Electrolysis - AQA

Electrolysis involves using electricity to break down electrolytes to form elements. The products of electrolysis can be predicted for a given electrolyte.

Part of

Combined Science

Chemical changes

Electrolysis of molten salts

Electrolytes

Electrolytes are ionic compounds that are:

in the molten state (heated so they become liquids), or
dissolved in water

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

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

Electrodes and ions

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

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

Two electrodes connected to a cell. The negative electrode attracts positive ions and the positive electrode attracts negative ions.

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

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.

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.

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.

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