Helping students to develop an hypothesis about electrochemistry: A

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GEORGE L. GILBERT Denisan University Granville,OH 43023

tested demonstrcrtions Helping Students to Develop an Hypothesis about Electrochemistry A Demonstration with a Lab Report and Supplemental Worksheet Submined by: Chester VanderZee Sioux Valley Schools Volga, SD 57071

of its depth. The agar seems to adhere to the glass better when it /R not too thick while etudnnta make their obsewations ofthe dishes the following day. Set the dish aside and do some other assimment or discussion for the rest of the period.

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Ideas and Facts to Help You Learn Electrochemistry 1. In water many common metals do this.

Checked by: Mel Mosher Missouri Southern State College Joplin, MO 64801

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I would like to share a successful demonstration and my way of using it to develop report work and worksheets to aid my students' comprehension of some basic chemistry. This demonstration is a further modification of several others previously discussed in this Journal ( 1 4 ) Teaching demonstrations are very effective i n getting the students' interest and helping them see chemistry in everyday life. I n my philosophy, instructors should relate demonstrations to real life--consumer chemistry. If such demonstrations are not available instructors should keep working until they find one or develop one. Students should be given demonstrations that they can relate to their life through personal experience.

2. Nu two metals have identical tendencies for this electrochemical activlty. (Recall the alkali metal demonstration in which tmy pircrs of Li. Na, and K u,ere dropped into jam of water.) 3. When positive metal ions, which come from the more active of the two joined pieces, enter the water, the excess electrons remain on the metal piece. (Merril Chemistry, A Modern Course, 1979; p 524). 4. Either of the two metals is a better conductor for the : freed electrons than the water is. What does this imply , when more electrons are freed at one end? In which di- ' ' rectian will they travel? 5. These free electrons react with HzO.

Experimental I use the demonstration described here after the class has already covered introductory atomic theory, the history of some elements, and trends in the periodic table.

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Materials

To prepare materials for the demonstration first get two pieces of zinc, 2 cm x 4 cm each will do. Solder one of these zinc pieces to an ungalvanized iron nail. (You may need to sand the mating from the nail to get down to the "iron", which is really low-grade steel). Solder the other piece of zinc to a piece of copper. A4-cm-long piece of 12-gage wire will do fine. Then solder a second iron nail to another piece of copper wire that has the same size a s the previous wire. Now you should have a Zn-Fe piece, a Zn-Cu piece, and a n Fe-Cu piece. Fe and Ag also work well if you are willing to sacrifice a US. dime minted before 1964. Once you have made these pieces they eanbe cleaned and reused for years in repeat demonstrations. Schedule

Day I Boil 3 g of plain agar in 225 mL of distilled water in a n Erlenmeyer flask. Place the 3 soldered pairs of metal into 3 separate 8.8-cm glass Petri dishes. You may want to burnish the metal pieces just before use. Add a squirt of phenolphthalein to the agar suspension while it is hot and still in the flask. Then swirl. Pour the agar Over the metal pieces in the Petri dishes; try to cover the metal, but do not fill the dish beyond half

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Journal of Chemical Education

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O K + phenolphthalein + pink complex

7. lky to correlate your findings with generalizations made for period 4 elements. Is there an inconsistency?

8. Use these suggestions for each of the three elements. a. Look up and record the mass of 1 mol of the element. b. UreTahlc A-],at the backnfMcrnlstext,tofind the density of each element. c. Use results of a and b to calculate the volume occupied by 1mol of each element. d. Calculate the volume by atomic number of the element. This will give you your volume per proton. Essentially, the smaller this number is, the greater the electronegativity of the element is relative to the other piece. 9. Next, use your fmdings from the above calculations to reevaluate your observations and conclusions If you are curious about nonpink colors, see Holt Modern Chemistry, 1978: pp 532,533, and 553. 'Activity predicted fmm the periodic table: Zn > Cu > Fe .Activity seen in dishes and supported by computation: Zn>Fe>Cu Summary of computational work Volumelprnton Zn = 5.07 x Fe = 4.5 x Cu = 4 1 x

cubic cm cubic cm cubic cm

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Day 2 Ask the students to write a standard demonstration report. The following format has served my purposes well with demonstrations I. Obseruations-labelled and colored sketches of what is actually seen 11. Interpretation-including as much explanation, chemical and intuitive, as the student can handle 111. Questions-an opened-ended part that can be used to prevent and detect boredom and lack of attention (Where there is intellectual life, Were are questions.)

