Electrolysis

Electrolysis is the decomposition of an ionic compound by passing an electric current through it. The compound must be molten or dissolved in water, so that its ions can move. The molten substance or solution undergoing electrolysis is called the electrolyte.

In a simple electrolytic cell, two electrodes are placed in the electrolyte and connected to a power supply. The supply makes the cathode negative and the anode positive. The charges on the electrodes attract oppositely charged ions:

• At the cathode, positive ions gain electrons. A metal or hydrogen forms.
• At the anode, negative ions lose electrons. A non-metal other than hydrogen forms.

The ions carry charge through the electrolyte as they move towards the electrodes. At the electrodes, the transfer of electrons changes the ions into substances that can form the products of electrolysis.

Electrolysis of molten binary compounds

A binary ionic compound contains two elements: a metal and a non-metal. When the compound is molten, its ions can move. Passing an electric current through it breaks the compound down into its elements.

The positive metal ions move to the negative electrode, called the cathode. There they gain electrons and form the metal. The negative non-metal ions move to the positive electrode, called the anode. There they lose electrons and form the non-metal.

The two elements in the compound therefore identify the products: the metal forms at the cathode and the non-metal forms at the anode.

Electrolysis of molten lead(II) bromide

Molten lead(II) bromide contains lead ions, $$\ce{Pb^{2+}}$$, and bromide ions, $$\ce{Br-}$$.

At the cathode, each lead ion gains two electrons to form lead. The product is a grey, molten metal:

At the anode, bromide ions lose electrons and form bromine. The product appears as a reddish-brown vapour:

Inert electrodes made of platinum or carbon, such as graphite, conduct the current without supplying the products. The lead and bromine come from the ions in the molten compound, not from the electrodes.

Electrolysis of concentrated aqueous sodium chloride

When an electric current passes through concentrated aqueous sodium chloride using inert platinum or carbon (graphite) electrodes, gas bubbles form at both electrodes. The electrodes provide surfaces for the reactions without being used up.

At the cathode, the negative electrode, water gains electrons and produces hydrogen gas. The hydrogen is colourless and comes from the water in the solution, not from sodium ions.

At the anode, the positive electrode, chloride ions lose electrons and form chlorine gas, which is greenish-yellow. The high concentration of chloride ions favours the formation of chlorine at the anode.

Electrolysis of dilute sulfuric acid

When an electric current passes through dilute sulfuric acid using inert platinum or carbon electrodes, bubbles of gas appear at both electrodes. The electrodes are inert: they do not take part in the reactions.

Colourless hydrogen gas forms at the negative electrode, the cathode. Hydrogen ions, $$\ce{H+}$$, in the acidic solution gain electrons there:

Colourless oxygen gas forms at the positive electrode, the anode. Water provides the oxygen: water molecules lose electrons at the anode to form oxygen and hydrogen ions.

Electroplating

Electroplating uses electrolysis to coat a metal object with a thin layer of another metal. The coating can improve the object’s appearance and make it more resistant to corrosion.

Electroplating: Forming the coating

The object to be coated is connected as the cathode, the negative electrode. It is placed in an electrolyte containing positive ions of the coating metal.

When current passes through the electrolyte, metal ions gain electrons at the cathode. They become metal atoms, which deposit on the object and build up a thin coating.

Electroplating: Replacing metal ions

The coating metal can be used as the anode, the positive electrode. Metal atoms at the anode lose electrons and enter the electrolyte as positive ions. This replaces metal ions removed from the electrolyte as the coating forms.