Electrolysis Experiments (Edexcel IGCSE Chemistry)

Electrolysis of molten compounds

• Binary ionic compound are compounds consisting of just two elements joined together by ionic bonding

  • E.g. lead(II) bromide

• When these compounds are heated beyond their melting point, they become molten and can conduct electricity as their ions can move freely and carry the charge

• These compounds undergo electrolysis and always produce their corresponding element

• To predict the products of any binary molten compound first identify the ions present

• The positive ion will migrate towards the cathode and the negative ion will migrate towards the anode

• Therefore the cathode product will always be the metal and the product formed at the anode will always be the non-metal

The electrolysis of molten lead(II) bromide 

Method

1. Add lead(II) bromide into a crucible and heat so it will turn molten, allowing ions to be free to move and conduct an electric charge

2. Add two graphite rods as the electrodes and connect this to a power pack or battery

3. Turn on the power pack or battery and allow electrolysis to take place

Diagram showing the electrolysis of lead(II) bromide

Lead ions are attracted to the cathode, and bromide ions to the anode

What happens at the anode?

• Negative bromide ions move to the positive electrode (anode)

• At the anode, they lose two electrons to form bromine molecules

• There is bubbling at the anode as brown bromine gas is given off

What happens at the cathode?

• Positive lead ions move to the negative electrode (cathode)

• At the cathode they gain electrons to form grey lead metal 

• The lead deposits on the bottom of the electrode

Electrolysis of aqueous solutions

• Aqueous solutions will always contain water molecules (H₂O)

• In the electrolysis of aqueous solutions, the water molecules dissociate producing H⁺ and OH^– ions:

H₂O ⇌ H⁺ + OH^–

• These ions are also involved in the electrolysis process and their chemistry must be considered

• We now have an electrolyte that contains ions from the compound plus ions from the water

• Which ions get discharged and at which electrode depends on the relative reactivity of the elements involved

What is produced at the anode?

• Negatively charged OH^– ions and non-metal ions are attracted to the positive electrode

• If halide ions (Cl^-, Br^-, I^-) and OH^- are present then the halide ion is discharged at the anode, loses electrons and forms a halogen (chlorine, bromine or iodine)

• If no halide ions are present, then OH^- is discharged at the anode, loses electrons and forms oxygen

• In both cases the other negative ion remains in solution

What is produced at the cathode?

• Positively charged H⁺ and metal ions are attracted to the negative electrode but only one will gain electrons

• Either hydrogen gas or the metal will be produced

• If the metal is above hydrogen in the reactivity series, then hydrogen will be produced and bubbling will be seen at the cathode

• This is because the more reactive ions will remain in solution, causing the least reactive ion to be discharged

• Therefore at the cathode, hydrogen gas will be produced unless the positive ions from the ionic compound are less reactive than hydrogen, in which case the metal is produced

The electrolysis of aqueous solutions

• The apparatus can be modified for the collection of gases by using inverted test tubes over the electrodes

• The electrodes are made from graphite which is inert and does not interfere with the electrolysis reactions