork in each tet tube and hake briefly. Cloudine indicate inolubility. Record your

3 Laboratory Experi~t~ent The difference between thee ~.~..~--two reading / i the volume of the olid. ettled on ~ concluion on the report heet uing the abbreviation (oluble), p (paringly oluble), and i (inoluble). Into each of three more clean, dry tet tube place 2 or 3 ml of thee ame olvent and add 4 or 5 drop of toluene in place of naphthalene. Record your obervation. The formation of two layer indicate immicibility (lack of olubility). Now repeat thee experiment uing each of the three olvent (water, cyclohexav~e, and ethyl alcohol) with your olid and liquid unknown and record your obervation. B. Denity Determine the denitie of your two unknown in the following manner. The Denity of a Solid Weigh about 1.5 g of your olid m~known to the nearet g and record the weight. Uing a pipet or a wah bottle, half fill a clean, dry 10-mL graduated cylinder with a olvent in whicli your unknown i inoluble. Be care~d not to get the hquid on the inide wall, becaue you do not want your olid to adhere to the cylinder wail when you add it in a ubequent tep. Read and record tlxi volm:ne to the nearet 0.1 ml. Add the weighed olid to the liquid in the cylinder, being careful not to loe any of the material in the tranfer proce and enuring that all of the olid i beneath tlie urface of the liquid. Carefully tapping tlie ide of the cylinder with your finger will help ettle the material to the bottom. Do not be cortcerned about a few crytal that do not ettle, but if a large quantity of the olid reit ettling, add one or two drop of a oap olution and continue tapping the cylinder witli your finger. Now read the new volume to the nearet 0.1 ml. The difference in thee two volume i the volume of your olid (ee Figure 2.1). Calculate the denity of your olid unknown. You may recall that by meauring the denity of metal in thi way Archimede proved to the king that the charlatan alchemit had in fact not tranmuted lead into gold. He did thi after oberving that he weighed le in the bathtnb than he did normally by an amount equal ~o the weight of the fluid diplaced. According to legend, upon making hi dicovery Archimede emerged from hi bath and ran naked through the treet houting Eureka! (I have found it). The Denity of a Liquid Weigh a clean, dry 50-mL Erlenmeyer flak to the nearet g. Obtain at leat 15 ml of the unkno*am liquid in a dean, dry tet tube. Uing a 10-mL pipet, pipet exactly 10 ml of the unknown liquid

4 24 Experiment 2 Identification of Subtance by Phyical Propertie FIGURE 2.2 Sealing one end of a capillary tube. into the 50 ml Erlenmeyer flak and quickly weigh the flak containing the 10 ml of unknown to the nearet g. Uing the calibration value for your pipet (from Experiment 1) and the weight of thi volume of unknown, calculate it denity. Record your reult and how how (with unit) you performed your calculation. (SAVE THE LIQUID FOR YOUR BOILING- POINT DETERMINATION.) C. Melting Point (for Solid Unknown) Obtain a capillary ~ube and a mall rubber band. Sea1 one end of the capil- Iary tube by carefully heating the end in the edge of the flame of a Bunen burner until the eud completely cloe. Rotating the tube during heating will help you to avoid burning yourelf (ee Figure 2.2). Pulverize a mall portion of your olid-unknown ample with the end of a tet tube on a clean watch gla; partially fib the capillary with your unknown by gently tapping the pulverized ample with the open end of the capillary to force ome of the ample inide. Drop the capillary into a gla tube about 38 to 50 cm h~ length, with the ealed end down to pack the ampie into the bottom of the capillary tube. Repeat thi procedure until the ample column i roughly 5 mm in height. Now et up a melting-point apparatu a illutrated in Figm e 2.3. Place the rubber band about 5 cm above the bulb on the thermometer and out of the hquid. Carefnlly inert the capillary tube under the rubber b~nd with the doed end at the bottom. Place the thermometer with attached capi11ary into the beaker of water o that the ample i covered by water, the thermometer doe not touch the bottom of the beaker, and the open end of the capillary tube i above the urface of the water. Heat the water lowly while gently agitating the water ~vith a tirring rod. Oberve the ample in the capillary tube while you are doing thi. At the mmnent that the olid melt, record the temperature. Alo record the melting-point range, which i the temperature range between the temperature at which the ample begin

