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Voltaic Cells
Voltaic cells are devices which use electrochemistry to generate electricity from chemical energy.
Overview:
Voltaic cells (or galvanic cells) are spontaneous reactions. This means that the reaction happens on its own, without needing anything to prompt it. The ions in the cell want to combine on their own. To understand voltaic cells, first learn oxidation and reduction.
These cells convert chemical energy into electrical energy using the principles of oxidation and reduction.
To the right is a voltaic cell. There are two separate beakers, one with a zinc rod in an ZnSO solution, and with a copper rod in a CuSO solution.
The left side is the anode, meaning this is where oxidation will take place. The right side is the cathode, which is where reduction takes place.
The two rods are connected by a wire in which electrons flow through. There is a salt bridge between the two beakers which is where ions flow through.
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Remember: oxidation is ALWAYS at the anode and reduction is ALWAYS at the cathode
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How it works:
Adapted from "Galvanic cell with no cation flow-tr.svg" by VectorVoyager, licensed under Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0). Changes were made. License: https://creativecommons.org/licenses/by-sa/4.0/
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The Anode:
The ZnSO will dissolve in the solution and separate into SO ions and Zn ions. The stable zinc atoms in the rod have the tendency to combine with the SO ions. To do this, the Zn atoms on the rod will leave two of their electrons on the rod and separate become a Zn ion. The two electrons left on the rod are sent through the wire. This is oxidation, as Zn has lost two electrons to become Zn .
The oxidation half reaction will be:
The flow of electrons through the wire will cause the bulb to light up. (This is the chemical energy converted into electrical energy!)
Zn → Zn + 2e
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The Cathode:
The solution on the right has CuSO which has dissolved into Cu and SO ions. The electrons are passed through the wire, to the cathode and onto the copper rod. The copper rod now has a surplus of electrons. The Cu ions in the solution will take two of these electrons to become Cu.
The reduction half reaction will be:
These newly formed copper atoms will collect on the rod causing it to expand.
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Cu + 2e → Cu
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Because the cathode is where reduction takes place, electrons will always flow from the anode to the cathode.
The Salt Bridge:
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Just as the wire is a passageway for electrons, the salt bridge is a passage way for ions. Inside the salt bridge is NO ions and K ions. These are inert salts, inert meaning they will not react or interfere with any other ions. But why is this necessary?
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After some time, the anode's solution will have too many positive Zn ions. NO ions are added to balance the charges.
The cathode's solution will eventually have too many negatively charged electrons, so positive K ions are added.
If there were no salt bridge, electrons would stop flowing causing the cell to stop.
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Remember:
- Oxidation happens at the anode
- Reduction happens at the cathode
- Electrons always move from the anode to cathode
- The wire is a passageway for electrons
- The salt bridge is a passageway for ions
- Voltaic cells are spontaneous reactions meaning they happen on their own.