1. Electrolytes, non-electrolytes and the particles present
- Electrolyte: a compound that conducts electricity in the molten state or in aqueous solution and is decomposed by it.
- Strong electrolyte: almost completely ionised; contains ions only (NaCl, KOH, NaOH, dilute HCl, H₂SO₄, HNO₃, CuSO₄, molten PbBr₂).
- Weak electrolyte: partly ionised; contains ions and molecules (ethanoic acid, carbonic acid, ammonium hydroxide solution).
- Non-electrolyte: contains molecules only; does not conduct (sugar, alcohol, kerosene, carbon tetrachloride).
Solid ionic compounds do not conduct: their ions are held in fixed positions. Metals conduct by electrons with no chemical change; electrolytes conduct by ions and are decomposed.
| Particles present | Type | Examples |
|---|---|---|
| Ions only | Strong electrolyte | Molten PbBr₂, NaOH solution |
| Ions and molecules | Weak electrolyte | CH₃COOH, NH₄OH solutions |
| Molecules only | Non-electrolyte | Alcohol, sugar solution, kerosene |
Key terms
- Electrolyte
A compound that conducts electricity in the molten state or in aqueous solution and is decomposed by it.
- Weak electrolyte
An electrolyte that is only partly ionised, so its solution contains both ions and molecules.
- Non-electrolyte
A substance that does not conduct electricity in the molten state or in solution, because it contains molecules only.
2. Terms of electrolysis; oxidation and reduction
- Electrolysis: decomposition of an electrolyte (molten or in solution) by an electric current.
- Electrodes: conductors through which current enters and leaves the electrolyte. Anode — joined to the positive terminal; cathode — joined to the negative terminal.
- Cation: positive ion, moves to the cathode. Anion: negative ion, moves to the anode.
- Oxidation = loss of electrons (at the anode). Reduction = gain of electrons (at the cathode).
| Electrode | Sign | Ions attracted | Process |
|---|---|---|---|
| Anode | Anions | Oxidation | |
| Cathode | − | Cations | Reduction |
Formulas
- Reduction at a cathode
- Oxidation at an anode
Key terms
- Electrolysis
The decomposition of an electrolyte in the molten state or in solution by passing an electric current through it.
- Anode
The electrode joined to the positive terminal of the battery; anions lose electrons there.
- Cathode
The electrode joined to the negative terminal of the battery; cations gain electrons there.
- Oxidation
Loss of electrons.
- Reduction
Gain of electrons.
3. Selective discharge
When several ions can reach an electrode, the one discharged depends on:
- Activity series (K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Ag): the lower the metal, the more easily its ion is discharged at the cathode. From aqueous solutions of Na⁺, Mg²⁺ or Ca²⁺ salts, hydrogen forms instead. At the anode, OH⁻ is discharged in preference to SO₄²⁻ and NO₃⁻.
- Concentration: a much more concentrated ion can be discharged in preference (Cl⁻ in concentrated brine gives chlorine).
- Nature of the electrode: an active anode (copper, nickel, silver) dissolves instead of an anion being discharged; platinum and graphite are inert.
| Solution (platinum electrodes) | Cathode | Anode |
|---|---|---|
| Dilute MgSO₄ | H₂ | O₂ |
| Dilute CuSO₄ | Cu | O₂ |
| Dilute Na₂SO₄ | H₂ | O₂ |
Key terms
- Selective discharge
The preferential discharge of one ion over others at an electrode, decided by the activity series, concentration and the nature of the electrode.
- Inert electrode
An electrode, such as platinum or graphite, that conducts but takes no part in the reaction.
4. Electrolysis of molten lead bromide
- Apparatus: silica crucible (not attacked by the melt, withstands high temperature); graphite electrodes (inert, not attacked by bromine); heat until molten; fume cupboard.
- Lead bromide conducts only when molten: then its ions are free to move.
- Ionisation: PbBr₂ → Pb²⁺ + 2Br⁻.
- Cathode: Pb²⁺ + 2e⁻ → Pb — silvery grey molten lead collects at the bottom.
- Anode: 2Br⁻ → Br₂ + 2e⁻ — reddish-brown fumes of bromine.
| Electrode | Reaction | Observation |
|---|---|---|
| Cathode | Pb²⁺ + 2e⁻ → Pb | Silvery grey metal at the bottom |
| Anode | 2Br⁻ → Br₂ + 2e⁻ | Reddish-brown fumes |
Formulas
- Overall change
Key terms
- Silica crucible
A crucible of silica, used because it is not attacked by molten lead bromide and stands high temperatures.
