Products of electrolysis

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

  • positive ions gain electrons from the negatively charged cathode
  • negative ions 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

Lead forms at the negative electrode and bromine forms at the positive electrode.

The half equations for the reactions are (higher tier):

At the cathode: Pb2+ + 2e- → Pb

At the anode: 2Br- → Br2 + 2e-

A silvery grey liquid will form at the cathode (molten lead) though this is difficult to see as it will sink to the bottom as it is denser than the electrolyte.

A red-brown gas will be produced at the anode (bromine gas – formed as a gas due to the heat).

Lead bromideLead and bromine form during the electrolysis of molten lead bromide

Example

Predict the products of electrolysis of molten calcium chloride.

Positively charged calcium ions move to the negative electrode.

Here, they gain electrons to form calcium atoms, so calcium is formed at the negative electrode.

Negatively charged chloride ions move to the positive electrode. Here, they lose electrons to form chlorine atoms.

The atoms join up in pairs to form Cl2 molecules, so chlorine gas is formed at the positive electrode.

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.