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Cambridge IGCSE Chemistry · 0620
Chapter 4: Electrochemistry — Part 2
Topic 4.1b · Specific electrolysis reactions and electroplating
Electrolysis of Molten Compounds
In molten salts, a metal forms at the cathode and a non-metal forms at the anode.
- Molten lead(II) bromide (PbBr2): Pb2+ ions are reduced to lead at the cathode, and Br- ions are oxidised to bromine at the anode.
- Prediction: For any binary compound (containing only two elements) in a molten state, the positive ion forms the metal at the cathode, and the negative ion forms the non-metal at the anode.
Exam Traps
- Do not apply aqueous rules to molten electrolytes — sodium metal forms from molten NaCl, not from brine.
- Metal always at cathode, non-metal at anode for molten binary compounds — do not reverse the products.
Electrolysis of Aqueous Solutions
In aqueous solutions, water provides H+ and OH- ions which compete with the compound's ions at the electrodes.
- At the Cathode: Hydrogen is produced if the metal is more reactive than hydrogen (e.g., sodium). The metal is produced only if it is less reactive than hydrogen (e.g., copper).
- At the Anode: Oxygen and water are produced from OH- ions unless halide ions (Cl-, Br-, I-) are present, in which case the halogen is produced.
| Electrolyte | Product at Cathode | Product at Anode | Observations/Notes |
|---|---|---|---|
| Conc. Aqueous Sodium Chloride (Brine) | Hydrogen gas (H2) | Chlorine gas (Cl2) | NaOH(aq) is left behind. |
| Dilute Sulfuric Acid | Hydrogen gas (H2) | Oxygen gas (O2) | Requires inert electrodes. |
| Aq. Copper(II) Sulfate (Inert electrodes) | Copper (brown deposit) | Oxygen gas (bubbles) | Blue colour of electrolyte fades as Cu2+ concentration decreases. |
| Aq. Copper(II) Sulfate (Copper electrodes) | Copper (plating) | Copper ions (Cu2+) | Blue colour remains constant as the copper anode dissolves to replace ions. |
Electroplating
Electroplating uses electrolysis to coat a thin layer of one metal onto another to improve appearance or resistance to corrosion.
- Method: The cathode is the object to be plated; the anode is the plating metal; and the electrolyte contains ions of the plating metal.
- Example: To plate steel with silver, use a silver anode and a silver nitrate electrolyte. Silver ions are reduced to silver atoms on the steel cathode.
Exam Traps
- Do not place the object to be plated at the anode — metal ions are reduced (deposited) at the cathode.
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