Electrolysis Basics
* Electrolysis: The process of using an electric current to drive a non-spontaneous chemical reaction.
* Cathode: The negatively charged electrode where reduction (gain of electrons) occurs.
* Anode: The positively charged electrode where oxidation (loss of electrons) occurs.
The Reactions
During the electrolysis of NaCl, the following reactions occur:
* At the Cathode (Reduction):
* 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq) This is the reduction of water to form hydrogen gas.
* At the Anode (Oxidation):
* 2Cl⁻(aq) → Cl₂(g) + 2e⁻ This is the oxidation of chloride ions to form chlorine gas.
Why Hydrogen, Not Sodium?
1. Electrochemical Series: The electrochemical series (or activity series) lists metals in order of their reactivity. Sodium is a highly reactive metal, meaning it readily loses electrons. Hydrogen is less reactive.
* Sodium is more likely to be oxidized (lose electrons) at the anode, NOT the cathode.
* Hydrogen is more likely to be reduced (gain electrons) at the cathode.
2. Overpotential: The overpotential is the extra voltage needed to overcome the energy barrier for a reaction to occur. The overpotential for the reduction of sodium ions is significantly higher than that for the reduction of water. This makes the reduction of water to hydrogen the more favorable process at the cathode.
In Summary
* Electrochemical reactivity: Hydrogen is less reactive than sodium, making it more likely to be reduced at the cathode.
* Overpotential: The overpotential for hydrogen reduction is lower, favoring its formation over sodium.
Note: If you were to use a mercury cathode, the overpotential for sodium reduction would be lowered, and you might get sodium metal produced at the cathode. However, this is a highly hazardous process.
