The properties of chlorine
Chlorine (Cl2) is one of the most reactive elements; it easily binds to other elements. In
the periodic chart chlorine can be found among the halogens. Other halogens are
fluorine (F), bromine (Br), iodene (I) and astatine (At). All halogens react with other
elements in the same way and can form a large quantity of substances. Halogens often react with
metals to form soluble salts.
Chlorine atoms contain 17 negative electrons (negatively charged particles). These move around
the heavy core of the atom in three shells. Within the inner shell there are two electrons, within
the middle shell there are eight and within the outer shell there are seven. In the outer shell there
is space left for another electron. This causes free, charged atoms, called ions, to form. It can also
cause an extra eletron to form (a covalent bond; a chlorine bond), causing the outer shell to
complete.
Figure 2: chlorine atoms contain 17 electrons
Chlorine can form very stable substances, such as kitchen salt (NaCl). Chlorine can also form
very reactive products, such as hydrogen chloride (HCl). When hydrogen chloride dissolves in
water it becomes hydrochloric acid. The hydrogen atom gives off one electron to the chlorine
atom, causing hydrogen and chlorine ions to form. These ions react with any kind of substance
they come in contact with, even metals that are corrosion resistant under normal circumstances.
Concentrated hydrochloric acid can even corrode stainless steel. This is why it is stored either in
glass or in plastic.
Chlorine is a very reactive and corrosive gas. When it is transported, stored or used, safety
precautions must be taken. In Holland for example, chlorine is transported in separate chlorine
trains.
Watery chlorine should be protected from sunlight. Chlorine is broken down under the influence
of sunlight. UV radiation in sunlight provides energy which aids the break-down of underchloric
2
acid (HOCl) molecules. First, the water molecule (H O) is broken down, causing electrons to be
released which reduce the chlorine atom of underchloric acid to chloride (Cl-). During this
reaction an oxygen atom is released, which will be converted into an oxygen molecule:
2HOCl -> 2H+ + 2Cl- + O2
Chlorine is produced from chlorine bonds by means of electrolytic or chemical oxidation. This is
often attained by electrolysis of seawater or rock salt. The salts are dissolved in water, forming
brine. Brine can conduct a powerful direct current in an electolytic cell. Because of this current
chlorine ions (which originate from salt dissolving in water) are transformed to chlorine atoms.
Salt and water are divided up in sodium hydroxide (NaOH) and hydrogen gas (H2) on the
cathode and chlorine gas on the anode. These cathode and anode products should be separated,
because hydrogen gas reacts with chlorine gas very agressively.
To produce chlorine, three different electrolysis methods are used.
1. The diaphragm cell-method, which prevents products to mix or react by means of a
diaphragm. The electrolysis barrel contains a positive pole, made of titanium and a negative pole,
made of steel. The electrodes are separated by a so-called diaphragm, which is a wall that only
lets fluids flow through, causing gasses that form during a reaction to be separated. The
application of the countercurrent principle prevents hydroxide ions from reaching the positive
pole. However, chlorine ions can pass through the diaphragm, causing the sodium hydroxide to
become slightly polluted with chlorine. This causes the following reactions to take place:
+ pole : 2Cl- -> Cl2 + 2e- pole : 2 H2O + 2 e- -> 2OH- + H2
2. The mercury cell-methode uses one mercury electrode, causing the reaction products to be
purer than those of the diaphragm cell-methode. With this method an electrolysis barrel is used
which contains a positive titanium pole and a negative flowing mercury pole. On the negative
pole a reaction with sodium (Na+) takes place, causing sodium amalgams to be formed. When
the amalgams flow through a second reaction barrel, sodium reacts with water to sodium
hydroxide and hydrogen. This causes the hydrogen gas to remain separated from the chlorine
gas, which is formed on the positive pole.
Within the electrolysis barrel the following reactions take place:
2
+ pole : 2 Cl- -> Cl + 2e- pole : Na+ + e- -> Na
second reaction barrel: 2Na + 2H2O -> 2 Na+ + 2OH- + H2
3. The membrane-method resembles the diaphragm method. The only difference is that the
membrane only allows positive ions to pass, causing a relatively pure form of sodium hydroxide
to form.
During the mercury electrolysis process a solution containing 50 mass-% of sodium hydroxide is
formed. However, during the membrane and diaphragm processes the solution must be
evaporated using steam.
Sixty percent of the European chlorine production takes place by means of mercury electrolysis,
whereass 20% takes place in the diaphragm process and 20% takes place in the membrane
process.
Chlorine can also be produced by means of hydrogen chloride oxidation with oxygen from
air. Copper(II)chloride (CuCl2) is used as a cathalyser during this so-called ‘Deaconprocess’:
4HCl + O2 -> 2H2O + 2Cl2
Finally, chlorine can be produced by means of molten salts electrolysis and, mainly in
laboratories, by means of hydrochloric acid and manganese dioxide oxidation:
MnO2 + 4HCl -> MnCl2 + 2H2O + Cl2
When gaseous chlorine is added to water the following hydrolysis reaction takes place:
Cl2 + H2O = H+ + Cl- + HOCl