ENVIROMENTAL CHEMISTRYMODULE 3

CHLORINE

OBJECTIVE NEEDED TO BE FOCUSED ON:

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.

DEFINITION AND THE CREATION OF CHLORINE

•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.

BACKGROUNG INFO./ ENVIRONMENTAL IMPACTS OF THE CHLORO-ALKALI INDUSTRY

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.

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.

ELECTROLYSIS OF BRINE 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).

THE DIAPHRAGM CELL OR HOOKER CELL

Figure 1

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)

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.

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.

ECONOMIC

ADVANTAGE OF

CHLORINE

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)

INDUSTRIAL IMPORTANCE OF HALOGENS AND THEIR COMPUNDS

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. 

They are used in a wide range of

production including the

manufacturing of

chlorofluorocarbons, disinfectants,

halogen lamps, oxidants, Teflon

and some hydrocarbons.

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.

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.

A CLIP SHOWING THE REACTION OF CHLORINE

THANK YOU ALL FOR WATCHING

QUESTIONS NEEDED TO BE ANSWERED:

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.

DID YOU SCORE A 100% ON THOSE

FANTASTIC QUESTION? PLEASE

COME FOR YOUR REWARD!!!