Electrolysis Notes

Cambridge IGCSE Chemistry: Revision notes

Key facts

  • Electrolysis decomposes a molten or dissolved ionic compound using an electric current.
  • The cathode (negative) is where reduction happens; the anode (positive) is where oxidation happens.
  • Molten binary compounds: metal at the cathode, non-metal at the anode.
  • Aqueous: hydrogen forms at the cathode if the metal is more reactive; oxygen at the anode unless a concentrated halide is present.
  • Copper electrodes dissolve at the anode; copper deposits at the cathode.
  • Electroplating: object as cathode, plating metal as anode.

How electrolysis works

An electric current splits a molten or dissolved ionic compound into its elements.

The electrolyte must be molten or dissolved so its ions can move freely; in a solid, ions are fixed in the lattice. The anode is positive and oxidation occurs there; the cathode is negative and reduction occurs there.

  1. 1

    Electrons leave

    the negative terminal of the power supply and travel to the cathode

  2. 2

    Cations move

    towards the cathode and gain electrons

  3. 3

    Anions move

    towards the anode and lose electrons

  4. 4

    Electrons return

    from the anode to the positive terminal

What happens during electrolysis

Which electrode do cations move to?

Molten compounds

Metals form at the cathode and non-metals at the anode.

In molten binary compounds, cations are reduced to metal at the cathode and anions are oxidised to a non-metal at the anode. One product forms at each electrode. For molten lead(II) bromide, lead forms at the cathode and bromine at the anode.

−Br35p 45n

Br⁻

2,8,18,8

Br35p 45n

Br atom

2,8,18,7

At the anode, 2Br⁻ → Br₂ + 2e⁻: each bromide ion loses its extra electron and becomes a bromine atom (oxidation).

Half-equation

Cathode:
PbX2++2 eX−→Pb\ce{Pb^2+ + 2e- -> Pb}
Anode:
2 BrX−→BrX2+2 eX−\ce{2Br- -> Br2 + 2e-}

Product

Cathode:
Lead
Anode:
Bromine

Observation

Cathode:
Shiny grey molten metal
Anode:
Red-brown gas with a pungent smell

What forms at the anode when molten PbBrX2\ce{PbBr2} is electrolysed?

Aqueous solutions

Water adds H⁺ and OH⁻ ions, so the products depend on the ions present and the concentration.

At the cathode, the metal forms if it is less reactive than hydrogen; otherwise hydrogen forms. At the anode, a concentrated halide gives the halogen; a dilute solution or no halide gives oxygen.

  1. 1

    Cathode

    metal if it is less reactive than hydrogen; otherwise hydrogen

  2. 2

    Anode

    halogen if a concentrated halide is present; otherwise oxygen

Predicting the products when an aqueous solution is electrolysed.

Dilute sulfuric acid

Cathode:
Cathode: 2 HX++2 eX−→HX2\ce{2H+ + 2e- -> H2}, hydrogen (squeaky pop)
Anode:
Anode: 4 OHX−→OX2+2 HX2O+4 eX−\ce{4OH- -> O2 + 2H2O + 4e-}, oxygen (relights a glowing splint)

Concentrated brine

Cathode:
Cathode: 2 HX++2 eX−→HX2\ce{2H+ + 2e- -> H2}, hydrogen
Anode:
Anode: 2 ClX−→ClX2+2 eX−\ce{2Cl- -> Cl2 + 2e-}, chlorine (yellow-green, toxic)

Concentration

ClX−\ce{Cl-}:
Concentrated
ClX−\ce{Cl-}:
Dilute
BrX−\ce{Br-}:
Concentrated
BrX−\ce{Br-}:
Dilute
IX−\ce{I-}:
Any

Anode product

ClX−\ce{Cl-}:
ClX2\ce{Cl2}
ClX−\ce{Cl-}:
OX2\ce{O2}
BrX−\ce{Br-}:
BrX2\ce{Br2}
BrX−\ce{Br-}:
OX2\ce{O2}
IX−\ce{I-}:
IX2\ce{I2}

