Electrolysis of molten salts

Electrolytes

Electrolytes are ionic compounds that are either:

Under these conditions, the ions in electrolytes are free to move within the liquid.

Electrolysis is a process in which electrical energy, from a direct current (dc) supply, decomposes (breaks down) electrolytes. The free ions in electrolytes are attracted to the oppositely-charged electrodes connected to the dc supply.

Cations

The negatively charged electrode in electrolysis is called the cathode. Positively charged ions are called cations. They move towards the cathode.

Anions

The positively charged electrode in electrolysis is called the anode. Negatively charged ions are called anions. They move towards the anode.

Two electrodes connected to a cell. The negative electrode attracts positive ions and the positive electrode attracts negative ions.Ions migrate (move) towards their oppositely charged electrode

Products of electrolysis

When ions reach an electrode, they gain or lose electrons. As a result, they form atoms or molecules of elements:

  • cations gain electrons from the negatively charged cathode
  • anions lose electrons at the positively charged anode

Molten lead bromide, PbBr2(l), is an electrolyte. During electrolysis:

  • Pb2+ ions gain electrons at the cathode and become Pb atoms
  • Br- ions lose electrons at the anode and become Br atoms, which pair up to form Br2 molecules

So lead forms at the negative electrode and bromine forms at the positive electrode.

Molten lead(II) bromide heated in beaker while electrodes inserted. Positive lead ions are attracted to the negative electrode, while negative bromide ions are attracted to the positive electrode.Lead and bromine form during the electrolysis of molten lead bromide

Example

Predict the products of electrolysis of molten calcium chloride.

Calcium will form at the negative electrode because metals form there from positively charged metal ions. Chlorine will form at the positive electrode because non-metals form there from negatively charged non-metal ions.

curriculum-key-fact
During the electrolysis of molten salts, a metal forms at the cathode and a non-metal forms at the anode.
Question

Predict the products of electrolysis of molten aluminium oxide.

Aluminium will form at the negative electrode and oxygen will form at the positive electrode.