Functions of Electrolysis | Chemistry | CSEC CXC lessons online – Study Guide

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

Uses

To extract reactive metals from their ores.

To extract active non-metals

In electroplating, corrosion protection, in chrome plating and anodizing.

 

Processes

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

Gasoline additive

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

Cathode: Mercury

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

 

 

Anodizing Aluminium

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

 

Anode: Chromium

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 copper

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.

 

Cathode reaction:

Cu2+ (aq) + 2e- à Cu0 (s)

 

Anode reaction:

Cu0 (s) – 2e- à Cu 2+ (aq)

 

 

Citation:

Lambert and Mohammed (1993), Chemistry for CXC, Halley Court, Jordon Hill, Oxford.