CHEMISTRY FORM THREE TOPIC 6: IONIC THEORY AND ELECTROLYSIS.


 To account for the phenomena of electrolysis the Ionic Theory was put forward by Arrhenius in 1880. The theory states that electrolytes are made up of ions, which are built up in certain patterns called crystal lattice. When these substances dissolve in water, the structure is destroyed and the ions are set free to move.Concentrated mineral acids such as sulphuric acid, hydrochloric acid and nitric acid do not contain ions but they consist of molecules. However, when they are diluted, the molecular structure is destroyed and ions are formed.

Electrolytes and Non-electrolytes

The main purpose of this chapter is to investigate the effects which electricity has on a range of substances, and to develop a thorough explanation of those effects in terms of our present knowledge of atomic structure. Before we begin, it is important that we familiarize ourselves with different terms that we are going to use to explain different phenomena. It is crucial that the definitions and meanings of these terms be understood at the outset in order that concepts defined in this chapter are easily and clearly apprehended. These terms are given hereunder:

  • Electrolysis: decomposition of a compound in solution or molten state by passing electricity through it.
  • Conductor: a solid substance that allows electricity to pass through it. All metals are included in this class.
  • Non-conductor or insulator: a solid substance that does not allow electricity to flow through it. All non-metals fall in this class.
  • Electrolyte: a substance which, when dissolved or molten, conducts electricity and is decomposed by it.
  • Non-electrolyte: a compound which cannot conduct electricity, be it in molten or solution state.
  • Electrode: a graphite or metal pole (rod) or plate through which the electric current enters or leaves the electrolyte.
  • Cathode: a negative electrode which leads electrons into the electrolyte.
  • Anode: a positive electrode which leads electrons out of the electrolyte.
  • Ion: a positively or negatively charged atom or radical (group of atoms).
  • Cation: a positive ion which moves to the cathode during electrolysis.
  • Anion: a negative ion which moves to the anode during electrolysis.

Weak and Strong Electrolytes

Weak electrolytes are compounds that are only partially or slightly ionized in aqueous solutions. Some substances, for example, ethanoic acid solution ionize partially.

CH3COOH(aq) ⇔CH3COO(aq) + H+(aq)

Most of the electrolytes exist in solution in the form of unionized molecules. For example, in ordinary dilute (2M) ethanoic acid, out of every 1000 molecules present, only 4 are ionized and 996 are unionized.

A solution of ammonia water is also a weak electrolyte, containing a relatively small proportion of ammonium and hydroxyl ions.

NH4OH(aq) ⇔NH4+(aq) + OH(aq)

Most of the organic acids are weak electrolytes, e.g. tartaric, citric and carbonic acids.

However, there is no sharp dividing line between weak and strong electrolytes.Water is also a weak electrolyte. It ionizes only slightly.

Study shows that for every molecule of water ionized, there are 6 million molecules of water not ionized. Strong electrolytes are compounds that are completely ionized in aqueous solutions. When sodium chloride is dissolved in adequate water it ionizes completely into Na+ and Cl ions. There are no NaCl solid particles left unionized. All strong electrolytes (salts, the mineral acids and caustic alkalis) ionize completely in solutions. 

The Mechanisms of Electrolysis

Electrolytic Cells of Different Electrolytes in the Molten and Aqueous States

Set up electrolytic cells of different electrolytes in the molten and aqueous states

The conductivity of ionic compounds is explained by the fact that ions move in a particular direction in an electric field. This can be shown in experiments with coloured salts. For example, copper (II) chromate (VI) (CuCrO4) dissolves in water to give a green solution. This solution is placed in a U-tube.

A colourless solution of dilute hydrochloric acid (HCl) is then layered on top of the salt solution in each arm. Graphite rods are fitted as shown in figure 13.3. These rods (electrodes) carry the current into and out of the solution.

After passing the current for a short time, the solution around the cathode becomes blue. Around the anode, the solution becomes yellow. These colours are produced by the movement (migration) of the ions in the salt.

The positive copper ions (Cu2+) are blue in solution. They are attracted to the cathode (negative electrode). The negative chromate ions (CrO42-) are yellow in solution. They are attracted to the anode (the positive electrode). The use of coloured ions in solution has shown the direction that positive and negative ions move in an electric field. Always positive ions (cations) move to the cathode and negative ions (anions) move to the anode.

Ionic Theory And Electrolysis

Purification of copper by electrolysis

Only pure copper sticks to the cathode. Most impurities fall to the bottom of the electrolytic cell. They form a solid material (anode sludge or slime) which contains small quantities of precious metals such as silver, gold and platinum. The precious metals recovered from the slime are purified and sold.

An Experiment on Electroplating of Metallic Materials

Carry out an experiment on electroplating of metallic materials

Electroplating is the coating of a metal with a layer of another metal by means of electrolysis. Electrolysis can be used to coat a thin layer of a less reactive metal onto a more reactive metal. The thin layer of less reactive metal will provide protection from corrosion for the more reactive metal underneath. It may also make the product more attractive.

The object to be coated should be made the cathode and the coating material should be the electrolyte. The most commonly used metals for electroplating are copper, chromium, silver and tin.

Steel can be electroplated with chromium or tin. This prevents the steel from rusting and gives it a shiny, silver finish. This is also the idea behind chromium-plating articles such as car bumpers, kettles, bath taps, etc. Chromium does not corrode, it is a hard metal that resists scratching and wear, and can also be polished to give an attractive finish.

Nickel can be electroplated with silver. This will make nickel more attractive.The diagram below shows how a steel jug is electroplated with silver. The jug becomes the cathode of an electrolytic cell. The anode is made of silver. The electrolyte is a solution of a silver compound, for example silver nitrate.

Ionic Theory And Electrolysis

Silverplating a steel jug

At the anode: The silver dissolves, forming ions in solution:Ag → Ag+ + e

At the cathode: The silver ions receive electrons, forming a coat of silver on the jug:Ag+ + e→Ag (s)

When the layer of silver is thick enough, the jug is removed.In general, to electroplate any object with metal M, the set up is:

  • Cathode – object to be electroplated
  • Anode – metal M
  • Electrolyte – solution of a soluble compound of M