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Electrolytic Cells
Electrolytic cells are non-spontaneous reactions which convert electrical energy into chemical energy using oxidation and reduction.
Overview:
Electrolytic cells differ from voltaic/galvanic cells as they are non-spontaneous reactions. This means that the reaction does not happen on its own, and must have an external power source to start. In this case, a battery.
Electrolytic cells convert electrical energy into chemical energy while voltaic cells convert chemical energy to electrical energy.
First read: oxidation and reduction (must read) voltaic cells (recommended).
To the right is an electrolytic cell. The beaker contains a solution molten(melted) NaCl. Because the NaCl is molten, its ions of Na and Cl are freely moving around.
There are two rods both made out of graphite, as electrons are able to flow through graphite easily. Both rods are connected to an external battery by a wire.
The battery has a negative terminal and a positive terminal, which will determine which side is the cathode and anode.
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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 Voltage source with electrolytic solution.svg by User:ARTE. Public domain.. License: https://creativecommons.org/licenses/by-sa/4.0/
The Cathode:
In this cell, the cathode is on the left side of the beaker. This graphite rod is connected to the negative terminal of the battery, which has a surplus of electrons. These excess electrons from the battery will flow through the wire and onto the rod. The electrons give this rod a negative charge which attracts the positive Na ions. The Na ions want to gain an electron, so they will take one from the negatively charged rod to become Na.
The newly formed Na will collect on the bottom of the graphite rod (not shown in diagram). The reduction half reaction will be:
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Na + e → Na
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The Anode:
The anode is the graphite rod on the right side of the beaker. This rod is connected to the the positive terminal of the battery having very few electrons. The negatively charged electrons from the graphite rod will be attracted towards the positive terminal, leaving the rod with a positive charge. Negative Cl ions in the solution will be attracted to the positive rod. Cl ions will give one electron to the rod to become Cl gas.
Newly formed Cl will collect on the bottom of the anode rod (not shown in diagram). The oxidation half reaction will be:
Cl → Cl + e
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You may want to associate the cathode and anode with being negative and positive, however this is incorrect. Not all cells in electrochemistry will have a negative cathode and positive anode. Anodes and cathodes have nothing to do with charges. They are determined by where oxidation and reduction take place. The anode is where oxidation occurs and the cathode is always where reduction occurs. This is the only thing that determines which electrode is which.
Test your understanding:
1). What happens at the anode vs the cathode?
2). The cell connected to the positive terminal of the battery will have a _______ charge.
3). Is this reaction spontaneous or non spontaneous and why?
4). List two differences between voltaic and electrolytic cells.
5). In an electrolytic cell, _____ energy is converted to _____ energy.
Remember:
- Oxidation happens at the anode
- Reduction happens at the cathode
- Electrolytic cells are non spontaneous reactions meaning they do not happen on their own.