1. CHLORINE

2.  Definition of chlorine and important facts.
 Assessment of the impact of the chlor-alkali
industry on the environment.
 Description of the chemical process involved
in the electrolysis of brine using the
Diaphragm cell.
 Discussion on the economic advantages of
chlorine production by the diaphragm cell
method.
 Discussion on the industrial importance of
the halogens and their compounds.

3. •CHLORINE, symbol Cl, greenish-
yellow gaseous element. The
element’s name comes from the
Greek word chloros, which means
“pale green.” In group 17 (or VIIa)
of the periodic table, chlorine is
one of the halogens. The atomic
number of chlorine is 17.
•Elementary chlorine was first
isolated in 1774 by the Swedish
chemist Carl Wilhelm Scheele,
who thought that the gas was a
compound; it was not until 1810
that the British chemist Sir
Humphry Davy proved that
chlorine was an element and gave
it its present name.

4.  About 100 to 200 g mercury was lost for each 1000 kg
chlorine produced-apparently a small quantity until one
realizes that 2 500 000 kg chlorine was produced by
mercury cells every day during 1960 in the United States.
Thus every 2 to 4 days 1000 kg mercury entered the
environment, and by 1970 sizable quantities were being
found in fish.

5. Since 1970 adequate controls have been
installed on mercury cells and most new
alkali plants use diaphragm cells, but the
very large quantities of mercury introduced
into rivers and lakes prior to 1970 are
expected to remain for a century or more.

6.  Three important chemicals, NaOH, Cl2 and H2, can be
obtained by electrolyzing an aqueous NaCl
solution (brine). This forms the basis of the chlor-
alkali industry, in which the electrolysis is carried out
(see fig. 1).

7. Figure 1

8.  At the cathode, water is reduced:
2H2O + 2e– → H2 + 2OH– (eqn 1a)

Chlorine is produced at the anode:
2Cl– → Cl2 + 2e– (eqn 1b)

Thus the overall reaction is:
 2H2O(l) + 2Cl–(aq) → H2(g) + Cl2(g) + 2OH–(aq) (eqn 1)

9. Since the H2(g) and Cl2(g) might
recombine explosively should they
come in contact, the cathode must
be entirely surrounded by a porous
diaphragm of asbestos. Hence the
name of this type of cell.

10.  Cl2(g) produced in Eq. (1) are dried, purified, and
compressed into cylinders. Fresh brine is continually
pumped into the cell, and the solution which is
forced out contains about 10% NaOH together with a
good deal of NaCl. [Remember that the spectator
ions, Na+(aq), are not included in a net ionic
equation such as Eq. (1).] H2O is allowed to evaporate
from this solution until the concentration of the
solution reaches 50% NaOH, by which time most of
the NaCl has crystallized out and can be recycled to
the electrolysis.

12.  The NaOH is sold as a 50% solution or further dried to
give crystals whose approximate formula is
NaOH•H2O.
 The considerable effort required to concentrate the
NaOH solution obtained from diaphragm cells can be
avoided by using mercury cells. The cathode in such a
cell is mercury, and the cathode reaction is:
Na+(aq) + e– + xHg(l) → NaHgx(l)

13. Halogens and their compounds are very
important in the chemical industry.
Halogens are used in the chemical, water
and sanitation, plastics, pharmaceutical,
pulp and paper, textile, military and oil
industries.

14. They are used in a wide range of
production including the
manufacturing of
chlorofluorocarbons, disinfectants,
halogen lamps, oxidants, Teflon
and some hydrocarbons.

15. Bromine, chlorine,
fluorine and iodine are chemical
intermediates, bleaching agents and
disinfectants. Both bromine and
chlorine are used in the textile
industry for bleaching and shrink-
proofing wool.

16. Bromine is also used in gold
mining extraction processes and
in oil- and gas-well drilling. It is a
fire retardant in the plastics
industry and an intermediate in
the manufacture of hydraulic
fluids.

19.  Which Greek word does chlorine come from
and what does it mean?
 How much kg of chlorine was produced by
mercury cells every day and in what year was
it recorded in the United States?
 What are the three important chemicals
which can be obtained by electrolyzing an
aqueous NaCl solution (brine)?
 What’s another name for the Diaphragm cell?
 State one industrial importance of halogens
and their compounds.

20. DID YOU SCORE A
100% ON THOSE
FANTASTIC
QUESTION? PLEASE
COME FOR YOUR
REWARD!!!