Which half-reaction occurs at the anode for chloride ions to form chlorine gas?

• A. 2Cl- → Cl2 + 2e-
• B. 2Cl- + 2e- → Cl2
• C. Cl- → Cl + e-
• D. Cl2 + 2e- → 2Cl-

Answer: (A)

The key idea is oxidation at the anode during electrolysis. Chloride ions lose electrons here to form chlorine gas. The balanced half-reaction is 2 Cl- → Cl2 + 2 e-. Each chloride ion donates one electron, and two such ions combine to make Cl2, with those two electrons flowing out through the external circuit.

Why this fits at the anode: oxidation means loss of electrons, which is exactly what's happening to the chloride ions to produce chlorine gas. The other possibilities represent reduction (gaining electrons) or forming a single chlorine atom, neither of which matches the oxidation process at the anode. Reduction of chlorine gas to chloride ions would occur at the cathode, not the anode.