C5

Definitions
- Electrolysis: breakdown of ionic compound (molten or solution) by passage of electricity through it
  - chemical effect of electricity on ionic compounds, causing them to break apart into simpler substances, usually elements
- Electrolyte: substance which is decomposed
  - Conducts electricity when molten or in solution
  - Electricity carried by ions
  - Ions free to move to electrodes in molten or solution form
    - Weakened attraction between them
- Non-electrolytes: substance that can’t conduct electricity in molten state or solution
- Weak electrolyte: substance that can conduct electricity a little in molten state of solution
- Electrodes: electric current enters = leaves electrolyte through it
  - Unreactive metals (ex. platinum) or non-metal (carbon)
- Inert electrodes: don’t react with product of electrolysis
- Electrodes:
  - Cathode (negative): attracted to cation (positive ions)
  - Anode (positive): attracted to anion (negative ion)
- Transfer of charge during electrolysis by:
  - Movement of electrons in metallic or graphite electrodes
  - Removal or addition of electrons from external circuit at electrodes
  - Movement of ions in electrolyte
- Reason for conduction in electrodes: movement of delocalised electrons
  - In electrolyte: charge carries ions
- Electrolysis: important in industry
Introducing electrolysis
- Solid ionic compound can’t conduct electricity
- Ions = locked in place → can’t move
- Ionic compound = melted or dissolved in water → forces of attraction are broken + ions free to move
- Liquid + solution: conduct electricity
  - Called electrolysis
Electrolysis of lead(II) bromine and aluminium oxide
- Molten lead bromide: PbBr2 (l)
  - Pb2+
  - 2Br-
- Electrolysis of molten lead bromide
  - Electrode: graphite or metal
  - 2Br- - e- → 2Br
    - Oxidation
    - Bromide goes to positive electrode (anode)
      - Positive terminal (lack of electrons)
    - Loses electrons
  - Pb2+ +2e- → Pb
    - Reduction
    - Goes to cathode (negative terminal)
      - Opposites attract
      - Covered with electrons from powerbank
- In reactivity series:
  - elements over carbon = extracted using electrolyses
  - Under carbon = reduction with carbon
- Aluminium
  - Extracted from compound (aluminium oxide) by electrolysis
  - Aluminium oxide
    - Contains: aluminium ion (Al3+) + oxide ion (O2-)
    - High melting point (2000°C)
  - Electrolysis of aluminium oxide
    - Steps
      - Aluminium oxide = mixed with cryolite → lowers melting point

Reduces energy needed + saves money

- - - - Apply electric current to molten aluminium oxide
      - Anode + cathode = graphite

