Electrolysis of Molten Ionic Compounds

Section 1 of 7Rules for molten binary compounds

Only two kinds of ion

• Only two kinds of ion A binary compound contains just two elements, such as lead(II) bromide, PbBr₂, so only two ions are present.

Cathode product

• Cathode product The positive metal ions move to the negative electrode and form the metal.

Anode product

• Anode product The negative non-metal ions move to the positive electrode and form the non-metal element.

Why molten

• Why molten Melting frees the ions to move, so the compound can conduct and be decomposed.

Section 2 of 7Electrolysis of molten lead(II) bromide

1

Heat lead(II) bromide in a crucible until it melts completely.

2

Place two graphite rods in the melt as electrodes and connect them to a power supply.

3

Switch on the current in a fume cupboard, because bromine vapour is toxic.

4

Pb²⁺ ions move to the cathode, gain two electrons and form a bead of grey lead metal.

5

Br⁻ ions move to the anode, lose electrons and form brown bromine gas, seen as bubbles and brown fumes.

Section 3 of 7Products at each electrode

Electrode	Ion attracted	Product	Observation
Cathode (−)	Pb2+	Lead	Silvery-grey metal deposit
Anode (+)	Br−	Bromine	Brown gas

Section 4 of 7Predicting other products

Molten binary ionic compound

Molten NaCl

• Molten NaCl Sodium forms at the cathode and chlorine gas at the anode.

Molten ZnCl₂

• Molten ZnCl₂ Zinc forms at the cathode and chlorine at the anode.

Molten KI

• Molten KI Potassium forms at the cathode and iodine at the anode.

General rule

• General rule Metal at the cathode, non-metal at the anode, whatever the compound.

Section 5 of 7Choice of electrodes

1Electrodes must be inert

• Electrodes must be inert Graphite or platinum is used because they conduct but do not react.

2Reactive electrodes

• Reactive electrodes A metal electrode could react with the products or the melt.

3Side reactions

• Side reactions Extra products would form and the results would not show the true products.

4Clean outcome

• Clean outcome Inert electrodes make sure only the electrolyte's ions are discharged.

Section 6 of 7Key terms

Term	Meaning
Molten	In the liquid state after being heated above the melting point.
Binary compound	A compound made of just two elements.
Inert electrode	An electrode that does not take part in the reaction.
Discharge	The process by which an ion gains or loses electrons at an electrode and becomes neutral.
Cation and anion	A positive ion and a negative ion respectively.

Section 7 of 7Exam technique

1

Writing that hydrogen is produced; there is no water in a molten electrolyte, so the cathode product is the metal.

2

Forgetting to mention the fume cupboard when bromine or chlorine is produced.

3

Using a reactive metal for the electrodes.

4

Saying the lead appears at the anode; metals always form at the cathode.