Environmental Chemistry Test - 3

The brown haze of photochemical smog is largely attributed to NO₂. It is a reddish brown gas, owing to the presence of an unpaired electron.

The products of photochemical smog are peroxyacetyl nitrate and ozone.

Formation of Photochemical Smog:

Step 1: People begin driving in the morning; nitrogen is burned or oxidised.
N₂ + O₂ → 2NO ... (1)
The oxidation number of N₂ is 0. The nitrogen in NO has acquired an oxidation number of +2.

Step 2: After a few hours, NO combines with O₂, in another oxidation reaction.
2NO + O₂ → 2NO₂ ... (2)
The nitrogen in NO has an oxidation number of +2. The nitrogen in NO₂ has an oxidation number of +4.

Step 3: Nitrogen dioxide absorbs light energy, resulting in a reduction reaction.
NO₂ → NO + O ... (3)
The nitrogen in NO₂ has an oxidation number of +4 and the nitrogen in NO has an oxidation number of +2.

Step 4: In sunlight, atomic oxygen combines with oxygen gas to form ozone.
O + O₂ → O₃ ... (4)

Step 5: Reaction is temperature and sunlight dependent.
O₃ + NO ⇌ NO₂ + O
This self consuming cycle is disrupted when there are unburnt hydrocarbons present in the atmosphere.
NO and NO₂ can also react with the hydrocarbons instead of ozone to form other volatile compounds known as PAN (peroxyacetyl nitrate). The accumulation of ozone and volatile organic compounds along with the energy from the sun forms the brown, photochemical smog seen on hot, sunny days.

A chlorofluorocarbon (CFC) is an organic compound that contains only carbon, chlorine, and fluorine, produced as volatile derivative of methane, ethane, and propane. They are also commonly known by the DuPont brand name Freon. The most common representative is dichlorodifluoromethane (R-12 or Freon-12).

Exposure to nitrogen oxides may result in changes of the pulmonary system including pulmonary edema. Several sensitive plants get adversely affected by NOx even at the concentration of 1 ppm for a day or 0.35 ppm for a few months. The affected plants start shedding their leaves and fruit. Nitrogen oxides cause fading of textile dyes and deterioration of cotton and nylon fabrics. Hence, option (1) is correct.
H₂S gas is extremely toxic to humans. In higher atmospheric concentrations, it can be fatal. The reaction of H₂S with essential proteins is primarily responsible for it's toxic effects.
Exposure to SO₂ causes deterioration of fabrics (cotton, rayon) paper and leather. Being corrosive, it has damaging effects on metals, such as iron, steel, aluminium and building materials, such as lime stone, marble, roof slate, etc. Marble structures, limestone and geological sources of ammonia act as a sink for atmospheric SO₂.
The high concentration of CO₂ in atmosphere has been the main component of the greenhouse effect that has caused global warming, i.e. the rise of atmospheric temperature in recent years.

Chlorine gas was used earlier for bleaching paper. It is nowadays being replaced by hydrogen peroxide with suitable catalyst.

The toxic effects of carbon monoxide on human beings and animals arise from its reversible combination with haemoglobin (Hb) in red blood cells, which cause displacement of oxygen. This results in oxygen starvation or anoxia. The maximum permissible concentration of CO in the atmosphere is 40 ppm for an exposure of 8 hours. A higher concentration of CO or a longer exposure starts causing adverse effects. In small amounts, CO causes headaches and fatigue, and in larger doses, it can be lethal.
Hence, option (4) is the correct answer.

Inhaling of nitrogen oxides by humans results in pulmonary edema and hemorrhage. Several sensitive plants get adversely affected by NOx, even at concentration of 1 ppm for a day or 0.35 ppm for a few months. The affected plants start shedding their leaves and fruit. Nitrogen oxides cause fading of textile dyes and deterioration of cotton and nylon fabrics.
SO₂ is poisonous and irritates the respiratory system of animals and humans. A continuous exposure to concentration of 0.2 ppm for 3-4 days or even 0.5 ppm for full day has been reported to damage lungs and increase the rate of mortality. In plants and grasses, SO₂ also produces leaf injuries (called necrotic blotching). Sulphur dioxide damages barley, alfalfa, cotton and wheat.
The high concentration of CO₂ in atmosphere has been the main component of the greenhouse effect that has caused global warming, i.e. the rise of atmospheric temperature in recent years.

Exposure to SO₂ causes deterioration of fabrics (cotton, rayon), paper and leather. Being corrosive, it has damaging effects on metals, such as iron, steel, aluminium, and on building materials, such as limestone, marble, roof slate, etc. Marble structures, limestone and geological sources of ammonia act as a sink for atmospheric SO₂.

Green chemistry, also called sustainable chemistry, is an area of chemistry and chemical engineering focused on the designing of products and processes that minimise the use and generation of hazardous substances. All of the above are the achievements of green chemistry.

The most common nutrients causing eutrophication are nitrogen and phosphorus. The main source of nitrogen pollutants is run-off from agricultural land, whereas most phosphorus pollution comes from households and industry, including phosphorus-based detergents.

Statement 2 is incorrect because the chlorofluorocarbons are highly stable molecules and reach the stratosphere without breaking down in the troposphere.
In the stratosphere, the halogen free radicals are produced, which destroy the ozone molecules through a variety of catalytic cycles.