Home CHEMISTRY TOPIC 6: IONIC THEORY AND ELECTROLYSIS | CHEMISTRY FORM 3

TOPIC 6: IONIC THEORY AND ELECTROLYSIS | CHEMISTRY FORM 3

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CHEMISTRY FORM THREE FULL NOTES

Ionic Theory

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
Distinguish electrolytes from 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.
Electrolytes and non-electrolytes
Liquids
such as ethanol, paraffin, petrol and methylbenzene do not conduct
electricity. The bonding in these compounds is covalent. These
substances consist of molecules. There are no free electronsor charged
particles to flow through them. Solutions of covalent compounds, for
example sugar solution, do not conduct electricity.
These compounds are non-electrolytes. Non-electolytes exist only in the form of molecules and are incapable of ionization.
Ionic
compounds contain charged particles (ions), but in solid state, the
ions are firmly held in place and they are not free to move. An ionic
solid does not conduct electricity. However, the ions present can become
free to move if the solid is melted or dissolved in water. Then they
can conduct electricity. For example, solid sodium chloride cannot
conduct electricity but when melted or dissolved in water, the ions, Na+ and Cl are set free. Then these ions are free to move in solution and hence conduct electricity. These compounds are called electrolytes.
Weak and Strong Electrolytes
Categorize 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.
H2O(l)⇔H+(aq) + OH(aq)
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.
Electrolysis
has several uses in industry. Its main application has been in the
fields of manufacture of chemicals and in the purification of metals for
which other purification methods prove either too difficult or highly
expensive to apply. Some applications of electrolysis are as discussed
below:
The Industrial Purification of Copper by Electrolysis
Outline the industrial purification of copper by electrolysis
Some
metals can be purified by means of electrolysis. This process is used
in industry to purify copper, which must be very pure 99.9% for
electrical wiring. Copper made by roasting the sulphide ore is about
99.5% pure (so it has an impurity level of 0.5%). This level of impurity
cuts down electrical conductivity significantly.
This
is how the electrolytic purification (refining) process is carried
out:The anode is made of a large block of impure copper. The cathode is a
thin sheet of pure copper. The electrolyte is copper (II) sulphate
solution.During the refining process, the copper atoms of the impure
block become ions (the anode dissolves).Cu → Cu2+ + 2e
The ions from the solution become atoms.
Cu2+ + 2e → Cu(s)
They
stick onto the cathode. A layer of pure copper builds up on the
cathode. As electrolysis takes place, the cathode gains mass as copper
is deposited on it. As a result, the cathode gets smaller while the
cathode gets bigger as electrolysis proceeds. Eventually the whole
cathode dissolves.

Copper 1455265435901

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.

Silver 1455265751604

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
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