5 Laboratory Experiment 25 No. 3 two-hole rubber topper with lit (topper and clamp may be replaced by a buret damp) 25~mL baaker melting point ttlbe Rubber band Wire gauze Place the capillary tabe and thermometer bulb at the ame Apparatu for melting-point determination. melt and the temperature at which all of the ample ha melted. Uing thermometer-calibration curve (from Experiment 1), correct thee ternrecord the melting point and melting- Thee temperature may differ by only 1 or le. Point (for Liquid Unknown) the boiling point of your liquid unknown (ue ome of the ame you ued to determine the denity) by adding about 3 ml to a clean, tet tube. Fit the tet tube with a two-hole rubber topper that ha one your thermometer into the hole with the lit and one of your right tube into the other hole, a hown in Figure 2.4. Add one chip to the tet tube to enure even boiling of your the thermometer o that it i about 1 cm above the urface unknown liquid. Clamp the tet tube in the ring tand and connect to tubing a length of rubber tubing that reache to the ~ _-~emble your appara~ a hown in Figure 2.4. (CAUTION: Be cer- e no contriction in the rubber tnbing. Your ample i flan,mable. graduaily and watch for change in temperature. The temt at the boiling point of the liquid. Record the the oberved boiling point to the true boiling a~ room atmopheric preure uing your thermometer-calibration The normal boiling point (b.p. at 1 atm = 760 ram Hg) can now be cal- (ee Example 2.1, below) uing the nomograph provided in Figure boiling-point correction hould not be more than +5.

6 26 Experiment 2 Identification of Subtance by Phyical Propertie paper Rubber topper with a lit for the thermometer Clamp to drain (avoid contriction) Wire gauze Ring FIGURE 2.4 Apparatu for boiling-point determination OO O 6O 50 3O Normal b.p. at 760 mm Hg A AT correction FI6[J~{E 2.5 Nomograph for boiling point correction to 760 mm Hg.

7 Laboratory Experiment 27 Phyical Propertie of Pure Subtance Denity Melting Boiling Subtance (g/ml) point () point () Solubillty ~ in Water Acetanilide p p Acetone Benzophenone I.I i Bromoform i 2,3-Butanedione t-butyl alcohol Cadmium nitrate 4H20 2, i Chloroform b i Cyclohexane i p-dibromobenzene i p-dichlorobenzene i m-di~ trober~zene i Diphenyl i Diphenylamine i Diphenylmethane i Ether, ethyl propyl Hexane i Iopropyl alcohol Laurie acid i Magneium nit-rate 6H ~ i Methyl alcohol p Methylene chloride b i Naphthalene i (x-naphthol i i Phenyl benzoate i Propionaldehyde i Sodium acetate 3H i Stearic acid I i Thymol p Toluene i p-toluidine p Zinc chloride i a ~ oluble; p ~ paringly oluble; i = ir~olub]e bto~dc, ivlot organic compotu/d ued in the lab are toxic. ~Boil with decompoition. EXAMPLE 2.1 What will be the boiling point of ethanol at 650 ivan Hg when it normal boiling point at 760 mm Hg i known to be SOLUTION: The anwer i eaily found by conulting the nomograph in Figure 2.5. A traight line drawn from 78.3 on the le~t cale of normal boiling point through 650 ~nm Hg on the preure cale interect the temperature correction cale at 4. Therefore, Normal b.p. - correction = oberved b.p. 78.3oc - 4.0oc = 74.3oc Cyclohexane Alcohol S p p p Similar calculation could be done for the compound in Table 2.1 at any preure lited on the homograph in Figure 2.5. In thi experiment you will oberve a boiling point at a preure other than at 760 mm Hg, and you with to know it normal boiling point. In order to etimate it normal boiling point, aume that

