The Theme of Batteries in Chemical Reactions
Batteries (and the Oxidation-Reduction Reaction)
There is a special class of reactions called oxidation-reduction reactions that play an important role in our everyday lives. This mouthful of a reaction has been conveniently shortened to the term "redox" (pronounced ree-dox). Not only are redox reactions vital for biochemical reactions occurring in our bodies, but they're also used in many industrial processes. They also power The Energizer Bunny's favorite energy source: the battery.
A redox (reduction-oxidation) reaction is any reaction in which the oxidation states of the atoms change. If you recall, the oxidation state of an atom is the formal charge that it possesses. Oxidation involves the loss of an electron and thus an increase in the oxidation state. Reduction involves the gain of an electron and thus a decrease in the oxidation state.
An easy way to remember this is Leo the Lion goes Ger.
LEO = loss of electrons is oxidation
GER = gain of electrons is reduction
Leo goes ger. (Image from here.)
Let's look at a simple reaction to make sure we all understand. Take a close look at the reaction between hydrogen and fluorine:
H2 + F2 → 2 HF
The H atoms and F atoms in H2 and F2 are in the zero oxidation state. Let's figure out what their oxidation states in HF. H is in the +1 oxidation state, and F is in the -1 oxidation state.
We can also break down the reaction into two components to make it simpler to see the redox reaction.
H2 → 2 H+ + 2 e-
This is the oxidation half of the reaction because H is losing electrons (LEO).
F2 + 2 e- → 2 F-
This is the reduction half of the reaction because F is gaining electrons (GER).
It's important to remember that oxidation is always coupled to reduction. We can't have one without the other.
What do batteries have to do with redox reactions? The answer is absolutely everything.
An alkaline battery. (Image from here.)
Batteries are devices that produce an electric current from a redox reaction. The battery basically converts chemical energy into electrical energy.
Batteries take advantage of the ease with which certain metals lose electrons. The most basic and traditional battery utilizes the transfer of electrons between zinc and copper. When a piece of zinc is placed in a solution containing copper ions (Cu2+) a redox reaction takes place. The copper ions become pure copper metal and the zinc metal becomes zinc ions (Zn2+).
Zn (s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s)
A simple battery creates this reaction inside a small container along with two electrodes. One electrode is positive and the other is negative. If you switch the electrodes, your Tickle Me Elmo won't work. At the negative electrode, the oxidation reaction takes place and electrons flow out of the cell. At the positive electrode, the reduction reaction takes place and electrons flow into the cell. Remember, the flow of electrons is what creates the electricity that we know and love.
This simple setup is outdated though since it only allows a battery to be used once. Many of today's batteries are much more advanced. They use more "high-tech" oxidation-reduction techniques that can even be used time and time again. Cell phone batteries, anyone?