Chemistry - 6 Electrolysis

Electrolysis

the decomposition of a substance by an electric current

Cathode

negative electrode

Anode

positive electrode

Electrolyte

an ionic compound whose aqueous solution/molten form conducts an electric current

Anion

negative ion

Cation

positive ion

Why can't solid ionic compounds conduct electricity?

ions are held in place by strong electrostatic forces of attraction

Which electrode has electrons?

cathode

Substances are ... at the cathode

reduced

Substances are ... at the anode

oxidised

... become ... at each electrode

ions, atoms

Water molecule ionising

H₂O ⇌ H⁺ + OH⁻

Which product of ionised water goes to the cathode?

H⁺

Which product of ionised water goes to the anode?

OH⁻

Hydrogen is produced at the cathode if...

the metal is more reactive than hydrogen

Electrodes should be made from ... materials

inert (unreactive)

In the electrolysis of aqueous solutions, ... is usually produced at the anode (positive electrode)

oxygen, O₂

Half equation: anode: aqueous solutions

4OH⁻ → O₂ + 2H₂O + 4e⁻

Half equation: cathode: (some) aqueous solutions

2H⁺ + 2e⁻ → H₂

Order of discharge at anode

halide ion > hydroxide > all other negatively charged ions

Oxygen is released at the anode unless...

the solution contains a halide ion

Uses of aluminium (and its alloys) [7]:

- pans
- overhead power cables
- aeroplanes
- cooking foil
- drink cans
- window and patio door frames
- bicycle frames and car bodies

What compound is electrolysed to procure aluminium?

aluminium oxide, Al₂O₃

Where is aluminium oxide found?

bauxite ore

Aluminium oxide melting point

2050°C

How do we reduce the melting point of aluminium oxide?

mix it with cryolite

Cryolite-aluminium oxide mixture melting point

850°C

Aluminium oxide electrolysis reaction

2Al₂O₃ → 4Al + 3O₂

Aluminium forms at the ... electrode

negative

Oxygen is produced at the ... electrode

positive

Aluminium electrolysis cell [4]

- lined with carbon negative electrode
- molten aluminium is tapped or siphoned off
- carbon dioxide and oxygen gas emitted from anodes
- steel case

At the cathode (aluminium extraction):

Al³⁺ + 3e⁻ → Al

At the anode (aluminium extraction):

2O²⁻ → O₂ + 4e⁻

Reaction of oxygen with hot carbon anodes:

C + O₂ → CO₂

... have to be replaced regularly

Carbon anodes

Brine

water saturated with salt (sodium chloride)

Products of electrolysis of brine [3]

- chlorine gas
- hydrogen
- sodium hydroxide

At the anode (brine electrolysis):

2Cl⁻ → Cl₂ + 2e⁻

At the cathode (brine electrolysis):

2H⁺ + 2e⁻ → H₂

What is left after brine electrolysis?

sodium hydroxide, NaOH