Electrolysis: Cambridge (CIE) IGCSE Chemistry Revision Notes

Electrolysis Principles

• Electrolysis is a process where a molten ionic compound is broken down by an electric current. This applies to aqueous solutions as well.
• Ionic compounds in solid state cannot conduct electricity due to lack of free ions.
• Covalent compounds do not conduct electricity, thus cannot undergo electrolysis.
• When ionic compounds are molten or in solution, ions are free to move and carry charge.

Key Terms in Electrolysis

• Electrode: A rod (metal/graphite) that conducts electricity into/out of an electrolyte.
• Electrolyte: Ionic compound in molten/dissolved form conducting electricity.
• Anode: Positive electrode.
• Anion: Negatively charged ion attracted to the anode.
• Cathode: Negative electrode.
• Cation: Positively charged ion attracted to the cathode.

Basic Electrolytic Cell Setup

• Metals/Hydrogen form positive ions; hence, a metal/hydrogen gas is formed at the cathode.
• Metal production at the cathode depends on reactivity series (less reactive than hydrogen = metal production).
• Non-metals (except H) form negative ions, resulting in non-metal formation at the anode.

Charge Transfer in Electrolysis

• Current flow requires charge transfer via charge carriers.
• Power supply gives cathode electrons, making it negatively charged.
• Cations move to cathode to gain electrons.
• Anions move to anode to lose electrons.
• Diagram displays movement direction of electrons and ions in NaCl electrolysis.

Electrolysis of Molten Compounds

• Binary ionic compounds: Consist of two elements joined by ionic bonding.
• Predict products by identifying ions: Positive ions go to cathode, negative ions go to anode.
• Cathode product: Metal; Anode product: Non-metal.

Example: Electrolysis of Lead(II) Bromide

• Heat lead(II) bromide to molten state.
• Utilize graphite electrodes connected to power supply.
• Bromide ions move to anode, losing electrons to form bromine gas.
• Lead ions move to cathode, gaining electrons to form lead metal.

Worked Example

• Electrolysis of molten potassium chloride produces chlorine gas at the anode and potassium metal at the cathode.

Electrolysis of Aqueous Sodium Chloride & Dilute Sulfuric Acid

• Brine (concentrated NaCl) electrolysed with inert electrodes.
• Produces chlorine and hydrogen gases.
• Sodium hydroxide remains in solution.
• Chlorine: Used in bleach production.
• Hydrogen: Used in margarine production.
• Sodium hydroxide: Used in soap/detergent production.

Electrolysis of Dilute Sulfuric Acid

• Electrolysis produces oxygen and hydrogen gases.
• Twice as much hydrogen as oxygen is produced.

Electrolysis of Aqueous Solutions (Extended Tier)

• Water dissociation produces H+ and OH- ions contributing in electrolysis.
• Products depend on relative reactivity of ions.
• Anode: Halide ions produce halogens; OH- ions produce oxygen.
• Cathode: Metal ions or H+ ions are reduced depending on reactivity.

Ionic Half Equations

• Oxidation: Loss of electrons.
• Reduction: Gain of electrons.
• Oxidation occurs at the anode; reduction occurs at the cathode.
• Ionic half-equations demonstrate electron transfer involving reactions.
• Example for metals: Cu2+ + 2e- → Cu
• Example for non-metals: 2H+ + 2e- → H2

Examiner Tips and Mnemonics

• PANIC: Positive Anode, Negative Is Cathode.
• OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).
• RED CAT and AN OX: REDuction at the CATode, ANode for OXidation.