Earlier in the week a neighbor called me to ask if I had jumper cables. Her children had hopped in the car while she remained inside to collect all of the necessities for a day at the pool: sunscreen, snacks, drinks, goggles, towels. If you get through all the prep work the pool is supposedly relaxing. Anyway, it was a warm day so the kids blasted the air condition and they also turned on the radio, but the ignition was not on, so they drained the battery.
I did not have jumper cables, but I walked over to my neighbors house equipped with something far more useful – a knowledge of electrochemistry.
Like a good chemist, I began providing my neighbor with bits of electrochemistry facts. “Electrons flow from the anode to the cathode.” “Oxidation occurs at the anode, reduction occurs at the cathode.” I am sure it set her mind at ease to have me there.
She lifted the hood and we hunched over the battery. My neighbor is an electrical engineer and while we stood there staring we conferred over our limited practical knowledge for the situation. “Red is positive, black is negative,” she said. “Right,” I said. “Current flows from black to red,” she said. “Yep,” I said. You can see how vital I am; it is really a good thing she called me.
A third neighbor then showed up, with no understanding of electricity, but with a pair of jumper cables and instructions on how to use them.
The first thing we were supposed to do was clamp one end of the red-handled clamp to the red electrode of the dead battery and the other end to the red electrode of the live battery. Then we were supposed to connect the black-handled clamp to the black electrode of the live battery and the other end to unpainted metal. Next we needed to start the car with the good battery followed by the car with the dead one.
We followed all those steps and nothing happened. “Somethings wrong,” I said. (Really, if you are ever in trouble you should call me. You can see how incredibly helpful I am.)
We – and by we I mean my one neighbor who was actually brave enough to place the cables on the battery – tried putting that last black clamp in a couple of locations but nothing happened until we(she) put it on the black electrode of the dead battery. Then the car turned over immediately. We congratulated ourselves and I was happy to have participated, although not directly, in jumping a car without a male present. But none of it made sense to me.
The practical didn’t braid with the theoretical. I could’t understand why we had connected red to red if we wanted electricity to flow from the negative electrode to the positive electrode. Also, shouldn’t connecting black electrodes have shorted the circuit? Instead it started the car. I turned to a General Chemistry text book for answers.
A car battery, like any battery, uses chemistry to generate electricity. A car battery exploits a sulfuric acid solution to balance out electrons transported between lead and lead oxide plates lined up in series inside the battery, thus it is aptly named a lead-acid storage battery.
The chemical reaction that occurs is as follows:
Pb(s) + PbO2(s) + 2HSO4-(aq) + 2H+(aq) —-> 2PbSO4(aq) + 2H2O(l)
Sulfuric acid is shown as ions because it is a strong acid, meaning it is completely dissociated in water. In the process of the reaction, lead loses two electrons to lead oxide. This flow of electrons is put to work to ignite the car’s engine.
The lead and lead oxide plates must continually be cleared of the lead sulfate produced in the reaction or current will no longer flow and the battery will die. When the engine is running, the car is fueled by gasoline and the battery is no longer in use. A mechanical wheel driven by the engine, called an alternator, creates a current that runs in reverse through the battery and sheds PbSO4 from the plates readying it for the next time you start your car. Pull current from the battery for too long while the engine is not running and you will find yourself identifying the people in your neighborhood who own jumper cables.
Ah, it is so satisfying to untangle the mess of what was stored in my head and be able to weave it neatly with what I observed in practice. This explains why you have to connect red electrodes with the cables, because you are using the current from the live battery to run the reaction in reverse and slough off lead sulfate from the dead battery.
But what about the black electrode question? Now that I understand how the current flows through the battery it makes sense that the black electrode should be connected to ground. In our first few attempts we(she) just must not have been able to make a good connection. Although, I have read connecting the black clamp to the black electrode for too long is a bad idea because apparently the current running through the water in the battery forms hydrogen gas by electrolysis and can cause an explosion.
I am sure you already knew all this about car batteries, but I am experiencing a magnificent amount of chemistry girl pride at the moment for having put it all together for myself.
Photo source: battery: cleantechtutor.com, charge: dummies.com, inside:gravitaexim.com