Discuss briefly the electrolysis of molten lead bromide
Answers
Lead bromide is an ionic compound and can conduct electricity when the ions are free to move - either molten or in aqueous solution. Despite lead bromide and the products of this reaction being hazardous, it is often used to show electrolysis of a molten ionic compound. This is because lead bromide has a relatively low melting point.
Positive lead ions (cations) move to the cathode and gain electrons to become lead metal. This is dense and can be collected at the bottom of the vessel as a molten metal. Negative bromide ions (anions) move to the anode and loose an electron to make chlorine atoms. Two bromine atoms then combine and the diatomic bromine gas is released at the anode.
Electricity flow and therefore electrolysis can only occur when the ions are free to move. The flow of electricity is shown by the lamp being on, when the lead bromide is molten and the power pack is on.
Electrolyte ➡
Molten lead bromide ( PbBr₂ )
Temperature ➡
Above 380°C , the melting point of PbBr₂ is maintained by continuous hearing by a burner
Electrolytic cell ➡
Crucible made of silica
Electrodes ➡
The cathode and anode are both made of graphite plates which are inert. They are connected by copper wires to the two ends of a battery
Ions present ➡
Pb²+ , Br⁻
Electrode relations ➡
Anode :
Br⁻ - e⁻ ➡ Br
Br + Br ➡ Br₂
Cathode :
Pb²+ + 2e⁻ ➡ Pb
Overall reaction ➡
PbBr₂ ➡ Pb + Br₂
Observations ➡
- Dark reddish brown fumes of bromine evolves at anode
- Silver grey metal lead is formed on the cathode
★ Solid lead bromide is a non - conductor of electric current since its ions are not free but held together by electrostatic force of attraction
★ The ions becomes free when lead bromide in the fused or molten state . Hence the crucible is heated from outside to keep lead bromide in the molten state
★ Graphite anode is preferred to other inert electrodes such as platinum since graphite is unaffected by the reactive bromine vapours