High melting point

Good conductor

- - - - Al3+ attracted to cathode

Al3+ + 3e- → Al

Reduction

Each ion gains 3 electrons → forms aluminium atom

- - - - O2- attracted to anode

2O2- → O2 + 4e-

2O2- - 4e- → O2

Oxidation

Loses 4 electrons → forms oxygen atom

- - - Anode must be replaced regularly
      - Oxygen produced at anode reacts with graphite → CO2
      - C + O2 → CO2
    - Producing metals by electrolysis: expensive
      - Melting compound requires a lot of energy
      - A lot of energy needed for producing current
Extraction of aluminium oxide
- Aluminium
  - Most abundant metal in earth’s crust (8%)
  - Found as bauxite (mainly aluminium oxide), minerals (cryolite + mica), clay
  - Reactive metal
  - Difficult to extract from ore
    - Hold combined element tightly
  - Extracted by electrolysis
- Use of aluminium
  - Alloy: aircraft construction, bicycle parts
  - Element: metal cans, cooking foils, car parts, saucepan, cables
- Process of extraction
  - Hall - Heroult process
  - Electrolysis of aluminium oxide (alumina)
  - Steps:
    - Bauxite (impure aluminium oxide form) = treated with sodium hydroxide → obtain pure aluminium oxide by removing impurities (ex. iron (II) oxide + sand)
      - Improves conductivity of molten aluminium oxide
    - Purified aluminium oxide = dissolved in molten cryolite (Na3AIF6) in electrolysis cell
      - Cryolite used to reduce temperature of Hall - Hérouit cell (2017°C → 800 - 1000°C)
      - Provides energy saving
      - Need to manufacture it =in short supply
    - Molten mixture = electrolysed in cell
      - Anodes = blocks of graphite → lowered into molten mixture
      - Cathode = graphite lining of steel vessel containing cell
    - Aluminium oxide = ionic compound
    - Once melted → ions = mobile (electrostatic forces of attraction = broken because of heat)
    - Negatively charged oxide ions = attracted to anode → lose electrons (oxidation)
      - oxide ions → oxygen molecules + electrons
      - 2O2- (l) →> O2 (g) + 4e-
    - Positive aluminium ions = attracted to cathode → gain electrons → form molten aluminium metal (reduction)
      - aluminium ions + electrons → aluminium metal
      - 2Al203 (l) → 4Al (l) + 302 (g)
      - Al3+ (l) + 3e- → Al (l)
    - Molten aluminium goes to bottom of cell → siphoned out at regular intervals
    - Problem: oxygen reacts with graphite anodes → CO2
      - C(s) + O2(g) → CO2 (g)
      - carbon + oxygen → carbon dioxide
    - Anodes burn away → replaced
    - Cheap form of electricity is required: Hydroelectric power
  - Metals & hydrogen: formed at cathode
  - Non-metals (usually gas): formed at anode
  - Metal ions + hydrogen ions = positively charged
  - Whether you get metal or hydrogen depends on position of metal in reactivity series
    - Metal produced if metal < reactive in water than hydrogen
    - Hydrogen produced if metal > reactive in water than hydrogen
Electrolysis of aqueous solution
- For industrial processes
- Electrolysis in water
  - Pure water: poor conductor
    - Few ions
  - Can be made to decompose if electric current passed through it in Hofmann voltameter
    - Add dilute sulfuric acid
    - Power turned on → electric current flows through solution → gases produced at two electrodes
      - Collected in side arms of apparatus
      - After 20 minutes: 2x as much gas produced at cathode than anode
      - Gas collected at cathode: contains hydrogen gas
      - Positive charged hydrogen ions go to cathode

hydrogen ions + electrons → hydrogen molecules

4H+(aq) + 4e- → 2H2(g)

- - - - OH (aq) ions: attracted to anode + loses electrons
      - Water + oxygen formed
      - Oxygen collects at anode

hydroxide → water + oxygen + electrons

4OH-(aq) → 2H20(l) + O2(g) + 4e-

- - Electrolysis of dilute sulfuric acid, platinum may be replaced by carbon
    - Carbon dioxide will also form at anode
  - Salt: one of most important raw materials
    - Found in sea
  - Sodium chloride
    - Ionic compound of sodium + chloride
  - Chlor-alkali industry
    - Electrolysis of saturated sodium chloride solution (brine): basis of major industry
    - When rock salt (sodium chloride) is underground → mined
      - Or evaporation of sea water in large shallow lakes
    - 3 important substances produced in electrolysis: chlorine, sodium hydroxide & hydrogen
    - Electrolysis: expensive
      - A lot of electricity
      - Economical: products → many uses
    - Electrolysing brine:
      - Diaphragm cell
      - Mercury cell
      - Membrane cell

Preferred: produces pure product

Less pollution

Cheaper

Brine purified with ion exchange: removes calcium, strontium, magnesium compounds

Membrane cell used continuously: fresh brine flows into cell while broken down

Products don’t mix

Ions in concentrated solution:

From water: H+(aq) & ОН- (aq)

From sodium chloride: Na+ (aq) Сl- (aq)

When current flows: chloride ions (Cl- (aq)) = attracted to anode

Electrode process: produces chlorine gas

Chloride ions → chlorine molecules + electrons

2CL- (aq) → Cl2 (g) + 2e-

Leaves high sodium ions (Na+ (aq)) concentration around anode

Hydrogen ions (H+(aq)): attracted to cathode → hydrogen gas produced

hydrogen ions + electrons → hydrogen molecules

2H+ (aq) + 2e- → H2 (g)

Leaves high hydroxide ions (OH- (aq)) concentration around cathode

Sodium ions (Na+ (aq)) → drawn through membrane + OH-(aq) → sodium hydroxide (NaOH) solution

60 million tones produced a year