Functions of Electrolysis
Electrolysis of pure water
Pure water is not a good conductor of electricity as it partially ionizes.
If pure water is used as an electrolyte no current will flow.
Inert Electrodes used in electrolysis are either platinum or graphite
When hydrogen is given off at cathode it ‘gives a pop’ with a lighted splint
When oxygen is given off at anode it relights a glowing splint
When water is removed during electrolysis the electrolyte becomes more concentrated
When electrolysis involves use of an inert electrode the electrode does not contribute ions to the solution
e.g. Electrolysis of aqueous copper (II) sulphate using inert electrodes (platinum or graphite) ions in solution:
H+ (aq) OH - (aq) from water
Cu + (aq) SO42- (aq) from copper sulphate
Copper is discharged at cathode
As the copper ions are being removed from the solution the blue color of the solution fades.
Hydroxide ions are discharged at anode.
Electrolysis with an active electrode results in the electrode contributing ions to the solution
e.g. Electrolysis of aqueous copper (II) sulphate using copper electrodes (active electrodes) ions in solution:
H+ (aq) OH- (aq) from water
Cu 2+ (aq) SO4 2- (aq) from copper(II)sulphate
Copper is discharged at cathode.
As the copper ions are being removed from the solution the blue color of the solution is maintained as copper atoms leave the anode and enter the solution as copper(II)ions.
The concentration of the electrolyte remains the same.
The mass of the anode decreases and the mass of the cathode increases by an equivalent amount
Hydroxide ions are discharged at anode.
N.B. Cations move towards cathode and anions move towards the anode.
Electrolysis is used in Industry
To extract reactive metals from their ores.
To extract active non-metals
In electroplating, corrosion protection, in chrome plating and anodizing.
The Down’s Process
This process involves the electrolysis of molten sodium chloride. The main purpose of this process is to produce Sodium.
Chlorine is also produced.
Conditions and materials used in process:
3:2 mixture of sodium chloride and calcium chloride (he calcium chloride is used to keep the temperature of the NaCl and CaCl mixture under the boiling point)
Temperature – 600o C
Na and Cl are very reactive with each other and the cell is therefore constructed to keep them from reacting.
A high current is used to keep the mixture molten
Uses of Na
Extraction of titanium
As a coolant in nuclear reactors
The Mercury Cathode Cell
This process involves the electrolysis of Brine or concentrated NaCl.
Main purpose: to manufacture Chlorine , NaOH and H2 also produced
Primary Products: Sodium and Chlorine
Anode: Titanium or Graphite
Anode: 2Cl- (aq) – 2e- à Cl2
Cathode: Na (l) + Hg (l) à NaHg (amalgam)
In this process the usual order of discharge has been reversed. On a ‘fresh metal surface’ this occurs. In this process Na has been discharged on the mercury cathode.
In the lower tank the NaHg amalgam reacts with water forming à NaOH (aq) + H2 (g) + Hg(l)
The use of mercury is both expensive and toxic
Source of Al: Al is extracted from its ore bauxite by Electrolysis
Anodizing is done to produce corrosion-resistant articles
Al forms an oxide coating (Al2O3) this coating is thick and tough
In this process Aluminium is made the anode of the cell. In this process the electrolyte used is dil. Sulphuric Acid or dil. Chromic (VI) acid or any electrolyte which releases O2 at the anode.
Reaction at anode: 4OH- – 4e- à 2H2O (l) + O2 (g)à This oxygen results in the formation of the thick Al2O3 coating is made to absorb dyes.
Electroplating or Electrodeposition
This process involves the passage of an electric current which causes the deposit of a metal on the cathode with the right electrode and electrolyte.
Conditions for Electrolysis:
The surface to be plated has to be clean
The surface is given an undercoat of another metal
Small currents are used
pH is controlled
temperature is controlled
small amounts of suitable substances are added to the plating solution to ensure that the plate has desirable properties
Uses of Electroplating:
To enhance the appeal of the plated article
To obtain an article which is less easily corroded
To avoid using less expensive metals for the object
Objects which are electroplated:
Cutlery, tools, taps, parts for cars, cans
Chrome and Nickel Plating
Characteristics of Chrome plates
Have good appearance
Increase the hardness of tools and machinery
Give protection from corrosion
Plating solution: chromic (VI) acid to which chromium (III) sulphate and fluorosilicate
Cathode : Metal to be plated
This process is carried out at high temperatures
In decorative chrome plating the article is given an undercoat of copper and nickel
Electrorefining is an electrolytic method which is used to remove impurities from copper
Anode: Impure copper
Cathode: Pure copper
Electrolyte: Copper(II) sulphate and sulphuric acid
A large current is used for this process.
Cu2+ (aq) + 2e- à Cu0 (s)
Cu0 (s) – 2e- à Cu 2+ (aq)
Lambert and Mohammed (1993), Chemistry for CXC, Halley Court, Jordon Hill, Oxford.