Chlorine, Cl, is a poisonous greenish-yellow gaseous non-metallic
element, found in
Group VIIb (i.e. the
Halogen Group of elements) of the periodic table.
It has two Isotopes
Chlorine is easily liquefied under pressure.
- Atomic Number : 17
- Atomic Mass : 35.453
- Melting Point : -101 degC
- Boilingg Point : -35 degC
- Density : 0.003
Chlorine was discovered by Scheele in 1774AD, by the action of
Manganese Dioxide on Hydrochloric Acid.
MnO2 + 4 HCl ==> Cl2 + MnCl2 + 2 H2O
In 1774AD, Sir Humphery Davy demonstrated that
the gas was an element and
he suggested the name Chlorine (Greek, greenish yellow) for the gas.
Because of its reactivity, Chlorine does not exist in the free
elemental state in nature, although it is widely distributed in
combination with other elements. The most common chlorine compound
is Common Salt (i.e. Sodium Chloride, NaCl) which occurs in
seawater and in rock salt.
The most common laboratory method for preparation of Chlorine is to
heat 100 gm. of Manganese Dioxide with 300 ml. of concentrated Hydrochloric
MnO2 + 4 HCl ==> MnCl2 + 2 H2O + Cl2
The gas is bubbled through water to remove any traces of hydrochloric
gas that may be present and then it is dried by bubbling it through
concentrated sulphuric acid.
Chlorine may also be prepared by dropping cold concentrated Hydrochloric
Acid on crystals of Potassium Permanganate.
2 KMnO4 + 16 HCl
==> 2 MnCl2 + 2 KCl + 8 H2O + 5 Cl2
The gas is bubbled through water to remove any traces of
Hydrochloric Acid gas that may be present and then it is dried
by bubbling it through concentrated Sulphuric Acid.
Chlorine is manufactured industrially as a by-product in the manufacture
of Caustic Soda by the electrolysis of brine.
2 NaCl + 2 H2O ==> Cl2 + H2 + 2 NaOH
This process was carried out in Kellner-Solvay Cells, using Mercury and
Carbon as the electrodes. However, due to the toxicity of mercury,
the modern version of the process uses metal electrodes with
special membranes in the electrolytic cells.
- a highly toxic greenish yellow gas,
- has a pungent odour, and
- fumes in moist air.
Chlorine is a highly reactive element, and undergoes reaction with a
wide variety of other elements and compounds.
Chlorine is a good bleaching agent, due to its oxidising properties.
Chlorine is soluble in water (which solution is called Chlorine Water)
and this loses its yellow colour on standing in sunlight, due to the
formation of a mixture of Hypochlorous Acid and Hydrochloric
Cl2 + H2O ==> HOCl + Hcl
Chlorine gas supports the vigorous combustion of many elements to form
their chlorides. For example, Sulphur and Phosphorus burn in the gas.
Cl2 + S ==> Scl2
Cl2 + P ==> PCl3 + PCl5
Reaction of Chlorine with Hydrogen
A mixture of Chlorine and Hydrogen explodes when exposed to sunlight
to give Hydrogen Chloride. In the dark, no reaction occurs, so
activation of the reaction by light energy is required.
Cl2 + H2 ==> 2 HCl
Reaction of Chlorine with Non-Metals
Chlorine combines directly with most non-metals (except with
Nitrogen, Oxygen and Carbon, C).
Reaction of Chlorine with Metals
Chlorine combines directly with all metals forming metal chloride salts.
Oxidising Reaction of Chlorine
Chlorine is a strong oxidising agent. Chlorine oxidises Iron (II)
Chloride, FeCl2, to the salt containing Iron in the higher oxidation
state Iron (III) Chloride, FeCl3. This is possible because Iron has
a variable valency.
2 FeCl2 + Cl2 ==> 2 FeCl3
Chlorine displaces the less electronegative Bromine and Iodine from
their respective salts.
Cl2 + 2 KBr ==> 2 KCl + Br2
Chlorine removes Hydrogen from the hydrides of non-metals, forming
Hydrogen Chloride, and leaving the non-metal element.
Cl2 + H2S ==> 2 HCl + S
Reaction of Chlorine with Water
When Chlorine Water (i.e. a solution of chlorine gas in water) in a
flask inverted in a basin of the same liquid is exposed to bright
sunlight, the Chlorine is decomposed and a solution of Hydrochloric
H2O + Cl2 ==> HCl + HClO
The Hydrochloric Acid, HClO, is not very stable and the solution
readily decomcomposes, especially when exposed to sunlight, yielding
2 HClO ==> 2 HCl + O2
Reaction of Chlorine with Sodium Hydroxide
Chlorine reacts with a cold solution of Sodium Hydroxide to give a
mixture of Sodium Chloride, NaCl, and Sodium Hypochlorite, NaOCl.
Cl2 + 2 NaOH ==> NaCl + NaOCl + H2O
Chlorine reacts with a hot solution of Sodium Hydroxide to give a
mixture of Sodium Chloride and Sodium Chlorate.
3 Cl2 + 6 NaOH ==> 5 NaCl + NaClO3 + 3 H2O
Chlorine is used
- for the manufacture of bleaching powder and liquid bleaches,
- to bleach fabrics (e.g. linen and cotton), wood pulp and paper,
- in the manufacture of a wide range of chloro-organic solvents,
including Methylene Chloride, CH2Cl2, Chloroform, CHCl3, Carbon
- in the manufacture of a number of important inorganic chemicals,
including Sulphur Chloride, S2Cl2, Thionyl Chloride, SOCl2,
Phosgene (i.e. Carbonyl Chloride), COCl2, and inorganic Chlorates,
(e.g. Sodium Chlorate, NaClO3),
- for the direct manufacture of Hydrochloric Acid by the
direct combination of its elements,
H2 + Cl2 ==> 2 HCl
- for the extraction of Gold from its ores,
- in the manufacture Sodium Hypochlorite (i.e. domestic bleach),
disinfectants, insecticides, plastics and Hydrochloric Acid,
- as a disinfectant used to kill bacteria in the preparation of
- Chlorine is also important in the manufacture of paints,
aerosol propellants and plastics.
Detection and Analysis
Chlorine is detected by its action on Iodide salts (e.g. Sodium Iodide),
which are oxidises to free elemental Iodine, I2.
2 NaI + Cl2 ==> 2 NaCl + I2
The Iodine liberated in this reaction turns starch indicator solution
to a blue colour.
Chlorine bleaches litmus paper.
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