An Alternative Methodology for General Chemistry Laboratories Chemical Equivalent of a Metal Carlos M. Bonatti, J o s e L. Zurita, and Horacio N. ~ 6 l i m o ' lnstituto de Fisica, Facultad de Ciencias Exactas y Tecnologia, Universidad Nacional de Tucuman, Av. lndependencia 1800. - 4000, Tucuman, Argentina Students of pure chemistry can be trained in scientific methodolow from the vew beginnine of their studies bv adopting zternative metgods-with respect to the tradftional ones. With this in mind, we have developed several conventional experiments for general chemistry laboratories where the students are induced to obtain bibLiographic information and decide on some a s ~ e c t of s the ex~erimentalwork. I n this paper, we describe a n experiment to determine the chemical equivalent of a metal using the proposed methodology. I n order to familiarize the students with the bibliography, and to increase their interest in the experiment, a n unknown metal will be given to the students, and they will be asked to identify it from the experimental value of its chemical equivalent. The metals that can be chosen for this experiment must have the following characteristics:
to atmosphere
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1. They must generate hydrogen when mineral acids react
with them. 2. They must possess only one oxidation state. 3. They must not react violently with water.
We propose to the students the following work sequence: 1. List
2.
3. 4.
5. 6.
all the metals that have the above specified characteristics. Collect bibliographic information about the physical and chemical properties of those metals. Calculate the maximum experimental error by comparing the two nearest chemical equivalents in order to avoid mistaking one metal for the other. .\lnke the el-ror propaxawn and venfy whether the srlrrred equipment is qprapnate toabwm a lower or rqunl error than that calculated above. Assemble the experimental device using the selected equipment and start the experiment. Finally, report all ohsewatians, results and eandusions.
Before starting the experiment, the students and the instructor are encouraeed to discuss the validitv of the ideal gas law for hydrogen and its tap water solubhity a t room temperature and pressure. Experimental Apparatus Figure 1shows a line drawing of the apparatus, and Figure 2 shows an e x ~ a n d e dview of the s t o ~ ~assemblv. er The chemical equivalent is obtained by collectingand measurine hvdroeen over water in laboratorv conditions " when a known mass of the unknown metal reacts with a HCI solution. Then, the metal may be identified from this value. We use the same type of gas collection apparatus as that described bv Shoemaker and Garland ( 1 ) .but we have developed the gas generation system illustrated in Figure 2 to introduce the metal into the HCI solution in a closed system so that the hydrogen evolved may be measured. I t consists of a 125-mL Erlenmeyer flask with a 19/26 female ground glass
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Figure 1. Line drawing of the apparatus. 1, Gas buret. 2. Leveling bulb. 3, Side tube. 4, Two-way stopcock. 5, Erlenmeyer flask with a 19126femaleground glass joint. 6, Stopper assembly with oil hydraulic seal (Quickfit ST1012 and ST 10127).7 , Sliding rule. 8, Tefloncap. joint that contains the HC1 solution. The stopper assembly has a n oil hydraulic seal and a glass rod is used as a piston t o introduce the metal contained in a Teflon cap attached to the end of a 19/26 male ground glass joint. This Teflon cap has two longitudinal grooves connecting the stopper assembly with the Erlenmeyer flask to prevent excess pressure when the Teflon cap is attached. A sliding rule is adapted to the gas collection apparatus to avoid parallax error between the buret and the side tube, thus improving the volume readings. Procedure After studying the properties of the metals and following a discussion with the instructor. students will be able to decide on the concentration of the HCl solution to obtain a smooth, steady evolution of hydrogen. A sufficient volume of this solution is introduced into the Erlenmeyer flask. Then the students decide what metal mass should be introduced into the Tdlon cap in order to perform a first trial, taking into account that the metal is unknown and that the volume of evolved hydrogen should not exceed the buret capacity (50 mL) a t room temperature and pressure. Two (or more) empty Teflon caps are weighed to perform a t least two independent determinations and the appropriate amount of metal is put into one of them and weighed again to obtain the real metal 'Author to whom correspondenceshould be addressed.
