10.09.1 Electrolysis of Molten compounds
Syllabus
When an ionic compound is melted or dissolved in water, the ions are free to move about within the liquid or solution. These liquids and solutions are able to conduct electricity and are called electrolytes.
Passing an electric current through electrolytes causes the ions to move to the electrodes. Positively charged ions move to the negative electrode (the cathode), and negatively charged ions move to the positive electrode (the anode). Ions are discharged at the electrodes producing elements. This process is called electrolysis.
(HT only) students should be able to write half equations for the reactions occurring at the electrodes during electrolysis, and may be required to complete and balance supplied half equations.
During electrolysis, at the cathode (negative electrode), positively charged ions gain electrons and so the reactions are reductions.
At the anode (positive electrode), negatively charged ions lose electrons and so the reactions are oxidations.
When a simple ionic compound (eg Lead Bromide or Zinc Chloride) is electrolysed in the molten state using inert electrodes, the metal (Lead/Zinc) is produced at the cathode and the non-metal (Bromine/Chlorine) is produced at the anode.
Students should be able to predict the products of the electrolysis of binary ionic compounds in the molten state.
What does this mean?
What is an Electric Current?
In Physics an electric current is "a flow of charge".
In practice this is almost always a flow of (negative) electrons in a wire.
But ions also have charge.
If they can move positive ions are attracted to the negative terminal & negative ions to the positive, allowing Ionic substances to conduct.
In an Ionic solid the ions are fixed in place by strong ionic bonds.
So, Ionic solids never conduct electricity.
But when melted all Ionic liquids can conduct electricity.
Most Ionic substances dissolve in water, allowing the ions to move.
All Ionic solutions conduct (but not every ionic substance is soluble)
Covalent molecules do not contain charged ions and the electrons are fixed in place (except in Graphite).
So Covalent substance (almost) never conduct electricity because there are no charged particles that can move.
Which substances are produced during electrolysis?
Electrolysis is the break-up of a molten or dissolved Ionic substance using DC electricity.
A different substance is produced at both electrodes and for molten electrolytes it easy to predict what will happen.
Metal ions are positive = Cations. These will be attracted to the negative electrode = Cathode.
Non-metal ions are negative= Anions. These will be attracted to the positive electrode = Anode.
So, when we melt an ionic substance and pass a current through it the anion will be released at the positive electrode, the cation at the negative electrode.
The examples below show how simple it is to predict the products of electrolysis of Molten salts
and we could easily predict that
Aluminium Oxide would make Aluminium and Oxygen,
Potassium Fluoride should make Potassium and Fluorine.
etc
Ionic half-equations (HT only)
At the Cathode the positive ions (Cations) pick up electrons (e-) from the electrode becoming neutral atoms.
This is Reduction.
eg. Pb2+ (l) + 2 e- --> Pb (l)
Cu2+ (l) + 2 e- --> Cu (l)
Na+ (l) + 1 e- --> Na (l)
Notice that 2+ ions need to gain 2 electrons, 1+ ions need to gain 1 electron etc
At the Anode the negative ions (Anions) lose electrons to the electrode to become neutral atoms.
This is Oxidation. eg. 2Br- (l) --> Br2 (g) + 2 e-
2O2- (l) --> O2 (g) + 4 e-
2Cl- (l) --> Cl2 (g) + 2 e-
Notice that simple gases made during electrolysis generally contain 2 atoms.
So, although a Bromide ion (Br-) only needs to lose one electron there are two electrons in the half-equation because two ions are needed to make Bromine (Br2)
And, although a Oxide ion (O2-) only needs to lose two electrons there are four electrons in the half-equation because two ions are needed to make Oxygen (O2)
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