edited by: RON DELORENZO

Middle Georgia College Cochran. Georgia 31074

A Visual Analogy for Metallic Deposition The original problem stated that there was a mass increase of 2 grams. Using dimensional analysis, we can compute:

Dacio R. Hartwig and Romeu C. Rocha Filho 64 g Cu depos~ted

Universidade Federal de S6-o Carlos. Caixa (2 g mass increase) ( , ' ) = 16 g Cu deposited Postal 676. 13560-SBo Carlos-SP. Brazil 8 g mass ,"crease

Although the ahove solution is very simple, few high school The fundamental concept of spontaneous depo- or even college students can do it satisfactorily; most students

sition stoichiometry problems is that atoms simultaneously give the value as the answer. Why? Normally the necessary arrive a t and depart from the same elect~ode. For example, emphasis is not given to the concept of simultaneous arrival when copper is deposited on an iron rod from a copper(I1) and departure of atoms at from the rod. consequently, sulfate solution, iron(I1) cations depart from the iron rod a t students visualize only the arrival of ions at the rod, Such an the same time that copper(11J arrive at the rod' erroneous notion leads students to amclude that the mass of However, this simultaneity is not easilyperceived hystudents; copper deposited is g, ohviously a wrong answer. Further. consequently, a problem such as the one shownhelow is sel- more, after the occurrence of a spontaneous rnetallic deposi- dom solved correctly. tion. it can also h a n ~ e n that the rod's final weight is smaller

An iron rod was lmrnersed in an aqueous copper(l1) sulfate solution. After a while it was observed that copper had heen deposited on the rod. Later the rod was removed from the solution, dried, and weighed. It was noticed that the rod was 2 g heavier. How many grams of capper were deposited on the rod?

Solution For mole quantities, the reaction can be written

1 mole Cuzt(aq) + 1 mole Fe(si - 1 mole Fez+(aq) + 1 mole Cu(si

Since only the mass change of the md material is given, mass change of rod due to one mole Cu depositing: +64 g mass change of rod due to one mole Fe dissolving: -56 g net mass increase of the r r d +64 g + (-56 gi = 8 g

We can see that for every 64g of Cu de~osited, the mass of the rod . ~

increases hy 8 g, i.e.,

64 g Cu deposited = 8 g mass increase

' This analogy simulates reality more effectively if the balls and the board corresponding to the initial "metal rod'' are of the same material.

lOOg boord

29 marble

59 steel boll

board iOOg 17 marbles 349

totol 1349

I

. . than before. This fact again leads to confusion, especially if once again students are thinking the phenomenon to he as- sociated only with the arrival of atoms a t the rod. .

This incorrect (partial) vision of spontaneous metallic de- position can he remedied through the use of the following vi- sual analogy. A thick hoard of expanded polystyrene, in which several hemispherical cavities are filled with marbles, plays the role of the metal rod; the metallic deposition phenomenon is simulated through the exchange of some of these marbles by small steel halls. "Assuming that after this exchange the mass of the "metal rod" has increased by 30 g, and knowing what each marble and steel hall weighs, e.g., 2 and 5 g, re- spectively, the students could he questioned as t o what was the mass of the steel balls on the "metal rod".

This feature presents a collection of descriptive applicatfons and an- aloaiesde~ionedio helo students understandsome of thedifficuitcon-

iOOg boord

board 1009 7 marbles 149

10 steel balls 509 totol 1649

t Volume 60 Number 7 July 1983 591

The solution to this example problem, analogously to the chemical one outlined before, is

initial "metal rod" + n steel balls - final "metal rod" + x marbles

Since only the mass change of the "metal rod" is given,

mass change of "metal rod" due to one steel ball put in: +5 g mass change of "metal rod" due to one marble taken out: -2 g net mass increase of the "metal rod": +5 g + (-2 g) = 3 g

We can see that for every exchange of a marble by a steel ball, the mass change of the "metal rod" increases by 3 g, i.e., (see figure)

5 g of steel balls = 3 g mass increase

The example problem stated that there was a mass increase of 30 g. Using dimensional analysis, we can compute:

( 5 g of steel balls) = (30 g mass increase) 50 g of steel balls put in

3 a mass ~ncrease

I t is clearly seen that ten marbles have been exchanged for ten steel balls. Once the students are able to solve this example problem, and once the analogy with the original chemical problem is established, the latter can be easily solved since the concepts and arithmetical operations involved in both prob- lems are exactly the same.

I t is important for the teacher to point out that the arrival and departure points are not identical. To stress this point, the steel balls should be placed randomly in the hemispherical cavities of the polystyrene hoard, always with enough care so as not to show to the students how many balls have been ex- changed.

In order to simulate the case in which the rod becomes lighter after the spontaneous deposition occurs, the same analogy can be used starting with the steel balls in the cavities of the polystyrene board.

592 Journal of Chemical Education