Half-equation

ClX−\ce{Cl-}:
2 ClX−→ClX2+2 eX−\ce{2Cl- -> Cl2 + 2e-}
ClX−\ce{Cl-}:
4 OHX−→OX2+2 HX2O+4 eX−\ce{4OH- -> O2 + 2H2O + 4e-}
BrX−\ce{Br-}:
2 BrX−→BrX2+2 eX−\ce{2Br- -> Br2 + 2e-}
BrX−\ce{Br-}:
4 OHX−→OX2+2 HX2O+4 eX−\ce{4OH- -> O2 + 2H2O + 4e-}
IX−\ce{I-}:
2 IX−→IX2+2 eX−\ce{2I- -> I2 + 2e-}

Which gas forms at the anode in concentrated aqueous sodium chloride?

Copper electrodes

With copper electrodes the anode dissolves and copper deposits on the cathode, so the solution stays blue.

With inert electrodes (carbon or platinum), copper forms at the cathode and oxygen at the anode. With copper electrodes, the anode itself reacts, so the anode loses mass and the cathode gains mass. The CuX2+\ce{Cu^2+} concentration stays constant, so the solution stays blue. This is the basis of electrorefining copper.

Cu29p 35n

Cu atom

2,8,18,1

2+Cu29p 35n

Cu²⁺

2,8,17

Copper anode: Cu → Cu²⁺ + 2e⁻, so the anode dissolves. The Cu²⁺ ions then plate out on the cathode.

Inert electrodes

Cathode:
Cathode: CuX2++2 eX−→Cu\ce{Cu^2+ + 2e- -> Cu}
Anode:
Anode: 4 OHX−→OX2+2 HX2O+4 eX−\ce{4OH- -> O2 + 2H2O + 4e-}, oxygen

Copper electrodes

Cathode:
Cathode: CuX2++2 eX−→Cu\ce{Cu^2+ + 2e- -> Cu}, gains mass
Anode:
Anode: Cu→CuX2++2 eX−\ce{Cu -> Cu^2+ + 2e-}, loses mass

During electrolysis of copper(II) sulfate with copper electrodes, what happens to the anode?

Electroplating

Electroplating coats an object in a thin layer of metal using electrolysis.

The object is the cathode and a piece of the plating metal is the anode. The electrolyte contains ions of the plating metal. These are reduced at the cathode, and the anode dissolves to replace them. The coating improves appearance and stops oxygen and water reaching the metal below.

  1. 1

    Cathode

    the steel fork, connected to the negative terminal

  2. 2

    Anode

    a copper block, connected to the positive terminal

  3. 3

    Electrolyte

    copper sulfate solution

  4. 4

    Cathode reaction

    CuX2++2 eX−→Cu\ce{Cu^2+ + 2e- -> Cu} coats the fork

  5. 5

    Anode reaction

    Cu→CuX2++2 eX−\ce{Cu -> Cu^2+ + 2e-} replaces the ions

Copper-plating a steel fork

In electroplating, the object to be plated is the:

Ions and half-equations

Electrons carry current in the wires; ions carry it through the electrolyte.

Cations move to the cathode and are reduced; anions move to the anode and are oxidised. Half-equations must balance for atoms and charge. Reduction has the electrons on the left (CuX2++2 eX−→Cu\ce{Cu^2+ + 2e- -> Cu}); oxidation has them on the right (2 ClX−→ClX2+2 eX−\ce{2Cl- -> Cl2 + 2e-}).

  1. 1

    Anode

    anions lose electrons (oxidation)

  2. 2

    Wires and power supply

    electrons move to the cathode

  3. 3

    Cathode

    cations gain electrons (reduction)

  4. 4

    Electrolyte

    ions move to complete the circuit

Then back to step 1

Electrons carry the current in the wires; ions carry it through the electrolyte.
  • Reduction (cathode)electrons on the left
  • Oxidation (anode)electrons on the right

In a half-equation for oxidation, where do the electrons appear?

Try an exam question

Concentrated aqueous sodium chloride is electrolysed using inert electrodes. State the product at each electrode and write the half-equation for the anode.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.