E l e c t r o c h e m i c a l Demonstration: Motor Driven by a S i m p l e G a l v a n i c Cell
J. F. Skinner' Williams College Williamstown, Massachusetts 01267 The measurement of the voltaees ~roducedin electroo i metals and different chemical cells utilizing different electrolyte concentrations is frequently part of introductory chemistry courses. The results, discussed in terms of standard reduction ~otentialsand the Nernst equation. illustrate the conversion of chemical potential energy into electrical potential enerev. As these measurements are usually made with little or no current flow, the student often does not realize that the assembly of metals and solutions is a "battery" which could be used under conditions permitting current flow (or electron flow, for a better understanding of the redox processes involved) converting the electrical potential energy to mechanical energy. A s i m ~ l edemonstration is described wherein a Zn/ % n 7 T u t T Cu t Uaniull) cell will operate a small motor. A 6.5 X 10.5-cmuiece of O2mm zinc sheet is curved to iust fit inside a 150-ml beaker. A glass tube (6.5 cm long, 4.5 cm. 0.d.) with a elass frit (3.8 cm in diameter, coarse wrositv) a t the bottom s&.; twth to hold the secondelectrolyte an&togive rhe liquid-liauid junction hetu,ern the rwoelertrol\tes. A Teflon ring fitted.aro;nd the top of this inner cell makes it possible to support it on the lip of the beaker. The inner cell may be fabricated from a straight fritted tube2 or froma wide-based porous filter crucible (coarse).
The current that can be delivered bv such a cell is very dependent on the method used to separate the l i q u i d s . ~ ~ h e common NH4NOs-filled 7-mm U-tube results in a large impedance and low current. Use of medium or fine porosity frits or reducing the diameter of the frit will alsoreduce the current below thelevel necessary for the motor. A 5.0 X 9.0-cm piece of 0.2-mm copper sheet is curved to fit inside the inner cell. Wire leads are fastened to eachelectrode and connected (check polarity) t o the terminals of a small dc rnotor.3 A small ~ i e c eof cardboard can be attached to the motor shaft to illustrate its rotation. About 90 ml of 0.5 M ZnS04 are added to the beaker and while placing the inner cell in position, about 50 ml of 0.5 M CuS04 are added, keeping the two electrolyte levels as even as possible to minimize diffusion. The motor shaft initially may need a gentle nudge to overcome its inertia. A steady current of about 10 ma is necessary. The voltage for copper and zinc electrodes in equimolar electrolytes will be approximately 1.10 V, whereas that for zinc and silver electrodes will be about 1.56 V. A faster rate of rotation of the motor will be noted with the latter electrode combination. Interested students or ones in an advanced laboratory can experiment further with different types of liquid-liquid separations and different electrolyte concentrations, measuring the current with an ammeter. Address during 1977-78 academic year: Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada. ZAvailable from Ace Glass Inc., Vineland, N.J. Catalog No. 7205-32 "vailable from Edmund Scientific Inc., Barrington, N.J. Catalog No. 40,872.
Volume 54, Number 10, October 1977 1 61s
