7.2    Electrolysis

Electrolytic cells

• Electrolysis is the decomposition of a compound into its elements by an electric current.
• Uses of electrolysis:
  • to extract useful metals from their ores
  • to produce useful by-products such as chlorine gas
  • to purify metals
• Electrolysis is carried out in an electrolytic An electrolytic cell consists of:
  1. the electrolyte which is the compound being decomposed. It is either a molten ionic compound or a concentrated aqueous solution of ions. Solid ionic compounds and covalent compounds cannot be used because they do not conduct electricity
  2. the electrodes which are rods made from graphite or a metal which conducts electricity to and from the electrolyte
     • anode is the positive electrode
     • cathode is the negative electrode
  3. external power supply of direct current and a complete d.c. circuit

 

Redox reactions in electrolysis

• During electrolysis, the cations are attracted to the cathode while the anions are attracted to the anode.
  • At the cathode, cations gain electrons and are Reduction occurs at the cathode.
  • At the anode, anions lose electrons and are oxidised. Oxidation occurs at the anode.
• A simple example is the electrolysis of molten sodium chloride:

1. At the cathode: Na⁺ + e⁻ → Na………..gain electrons, reduction.
2. At the anode: 2Cl⁻ → Cl2 + 2e⁻………..lose electrons, oxidation.
3. Sodium metal and chlorine gas are produced

• For electrolysis of aqueous sodium chloride:
  1. In an aqueous solution of sodium chloride, four types of ions are present (Na⁺, Cl⁻, H⁺ and OH⁻).
  2. However, only one type of ion can be discharged at each electrode
• Factors that determine the selective discharge of ions in an electrolytic cell:

• i.  Position of  the ion in the electrochemical series

  • the lower it is in the electrochemical series, the higher the possibility for it to be discharged

  ii. Concentration of the ions

  • the higher the concentration of the ion, the higher the possibility for it to be discharged.

 

• For electrolysis of aqueous sodium chloride:

1. At the cathode: 2H⁺ + 2e⁻ → H2…………because H⁺ is lower in the electrochemical series.
2. At the anode: 4OH⁻ → O2 + 2H2O + 4e⁻………….because OH⁻ is lower in the electrochemical series.

 

Extraction of aluminium from bauxite ore

• Bauxite is impure aluminium oxide, Al2O3. The impurities are iron oxides, silicon dioxide and titanium dioxide. Bauxite is the major aluminium ore
• The first step is to purify the bauxite to get pure aluminium oxide, Al2O3.
• Molten aluminium oxide is electrolysed in a solution of cryolite, Na3AlF6 to give out the required aluminium metal
• The function of dissolving aluminium oxide in large amount of cryolite is to:
  1. lower the melting point of the electrolyte from about 2140 °C to 970  °C.
  2. improve the electrical conductivity of the electrolyte

• Electrolysis of aluminium oxide is carried out in long narrow cells using carbon (graphite) electrodes

 

• At the cathode: Al³⁺ + 3e⁻ → Al………..reduction At the anode: 2O²⁻ → O2 + 4e⁻…………oxidation
• Overall equation: 4Al³⁺ + 6O²⁻ → 4Al + 3O2    or      2Al2O3 → 4Al + 3O2 
• Aluminium is denser than the electrolyte. It drops to the bottom of the electrolytic cell and is removed using a suction tube or through a tap hole
• The oxygen gas released will oxidise the hot carbon electrodes to carbon dioxide and carbon So the electrodes have to be replaced from time to time.
• Some environmental issues related:

1. The carbon monoxide gas produced is poisonous
2. Large amount of energy is needed because the current needed to electrolyse the electrolyte is very high

 

Electrolysis of  brine

• Brine is concentrated aqueous solution of sodium chloride. It is obtained from sea water or by dissolving rock salt in water
• Electrolysis of brine is used to produce chlorine, hydrogen and sodium hydroxide. A diaphragm cell is used to carry out this reaction

 

• The electrolyte level in the anode compartment is kept higher than in the cathode This ensures the flow of the solution is always towards the cathode compartment. This reduces the possibility of sodium hydroxide solution formed moving back as they are to be collected.
• The ions present in the aqueous solution of sodium chloride are Na⁺, Cl⁻, H⁺ and OH⁻. H⁺ and OH⁻ comes from the slight ionisation of  water.

• H2O ⇌ H⁺ + OH⁻

 

• At the cathode: 2H⁺ + 2e⁻ → H2………because H⁺ is lower in the electrochemical series.At the anode: 2Cl⁻ → Cl2 + 2e⁻………..because Cl⁻ is in a far more greater concentration than OH⁻.Overall: 2NaCl + 2H2O → 2NaOH + H2 + Cl2
• As more and more H⁺ are removed, more are formed from the ionisation of water because the position of equilibrium shifts to the The H⁺ formed are being removed again until none is left.
• At the same time, more and more OH⁻ are being produce When all the H⁺ are removed, only OH⁻ and Na⁺ are left in the electrolyte and NaOH is formed.

 

Electrolytic purification of copper

• Copper is purified to be used for electrical wiring because 99.99% of purity is required for efficient transmission of electrical Even small amounts of impurity reduces its conductivity greatly.
• Electrolytic purification of copper is carried out in cells using:
  • an electrolyte of copper(II) sulfate solution acidified with sulfuric acid
  • an anode of impure copper.
  • a cathode of pure copper.
• At the anode: Cu → Cu²⁺ + 2e⁻ At the cathode: Cu²⁺ + 2e⁻ → Cu
• The result is the transfer of copper from the anode to the cathode. As a result, the cathode gets thicker and the anode get thinner. The impurities in the anode settle down as anode ‘sludge’.
• The concentration of copper(II) sulfate solution does not change because the number of moles of Cu²⁺ ions formed at the anode is exactly the same as the number of moles of Cu²⁺ ions discharged at the cathode