Typical Experimental Results for One of the Metalsa
Figure 2. Expanded view of the stopper assembly. 1, Teflon cap. 2, 19/26male ground glass joint. 3, Nylon collar plate. 4, Glass rod piston. 5, Latex collar seal. 6, Screw thread. 7, Plastic female screw. 8, Oil hydraulic seal. 9, Longitudinal Teflon cap groove. mass. This quantity should be slightly less than or equal to that previously calculated. Then the Teflon cap is attached gently to the stopper assembly and inserted carefully into the mouth of the Erlenmeyer flask taking care that the piston rests on the Teflon cap. The gas collection apparatus, filled with tap water a t room temperature, is connected through a two-way stopcock with a flexible tubing to the gas generation system, and the system is checked for air leaks by raising and lowering the leveling bulb. After checking that the system is gas tight, the stopcock is opened to the atmosphere and the level of water in the buret is adjusted to approximately the zero mark by moving the leveling bulb. Then the stopcock is turned to connect the gas collection and the gas generation systems. The leveling bulb i s adjusted again until i t s water level matches that of the buret. The water level in the buret is read by using the sliding rule and recorded. Room temperature and barometric pressure also are recorded. By pushing the Teflon cap with the piston into the HC1 solution, the reaction is initiated. The reaction is completed when the water level becomes stationary. The water levels in the buret and bulb are matched again and the volume of water remaining in the buret is read and recorded. Then, the apparatus is cleaned and a second experiment is started using the other weighed Teflon cap with a metal mass based on results of first run. in order to displace almost completely the maximum buret volume. Knowing the volume of hydrogen collected over water a t . room temperature and pressure from a given mass of the metal, the volume of hydrogen a t STP is calculated. With this value it is possible to calculate the chemical equivalent and to identify the metal. Theoretical equivalents (quotient between the atomic weight and the oxidation state of the identified metal) can be compared with the experimental one to estimate the error. Barometric corrections must be made to calculate the chemical equivalent of the metal. The partial hydrogen
Item 1st test 2nd test Estimated metal mass (g) 0.0320 0.0960 Cap + metal mass (g) 0.7121 0.7874 Empty cap mass (g) 0.6800 0.6915 Real metal mass (g) Remaining water volume (mL) Initial water volume (mL) Hydrogen involved volume (mL) Barometric pressure (Torr) Room temperature (OC) Temperature correction for barometric reading (Torr) Latitude correction for barometric reading (Torr) Corrected barometric pressure (Torr) Water vapor pressure (Torr) Partial pressure of H2 (Torr) STP volume of H2 (mL) Experimental chemical equivalent Identified metal Error (%) Permissible error (%) 3.0 'Similar experimental errors were obtained with other selected metals. pressure is obtained by subtracting the water vapor pressure from the corrected barometric pressure (2). Results The physical and chemical properties of the metals selected according to the characteristics previously described should be listed in tabular manner. Typical experimental results are shown in the table. If the piston rests on the cap, no volume correction is necessary due to the insertion of the piston to push the Qflon cap into the Erlenmeyer flask because the glass rod is introduced only a few millimeters and consequently the change of volume produced is negligible within experimental error. Students are encouraged to report all detected sources of systematic errors and to present some alternatives to minimize them. We also suggest that the instructor should show the students if the identified metal is right by means of a characteristic chemical reaction andlor any other fast analytical method. Conclusions We describe a laboratory experiment for chemical equivalent determination of a n unknown metal that can react with mineral acids evolving hydrogen. The goals of this methodology are
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students must devise their own aspect of the experiment discuss this with the instructor, and learn t o use the literature in order to introduce them to scientific methodology.
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Because of the relatively short reaction time it is quite feasible for the students to carry out the experimental procedure three times during a two-hour laboratory period in order to get a n average value and a standard deviation. Literature Cited 1. Shoemaker D. P;Garland, C. W . Erperirnmls in Physicol Chemistry. 2nd. ed.; Mdjraw-Hill: New York, 1967; p 49. 2. Weast,R. C.,Ed. CRCHondbook ofCl~smisfryandPhysics;69thed.;CRC Press:Boea Raton. FL, 198E-1989, p E-36.
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