How Batteries Work: An Electrochemical Explanation

[Alex Hughes]

So, I'm a little confused on how batteries work. I understand they convert chemical energy into electrical energy. I also understand that the battery consists of an anode and a cathode with an electolyte in the middle to force the electrons from the chemical reaction to have to follow the wires in the circuit. But everywhere I read it says the chemical reactions in the battery are triggered as soon as you connect the two terminals with a conducting wire. How does completing the circuit all of a sudden trigger the chemical reactions? Also, in my physics class they always told us to think of the electrons as sliding down to a lower potential and being driven back up to the higher potential by some electromotive force. However, this goes against what I've been reading. It seems once the electrons flow from the anode to the cathode, they stay there until all the electrons produced from the chemical reaction are equally distributed between the two leads and no potential difference occurs anymore. Is what they tried to explain to us in physics only for conventional current? Or do the electrons actually go back to the anode after they have been dumped into the cathode. Lastly, what do these chemical reactions consist of and how do they work? Sorry for rambling, just didn't know the order to ask my questions. Would appreciate a detailed response. Thanks!

 

[Borek]

There are two separate reactions in any battery - one is oxidation (produces electrons) the other is reduction (consumes electrons). Neither can work on itself, they require transfer of electrons. When you close the circuit you allow this transfer of electrons so both reactions can go on.

These separate reaction are called half reactions, as opposed to the overall reaction describing whole system at once. So, in a simple Zn/Cu battery copper is reduced:

Cu²⁺ + 2e^- → Cu

and zinc is oxidized:

Zn → Zn²⁺ + 2e^-

overall reaction being

Cu²⁺ + Zn → Cu + Zn²⁺

Now, when you stick copper wire into copper ions solution it will have some potential (reaction goes ahead a bit, charging the wire). Same happens when you stick the zinc rod into the solution of zinc ions. Connect them, and electrons will flow. Not for long though, you need the circuit to be closed on the "other end" as well (but there it is just a matter of chemically inert ions traveling, not electrons).

 

[Windadct]

The chemical reaction achieves equilibrium with no electrical path from anode to cathode (in side is flooded with free electrons, the other has a deficit) - when you complete the circuit, it comes out of equilibrium and the chemical reaction continues converting the energy.

 

[Borek]

The chemical reaction achieves equilibrium with no electrical path from anode to cathode

To be more precise: each half reaction achieves a separate equilibrium. As long as there is no charge exchange they can't be thought of as a single reaction.

It may look like a nitpicking, but I know from experience that these details is what many people miss when they try to understand the batteries.