ROBERT C. PLUMB
chemical principles exemplified
Worcester Polltechnic Institute Worcester, Manachvsettr 01 609
Car Won't Start? lllusfrafing principles of fhermodynomics ond elecfrochemisfry
Suggested by Professor L e m a r d K. Nash, Harvard University It's a cold morning and your car won't start? Some knowledge of the chemical effects causing the trouble can help you get to school on time! The chemistry involved can he quite varied, iucluding such things as the electrical conductivity of graphitic carbon in spark plug deposits, the low volatility of gasoline a t low temperatures, and the effect of thermal molecular motion in imparting fluidity to lubricating oils. These effects all put additional burdens on the battery, which itself is affected by temperature, so that here we will consider only the battery part of the problem. Does the battery voltage drop as the temperature drops? The cell reaction is and from the standard free energies of formation and entropies of the reactants and products one finds for this reaction A@ = -87.5 keal A#" = +7 cel/"K
and &" = 1.9 V
The effect of temperature is
and for a dropin temperaturefrom68'F toOaF the consequent change in voltage for one cell will he -5.7 X V. A 0.3% decrease in voltage cannot be a very significant effect,, and we must look more deeply into the problem. Does the ability of a battery to deliver cuwent decrease as the temperature drops? For each mole of electrons transferred from the anode to the cathode through the
The exempla are designed to show fundamental chemical principles in operation. They deal with phenomena in which students have intrinsic interest; they apply abstract ideas to easily visltslised situations. All of us have our pet anecdotes and illnslrations which we know will attract the students' interest. Your contributions and suggestions are invited. They may be sent to the author. ' RICK~,:NBACKEE, Eowam V., "Rickenbacker," Prentice-Hall, Englewood Cliff8, New Jersey, 1967 p. 30,31.
starter motor, a mole of ions must migrate between the anode and cathode through the electrolyte to preserve electrical neutrality. Both anions and cations move hut because of the l o ~ ~ conductivity er of HSOa- compared to H+ the major charge transport is by the Hf. The unequal movement of anions and cations causes the H,S04 concentration in the vicinity of the anode to be lower than in the hulk of the electrolyte. Two electrical effects are associated with t,hese migrations. One is the internal resistance of the cell ~ ~ h i is c hdetermined in large part (in a reasonably new battery) by the conductivity of the electrolyte; the other is concentration polarization of the electrodes xhich occurs because concentration changes in the electrolyte near the electrode surfaces \ d l change the voltage of the cell. The effects of the concentration changes are ameliorated by diffusion which tends to dissipate concentration gradients in the solution. The current, polarization, and internal resistance of an automotive battery depend upon design and load, hut in a typical situation -1/100th of the chemical reserve of a cell is consumed in a start and a current of 50 amp is drawn on a 12 V system. The internal resistance is of the order of 0.04 ohms but can be as high as 0.1 ohms. Thus the voltage during starts can drop to 10 V or lover. How will the internal resistance and concentration polarization depend on temperature? Both the ionic conductance and the ionic diffusivity decrease with decreasing temperature, as a result of the changes of viscosity of the solvent with temperature. For ions in water this amounts to about 2% per degree in the neighborhood of 25"C, and TTe see that there can be substantial increases in internal resistance losses (decreased conductivity) and concentration polarization (decreased diffusivity) as the temperature drops. Both are important effects in decreasing the pover output of your battery on a cold morning. (An old-fashioned trick
to beat these effects is to remove the battery and store i t in a warm place on a cold night.) Perhaps you have noted that if you have been trying to start the car and you let i t "rest" for a few minutes the battery will have renewed zest. Why? A Related Anecdote--from the Life of Eddie Rickenbacker' Concerning his trip in a Waverly Electric Car taken without permission from his employer. He was about 14 years old of the time ( 1 904).
Contributed by Arra Nergararian, Worcester Polytechnic Institute "After supper I started back to the garage. Mr. Evans would be in next morning. . . But I hadn't gone one quarter of the way when the little car began to give signs that I had driven it too much. I t slowed down and came to s stop. It was out of juice. The battevies were dead. Darkness was coming on and I had a mile and a half to go . no wrecker service . . only the Evans Garage which was going to be minus its one employee should its irate owner return to find said employee out with a customer's ezr.
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"In discussing electrical energy with me, Evans had observed that frequently a battery would regsin some current if allowed to sit idle for a while . . . I decided to wait an hour. Never had time dragged so slowly. My Ingersoll dollar watch was in and out of my pocket a. dozen times . . I gingerly pushed the oontrol lever, fully expecting nothing to happen, but . . . the oar lurched forward . . several blocks before it died. Again I sat for an the refreshed batteries took me a few more blocks. As hour the night wore on, the hope became shorter and the waits longer, hut finally about 3:00 A.M. I reached the garage. I hooked up the battery charger and took the streetcar home. ."
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Volume 47, Number 5, May 1970
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383
