Pass a current through a molten ionic compound and it breaks down into its elements: the metal at one electrode, the non-metal at the other. The ions do the carrying inside the liquid, the electrons do it in the wires, and the electrodes are where the two meet.
Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current. The electrolyte is the liquid that conducts; the anode is the positive electrode and the cathode the negative electrode. In a molten binary compound, positive metal ions are attracted to the cathode and form the metal; negative non-metal ions are attracted to the anode and form the non-metal. Molten lead(II) bromide gives lead at the cathode and bromine at the anode.
Select molten lead(II) bromide in the simulator and switch on the current. Watch which way each kind of ion moves and what appears at each electrode.
A binary compound contains only two elements. When molten it contains only two kinds of ion, so the products are easy to predict: the metal at the cathode, the non-metal at the anode.
| Molten electrolyte | Cathode (−) | Anode (+) | Observations |
|---|---|---|---|
| lead(II) bromide, PbBr2 | lead | bromine | red-brown vapour at the anode; silvery molten lead collects under the cathode |
| sodium chloride, NaCl | sodium | chlorine | pale green gas at the anode |
| zinc iodide, ZnI2 | zinc | iodine | purple-brown iodine at the anode |
Metals always form at the cathode and non-metals at the anode in a molten compound. Remember that opposite charges attract: positive metal ions go to the negative electrode. Also, a solid electrolyte does not conduct; if the question says the compound is solid, nothing happens until it melts.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In electrolysis, the anode is:
Why must an ionic compound be molten or dissolved before it can be electrolysed?
Molten lead(II) bromide is electrolysed. What forms at the cathode?
Which material is commonly used for inert electrodes?
Extended. Charge is carried by electrons in the external circuit and by ions in the electrolyte. At the electrodes, ions gain or lose electrons and become atoms or molecules.
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Molten zinc iodide is electrolysed with inert electrodes. Which row is correct?
(Extended) Which half-equation shows the reaction at the cathode during the electrolysis of molten lead(II) bromide?
(Extended) During electrolysis, how is charge carried through the wires and through the electrolyte?
Solid lead(II) bromide is placed in the cell and the power is switched on. Nothing happens until it is heated. Why?
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