Oxygen
Explanation
Electrolysis of water is the process of decomposing water molecules ($H_2O$) into their constituent gases, hydrogen and oxygen, by passing a direct electric current through water containing a small amount of acid or base (electrolyte) to increase conductivity.Process Analysis
- Electrode Polarity: In an electrolytic cell, the anode is connected to the positive terminal of the battery, and the cathode is connected to the negative terminal.
- Reaction at the Anode (Positive Terminal): Oxidation takes place at the anode. Water molecules or hydroxide ions lose electrons to form Oxygen gas ($O_2$).
Reaction: $2H_2O(l) \rightarrow O_2(g) + 4H^+(aq) + 4e^-$ - Reaction at the Cathode (Negative Terminal): Reduction takes place at the cathode. Hydrogen ions gain electrons to form Hydrogen gas ($H_2$).
Reaction: $4H^+(aq) + 4e^- \rightarrow 2H_2(g)$ - Stoichiometry: The overall reaction shows that for every two molecules of water decomposed, two molecules of hydrogen and one molecule of oxygen are produced. Consequently, the volume of hydrogen gas collected is double the volume of oxygen gas.
Key Takeaway: During water electrolysis, Oxygen is evolved at the Anode (Positive), and Hydrogen is evolved at the Cathode (Negative). A useful mnemonic is "AnOx" (Anode Oxidation) and "RedCat" (Reduction Cathode).