Reference- 100% From NCERT XI & XII
➡️Tetrachloromethane(Carbon tetrachloride)
It is produced in large quantities for use in the manufacture of
refrigerants and propellants for aerosol cans.
It is also used as
feedstock in the synthesis of chlorofluorocarbons and other chemicals,
pharmaceutical manufacturing, and general solvent use. Until the mid
1960s, it was also widely used as a cleaning fluid, both in industry,
as a degreasing agent, and in the home, as a spot remover and as fire
extinguisher.
There is some evidence that exposure to carbon
tetrachloride causes liver cancer in humans. The most common effects
are dizziness, light headedness, nausea and vomiting, which can cause
permanent damage to nerve cells. In severe cases, these effects can lead
rapidly to stupor, coma, unconsciousness or death. Exposure to CCl4
can make the heart beat irregularly or stop. The chemical may irritate
the eyes on contact.
When carbon tetrachloride is released into the air,
it rises to the atmosphere and depletes the ozone layer. Depletion of the
ozone layer is believed to increase human exposure to ultraviolet rays,
leading to increased skin cancer, eye diseases and disorders, and
possible disruption of the immune system.
➡️Freons
The chlorofluorocarbon compounds of methane and ethane are collectively
known as freons.
They are extremely stable, unreactive, non-toxic, noncorrosive
and easily liquefiable gases.
Freon 12 (CCl2F2) is one of the
most common freons in industrial use. It is manufactured from
tetrachloromethane by Swarts reaction.
These are usually produced
for aerosol propellants, refrigeration and air conditioning purposes. By
1974, total freon production in the world was about 2 billion pounds
annually.
Most freon, even that used in refrigeration, eventually makes
its way into the atmosphere where it diffuses unchanged into the
stratosphere. In stratosphere, freon is able to initiate radical chain
reactions that can upset the natural ozone balance
➡️Classification Based on Physical State of Dispersed Phase and Dispersion Medium
➡️Explain the terms with suitable examples:
(i) Alcosol (ii) Aerosol (iii) Hydrosol.
(i) alcosol:
- It is a colloidal solution which has a dispersion medium as alcohol and dispersion phase a solid substance.
- Dispersed phase is the phase that is scattered or present in the form of colloidal particles in the dispersed medium.
- Dispersed medium is the medium in which the colloidal particles are dispersed is called the dispersion medium.
- Example: We can see that in a starch solution, water represents the dispersion medium and starch represents the dispersed phase.
- It is used as a disinfectant.
(ii) aerosol:
- It is a colloidal solution having gas as dispersion medium and solid as a dispersion phase
- Example of aerosol is: Fog
An aerosol is a suspension system of solid or liquid particles in a gas. It is found that an aerosol contains both the particles as well as air.
(iii) Hydrosol:
- Hydrosol is a colloidal solution which has a dispersion medium as water and dispersion phase a solid.
- For example: Starch sol or gold sol is hydrosol.
- It is found that hydrosol has similar healing properties as essential oils, but they are much less concentrated than essential oils and can be used internally and externally.
➡️Ozone
Ozone is an allotropic form of oxygen. It is too reactive to remain for
long in the atmosphere at sea level. At a height of about 20 kilometres,
it is formed from atmospheric oxygen in the presence of sunlight. This
ozone layer protects the earth’s surface from an excessive concentration
of ultraviolet (UV) radiations.
Pure ozone is a pale blue gas, dark blue liquid and violet-black solid.
Ozone has a characteristic smell and in small concentrations it is harmless.
However, if the concentration rises above about 100 parts per million,
breathing becomes uncomfortable resulting in headache and nausea.
Ozone is thermodynamically unstable with respect to oxygen since
its decomposition into oxygen results in the liberation of heat (DH is
negative) and an increase in entropy (DS is positive). These two effects
reinforce each other, resulting in large negative Gibbs energy change
(DG) for its conversion into oxygen. It is not really surprising, therefore,
high concentrations of ozone can be dangerously explosive.
Due to the ease with which it liberates atoms of nascent oxygen
(O3 →O2 + O), it acts as a powerful oxidising agent. For example, it
oxidises lead sulphide to lead sulphate and iodide ions to iodine.
PbS(s) + 4O3(g) → PbSO4(s) + 4O2(g)
2I–(aq) + H2O(l) + O3(g) → 2OH–(aq) + I2(s) + O2(g)
Experiments have shown that nitrogen oxides (particularly nitrogen
monoxide) combine very rapidly with ozone and there is, thus, the
possibility that nitrogen oxides emitted from the exhaust systems of
supersonic jet aeroplanes might be slowly depleting the concentration
of the ozone layer in the upper atmosphere.
NO g + O3 g →NO2 g + O2 g
Another threat to this ozone layer is probably posed by the use of
freons which are used in aerosol sprays and as refrigerants.
The two oxygen-oxygen bond lengths in the ozone
molecule are identical (128 pm) and the molecule is angular
as expected with a bond angle of about 117°. It is a resonance
hybrid of two main forms:
➡️Acid rain
When the pH of the rain water drops below
5.6, it is called acid rain.
Ammonium salts are also formed and can
be seen as an atmospheric haze (aerosol of fine
particles). Aerosol particles of oxides or
ammonium salts in rain drops result in wet deposition.
SO2 is also absorbed directly on
both solid and liquid ground surfaces and is
thus deposited as dry-deposition.
