Electrolysis

Electrolysis is the chemical breakdown of an ionic compound, when molten or in aqueous solution, by passing an electric current through it.

Parts of a simple electrolytic cell

• Cathode (negative): where reduction happens (ions gain electrons).
• Anode (positive): where oxidation happens (ions lose electrons).
• Electrolyte: a molten ionic compound or an aqueous ionic solution that conducts electricity.

How charge moves

• Electrons flow in the external circuit from the anode to the cathode.
• Positive ions (cations) move to the cathode; negative ions (anions) move to the anode.

Simple rules for products

• At the cathode: metals or hydrogen form.
• At the anode: non-metals (not hydrogen) form.

Molten ionic compounds (binary: made of two elements)

• Cathode: metal forms.
• Anode: non-metal forms (e.g., halogen).

Aqueous solutions (water is present)

• Cathode: if the metal is less reactive than hydrogen (e.g., Cu, Ag), the metal forms; otherwise hydrogen gas forms.
• Anode (inert electrodes): concentrated halide gives the halogen (Cl− → Cl₂); otherwise oxygen forms from water.

Half-equations (show electron transfer)

Examples:

• Metal at cathode: $$\mathrm{Cu^{2+} + 2e^- \to Cu}$$
• Hydrogen at cathode (from water): $$\mathrm{2H_2O + 2e^- \to H_2 + 2OH^-}$$
• Halogen at anode: $$\mathrm{2Cl^- \to Cl_2 + 2e^-}$$
• Oxygen at anode (from water): $$\mathrm{4OH^- \to O_2 + 2H_2O + 4e^-}$$

Key examples and observations

• Molten lead(II) bromide (PbBr₂) with graphite: lead metal at cathode (grey deposit); bromine at anode (brown gas). $$\mathrm{Pb^{2+}+2e^-\to Pb}$$ and $$\mathrm{2Br^-\to Br_2+2e^-}$$
• Aqueous copper(II) sulfate with graphite: copper at cathode (reddish coat); oxygen at anode (bubbles), blue solution fades.
• Aqueous copper(II) sulfate with copper electrodes: anode copper dissolves (Cu → Cu²⁺ + 2e⁻), cathode gains copper. Solution stays blue (used for electroplating/purifying Cu).
• Concentrated brine (NaCl(aq)): hydrogen at cathode; chlorine at anode; sodium hydroxide remains in solution.
• Dilute sulfuric acid: hydrogen at cathode; oxygen at anode.

Gas tests (observations)

• Hydrogen: lighted splint gives a ‘pop’.
• Oxygen: glowing splint relights.
• Chlorine: bleaches damp litmus.