8 28 Experiment 2 Identification of Subtance by Phyical Propertie your oberved boiling point i 57.0 and the oberved prettre i 650 mm Hg. Ue your oberved boiling point of 57.0 a if it were the normal boiling point and find the correction for a preure of 650 mm Hg. Uing the nomograph, yon can ee that the correction i 3.8. You would then add thi correction to your oberved boiling point to obtain an approximate normal boiling point: =60.8, or 61 By conulting Table 2.1, you can find the compound that bet fit yottr data; in tlii example, the data are for chloroform. E. Unknown Identification Your unknown are ubtance contained in Table 2.1. Compare the propertie that you have determined for your unknown with thoe in the table. Identify your unknown and record your reult. REVIEW QUESTIONS Before beginning tlii experiment in the laboratory, you hould be able to anwer the following quetion: 1. Lit five phyical propertie. 2. A mL ample of an unlcnown weighed g. What i the denity of the unknown? 3. Are the ubtance bromoform and laurie acid olid or liquid at room temperature? 4. Could you determine the denity of cadmium nitrate uing water? Why or why not? 5. What would be tlie boiling point of acetone at 670 mm Hg? 6. Why do we calibrate thermometer and pipet? 7. I bromoform lnicible with water? witli cycloliexane? 8. When water and toluene are mixed, two layer form. I the bottom layer water or toluene? (See Table 2.1.) 9. What olvent would you ue to determine the denity of cadmium nitrate? 10. The denity of a olid with a melting point of 52 to 54 wa determined to be g/ml. What i the olid? 11. The denity of a liquid whoe boiling point i wa determined to be 1.36 ± 0.05 g/ml. What i the liqnid? 12. Which ha the greater volume, 10 g of bromoform or 10 g of acetone? What i the volume of each?

9 Name Date Laboratory Intructor Dek Liquid unknown no, Solid unknown no. REPORT SHEET Identification of Subtance by Phyical Propertie EXPERIMENT 2 A. Solubility Naphthalene Toluene Liquid unknown Solid unknown B. Denity Solid Water Cyclohexane Alcohol Final volume of liquid in cylinder Initial volume of liquid in cylinder Volume of olid Weight of olid (how calculation) ml ml ml g Denity of olid (how calculation) g/ml Liquid Volume of liquid Volume of lic~uid corrected for the pipet correction Weight of 50-mL Erlenmeyer plu ~ 10 ml of unknown Weight of 50-mL Erlenmeyer WCtght of liquid (how calculation) ml ml g g g Denity (how calculation) g/ml 29

10 30 Report Sheet Identification of Subtance by Phyical Propertie C. Melting Point of Solid Unknown Oberved melting point Corrected (apply thermometer correction to obtain) Oberved melting-point range Corrected (apply thermometer correction to obtain) D. Boiling Point of Liquid Unknown Oberved Corrected (apply thermometer correction to obtain) Etimated true (normal) b.p, (apply preure correction to obtain) o C o C E. Unknown Identification Solid unknown Liquid unknown QUESTIONS 1. I thymol a olid or a liquid at room temperature? 2. What olvent would you ue to meaure the denity of magneium nitrate? 3. Convert your denitie to kg/l and compare thee value with thoe in g/ml. 4. If air bubble were trapped in your olid beneath the liquid level in your denity determination, what error would reult in the volume meaurement, and what would be the effect of thi error on the calculated denity? 5. A liquid unknown wa found to be inoluble in water and oluble in cyclohexane and alcohol; the unknown wa found to have a boiling point of 107 at 658 mm Hg. What i the ubtance? What could you do to confirm your anwer? 6. A liquid that ha a denity of g/ml i inoluble in cydohexane, What i the liquid?

11 Laboratory Experiment What i the boiling point of cydohexane at 600 mm Hg? Conult a handbook for the following quetion and pecify handbook ued. Handbook: 8. Omium i the denet element known. What are it denity and melting point? 9. What i the color of MnSO4? What i it olubility in cold water? 10. What are the formula, molecular weight, and color of lead azide?