5. Electrolysis of acidified water
- Pure water is a very weak electrolyte; dilute sulphuric acid is added to increase the number of ions. Platinum electrodes are used (inert).
- Ions: H⁺, SO₄²⁻, OH⁻.
- Cathode: 4H⁺ + 4e⁻ → 2H₂. Anode: 4OH⁻ → 2H₂O + O₂ + 4e⁻ (OH⁻ discharged in preference to SO₄²⁻).
- Hydrogen : oxygen = 2 : 1 by volume: 4 electrons give 2 molecules of H₂ but 1 molecule of O₂ (Avogadro's law).
- The acid is not used up; only water decomposes, so the acid slowly becomes more concentrated.
| Electrode | Gas | Volume |
|---|---|---|
| Cathode (−) | Hydrogen | 2 volumes |
| Anode (+) | Oxygen | 1 volume |
Formulas
- Anode reaction
- Overall change
Key terms
- Voltameter
The vessel in which electrolysis is carried out, holding the electrolyte and the electrodes.
6. Copper(II) sulphate solution: copper and platinum electrodes
| Copper electrodes | Platinum electrodes | |
|---|---|---|
| Cathode | Cu²⁺ + 2e⁻ → Cu (copper deposited) | Cu²⁺ + 2e⁻ → Cu (copper deposited) |
| Anode | Cu → Cu²⁺ + 2e⁻ (anode dissolves, no gas) | 4OH⁻ → 2H₂O + O₂ + 4e⁻ (oxygen) |
| Colour | Stays blue | Fades (Cu²⁺ not replaced) |
| Solution | Unchanged | Becomes acidic (H₂SO₄ left) |
Copper electrodes transfer copper from anode to cathode: the basis of copper refining and copper plating.
| Electrode used as anode | What happens at the anode |
|---|---|
| Copper (active) | Copper dissolves as Cu²⁺ |
| Platinum (inert) | OH⁻ discharged; oxygen given off |
Key terms
- Active electrode
An electrode, such as a copper anode, that takes part in the reaction and dissolves.
7. Electroplating and electrorefining
Electroplating: article = cathode; plating metal = anode; electrolyte = solution containing ions of the plating metal. Conditions: clean article, low current for a long time, suitable electrolyte.
- Nickel plating: nickel anode, nickel sulphate solution. Cathode Ni²⁺ + 2e⁻ → Ni; anode Ni → Ni²⁺ + 2e⁻.
- Silver plating: silver anode, sodium argentocyanide, Na[Ag(CN)₂], solution. Cathode Ag⁺ + e⁻ → Ag; anode Ag → Ag⁺ + e⁻. Silver nitrate is avoided: it deposits silver too fast, giving a rough coat; the argentocyanide releases Ag⁺ slowly for a smooth, even, firm coat.
Electrorefining of copper: impure copper anode, pure copper cathode, copper(II) sulphate solution acidified with dilute sulphuric acid. Zinc and iron impurities stay in solution; silver and gold collect below the anode as anode mud.
| Process | Anode | Cathode | Electrolyte |
|---|---|---|---|
| Nickel plating | Nickel | Article | Nickel sulphate |
| Silver plating | Silver | Article | Sodium argentocyanide |
| Refining copper | Impure copper | Pure copper | Acidified copper(II) sulphate |
Formulas
- Nickel deposited at the cathode
- Silver deposited at the cathode
Key terms
- Electroplating
Coating an article with a thin layer of a metal by electrolysis, the article being the cathode.
- Anode mud
The insoluble, less reactive impurities (such as silver and gold) that settle below the anode during electrorefining.
Exam tips
Common mistakes
- Writing anode equations with "− e⁻" on the left; write the electrons as a product: 2Br⁻ → Br₂ + 2e⁻.
- Saying the cathode is positive.
- Forgetting that a copper anode dissolves (no oxygen) while a platinum anode gives oxygen.
- Making the article the anode in electroplating.
- Saying silver nitrate is used for silver plating.
How marks are usually awarded
- Electrode equations (1 mark each): correct ions, electrons and charges.
- Observations (1 mark): colour and state, e.g. "reddish-brown fumes at the anode".
- Reasons (1 mark): name the property, e.g. "silica is not attacked by the melt".