GCSE Science | Electrolyis

15 Apr 2025 1 min read

What is the purpose of electrolysis?

An ionic compound is the electrostatic attraction between a metal ion and a non-metal ion
Electrolysis can be used to separate the metal and the non-metal ions.

Separating metals

If the metal is more reactive than carbon, we can react the metal oxide with carbon
e.g. sodium oxide + carbon --> sodium + carbon dioxide
If the metal is less reactive than carbon, we can use electrolysis

Reactivity series

MOST REACTIVE
potassium (go up group 1)
sodium
lithium
calcium (go up group 2)
magnesium
aluminium (go up group 3)
CARBON
zinc (transition metals)
iron
HYDROGEN
copper
silver
gold
MOST UNREACTIVE

Common ions

hydroxide - OH^-
nitrate NO₃^-
carbonate CO₃^2-
sulfate SO₄^2-
Phosphate PO₄^3-

What is electrolysis?

we use electricity to separate the metal and non-metal in an ionic compound
the positive ions get attracted to the negative electrode
the negative ions get attracted to the positive electrode
we can start electrolyis if the ionic compound is
- molten
- or
- aqueous (disssolved in water)

What is cryolite?

cryolite can be added to lower the melting point of the ionic compound
so less energy is needed to melt it

Electrolyis of molten ionic compounds

Molten NaCl contains the ions:
- Na⁺
- Cl^-
Half equations
- Na⁺ + e^- → Na
- 2Cl^- - 2e^- → Cl₂

Electrolysis of aqueous ionic compounds

Aqueous compounds also have the ions of water
- H⁺ and OH^- → H₂O
Electrolyis of aqeuous sodium chloride
- Ions present are:
  - H⁺
  - OH^-
  - Na⁺
  - Cl^-
  - only two of these will react at the electrode
  - follow the rules below
- Half equations
  - 2H⁺ + 2e^- → H₂
  - 2Cl^- - 2e^- → Cl₂

Rules for electrolysis of aqueous ionic compounds

there will be two positive ions - the one that reacts is the one that is
- LEAST REACTIVE
there will be two negative ions - the one that reacts will always be the
- HALOGEN (group 7)
- if there is no halogen, then it will be oxygen