Electrolysis of ionic compounds is used to produce alkalis and elements such as aluminium, chlorine and hydrogen.
Making ionic compounds electrically conduct
When an ionic compound is melted or dissolved in water, the ions are mobile within the liquid or solution.
Passing an electric current through ionic substances that are molten, for example lead bromide, or in solution breaks them down into elements. This process is called electrolysis and the substance that is broken down is called the electrolyte.
Movement of charged ions within the electrolyte during electrolysis
Positively-charged ions (cations e.g. Na+ ) move to the negative electrode (cathode)(−)
Negatively-charged ions (anions e.g. OH–) move to the positive electrode (anode)(+)
Reduction at Cathode, Oxidation at Anode
At the negative electrode (cathode), positively charged ions gain electrons (reduction)
At the positive electrode (anode), negatively charged ions lose electrons (oxidation).
A useful way to remember this is RedCat (Reduction at Cathode)
Rules of electrolysis for aqueous solutions using inert electrodes: selective discharge of ions
Aqueous solutions contain H+ and OH– from the hydrolysis of water. The products formed by electrolysis depend on the reactivity of the elements that comprise the electrolyte.
At the (−)Cathode (where reduction of positive ions occurs):
1. In aqueous solution, metal is produced if it is less reactive than hydrogen e.g. electrolysis of CuCl2(aq) yields Cu(s); AgCl(aq) yields Ag(s)
2. In aqueous solution, hydrogen will be produced if the metal is more reactive than hydrogen e.g. electrolysis of NaCl(aq) yields H2(g)
3. If electrolysing a molten metal salt then there is no other cation apart from the metal, so the metal is produced.
For example lead (II) bromide (PbCl2) is a solid ionic compound. When solid, the compound does not conduct electricity as the ions (Pb2+ and Cl−) are relatively fixed in position due to being held in giant ionic lattice structure. However, when heated and made molten, the ions become mobile and able to transfer electrical charge; electrolysis of molten PbCl2 will therefore yield Pb at the cathode.
At the (+)Anode (where oxidation of negative ions occurs):
1. All molten or highly concentrated halide ion solutions generate their respective halogen (e.g. electrolysis of molten NaCl will yield Cl2(g))
2. All dilute aqueous solutions, including dilute halide ion solutions, will generate oxygen gas as the OH– is preferentially discharged
3. All aqueous solutions containing SO42- and NO3– ions will generate oxygen gas from OH−(aq) (SO42- and NO3– are relatively unaffected by electrolysis and OH− is preferentially discharged).
Half-equations: Reactions at each electrode (cathode and anode) can be represented by half-equations, for example:
Electrolysis of copper (II) sulfate solution:
(-) Cathode: Cu2+(aq) + 2e– → Cu(s)
(+)Anode: 4OH–(aq) → 2H2O(l) + O2(g) + 4e–
Electrolysis of dilute sulfuric acid solution:
(-) Cathode: 2H+(aq) + 2e– → H2 (g)
(+) Anode: 4OH–(aq) → 2H2O(l) + O2(g) + 4e–
Uses in electroplating
Electrolysis is used to electroplate objects. This may be for a variety of reasons and includes copper plating and silver plating.
Uses in the extraction of aluminium
Aluminium is manufactured by the electrolysis of a molten mixture of aluminium oxide (made from the ore, bauxite) and cryolite.
Aluminium forms at the cathode and oxygen gas at the anode.
The anode is made of carbon, which reacts with the oxygen to produce carbon dioxide.
Aluminium oxide has a very high melting point (over 2000ºC)
Molten cryolite (Na3AlF6, sodium hexafluoroaluminate), which is an aluminium containing compound, is used as a solvent for molten aluminium oxide (Al2O3) as it lowers the melting point of the aluminium oxide electrolyte, thereby reducing the energy needs and costs to melt aluminium oxide.
Useful products formed from the electrolysis of brine (concentrated sodium chloride solution)
Electrolysis of concentrated sodium chloride solution produces:
hydrogen gas (at cathode)
chlorine gas (at anode)
sodium hydroxide is formed in the electrolyte solution that remains after electrolysis.
Cathode: 2H+(aq) +2e− → H2 (g)
*Anode: 2Cl– (aq) → Cl2 (g) + 2e−
Residual electrolyte: Na+(aq) OH−(aq) → NaOH(aq)
*The chloride ions are selectively discharged over the hydroxide ions because the chloride ions are at a much higher concentration in brine.
For every unit of electricity (say 2 moles of electrons) used, equal amounts of hydrogen and chlorine gas are produced.
As electrolysis proceeds, the electrolyte solution becomes more alkaline due to the removal of H+(aq) and Cl–(aq) to leave behind more and more Na+(aq) and OH−(aq).
These are important reagents for the chemical industry:
- hydrogen is used as a fuel and for making ammonia
- sodium hydroxide for the production of soap
- chlorine for the production of bleach and plastics and disinfecting water.
Boardworks simulation (electrolysis of brine)
Do not confuse the electrolysis of dilute NaCl(aq) solution with electrolysis of concentrated NaCl(aq) solution (known as brine)
Electrolysis of dilute NaCl(aq) solution gives:
Cathode: 2H+(aq) +2e− → H2(g) OR 4H+(aq) + 4e− → 2H2(g)
Anode: 4OH−(aq) → 2H2O(l) + O2(g) + 4e−
Residual electrolyte: Na+(aq) + Cl−(aq) → NaCl(aq)
For every unit of electricity (say 4 moles of electrons) used, twice as much hydrogen is produced compared to oxygen.
As electrolysis proceeds, the solution becomes more concentrated due to the removal of H+(aq) and OH–(aq) to leave behind Na+(aq) and Cl−(aq) in less and less water. The overall pH of the electrolyte solution remains unchanged during electrolysis as there is equivalent and simultaneous removal of both H+(aq) and OH−(aq) from solution .
The electrolysis of dilute NaCl(aq) forms the same products as the electrolysis of water and electrolysis of dilute sulfuric acid.
Purification of copper: electrolysis of impure and pure copper electrodes in copper (II) sulfate solution electrolyte
Pure copper cathode (reduction) : Cu2+(aq) + 2e– → Cu(s)
Impure copper anode (oxidation): Cu(s) → Cu2+(aq) + 2e–
Importantly, copper (II) sulfate solution remains the same intensity of blue throughout electrolysis.
For each unit of electricity applied, a Cu2+(aq) is generated by the anode and simultaneously removed from the electrolyte solution at the cathode: thus, the concentration of Cu2+(aq) ions in the electrolyte solution remains constant.
Hence, the intensity of blue colour remains the same.
Ultimately, the (impure) copper anode mass decreases, while (pure) copper cathode mass increases.
The impurities at anode will fall off to the bottom of the electrolysis container.
The essential features of electrolysis are:
- ions are attracted to the electrodes
- negative non-metal ions are attracted to the positive electrode
- positive metal ions are attracted to the negative electrode
- at the electrodes, electrons are gained or lost by the ions involved
- products are formed at each electrode