I evaluate the reports as follows. +2: very complete, careful work +1: a fair try, some comprehension

4: some effort, minimal or no comprehension

-1: no paper or no effort

In my grading system these points are added to the numerator for averaeine. -. but the denominator is not changed; the denominator comes from the number of letter marks earned on tests. auizzes. etc. I also hand out the fa& sheet shown in the box. Manv students can see the inconsistencv between the genera"lization of greater activity from l e ~ i right o in period 4 and how these three pairs of metals actually perform in water. This agar mix is essentially "semisolid" water. After the calculations (see item 8 on fact sheet) are completed, it all fits a pattern, a satisfying model to fit the observations.

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This demonstration is very popular with my classes and successful in meeting its goals of involving the students. Students eniov actuallv secine evidence that the electrons have migratki to the s l k p l y eolored areas. It is very exhilarating for them when the calculations are completed and the volume per proton patterns are seen. The look of pride on their faces at their achievment is so satisfvine that I always feel that the work involved in the materials is well worth the llttle effi~rt! The students get that "eureka" feeling, as if to say Wow I understand hGw electrochemistry with some metals works!" This is great. Literature Cited

Demonstrating The Conservation of Matter A Trilogy of Experiments Submitted by:

David Martin, Randy D. Russell, and Nicholas C. ~homas' Auburn University at Montgomery Montgomery, AL 36117 Checked by:

Galen Meil University of Montana Missoula. MT 59812

The law of conservation of matter is one of the first principles introduced in many science courses, including general chemistry. Students learn that during a chemical change there is no detectable increase or decrease in the total amount of matter in a system. Because the amount of matter can be determined easily by measuring its mass, this law can be demonstrated and, therefore, verified by

following the total mass of a system during the course of a chemical change. The following three short demonstrations, which should be presented in succession, utilize the reaction between two common reagents, dilute hydrochloric acid and calcium carbonate, and illustrate the validity of this important law. Demonstration 1: The Loss of Mass During a Chemical Change One hundred milliliters of 1 M hydrochloric acid is placed in a 250-mL Erlenmeyer flask. Powdered calcium carbonate (2.0 g) is weighed into a small plastic or glass vial. The vial should be small enough to fit into the flask and large enough to hold 2 g of the carbonate. The vial is then carefully lowered into the flask so that it does not tip over in the acid. (It may be necessary to place a marble, or some other heavy, nonmetallic object, in the vial ta prevent it tipping prematurely.) The flask is then placed on an electronic pan balance that is tared to read zero. Very accurate balances are not essential, and a readability of 0.1 g is satisfactory. The flask is gently shaken, causing the vial to fall, and the acid and carbonate to mix, and quickly relaced on the balance. There are no surorises here. In ;bout two minutes all the carbonate dissoives (with occasional eentle shaking) and a mass reduction of about 0.6 e is obseked due to l& of carbon dioxide, as expected. 1; large classes, the mass change can be observed by a student who can inform the rest of the class of the mass loss. If available, a video camera and television would be ideal to project the digital balance readout for the entire class to follow. At this point students can be mvited tucalculatc the theoretical m o u n t of carbon dioxide that should be oroduced during the reaction (0.88 g) and to explain the h a t i o n from the observed mass loss. Because carbon dioxide is soluble in water, some gas remains dissolved in solution. Demonstration 2: The Conservation of Mass Observed -Or is it? The above experiment is repeated with fresh acid and carbonate, but this time the carbonate is placed inside a rubber balloon (using a powder funnel). The balloon is then carefully stretched over the neck of the flask making sure that none of the carbonate falls into the acid. The flask is placed on the balance that is tared to zero. At this point the class can be informed that your intention is to pour the carbonate directly from the balloon into the acid. However- because the carbon dioxide cannot escapewhat will happen to the total mass of the system? The most popular response will be that no mass loss should be observed, because the gas is now contained in the balloon-no surprises here either, right? Wrong! Amass loss of about 0.5 g is obsewed in this experiment. Most students will be puzzled by this observation and scratch their heads as they endeavor to produce an explanation for the apparent breakdown in the conservation of matter law. A hint to resolve this dilemma can be offered by suggesting that students calculate the mass of air dis~lacedfrom the atmosphere as the balloon expands. To do this, they will need the density of air, approximately 0.0012 g/mL at 20 'C and 1atm, and the volume of the balloon, about 400 mL, which can be measured by determining the volume of water displaced when the balloon is hubmerged in a large beaker full of water. Students will find that the calculated mass of displaced air is equal to the mass loss observed during the reaction, about half a gram. Is this a coincidence? The mystery finally may be solved by reference to Archimedes' s principle: "A body immersed in a fluid will

' Author to whom colrespondence should be addressed. Volume 69 Number 11 November 1992

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