JEE Chemistry Environmental Chemistry MCQs Set 09

Answer: (a) These are chlorofluorocarbon compounds

Explanation:
1. Freons are chlorofluorocarbons, such as CCl2F2 (Freon-12), used as refrigerants and aerosol propellants.
2. They are not radicals themselves, but they release Cl radicals in the stratosphere, which destroy ozone.

Teacher's Note:
1. Freons are CFCs.
2. Chlorine radicals from CFCs deplete the ozone layer.

Answer: (b) An average human being consumes nearly 15 times more air than food

Explanation:
1. A human being takes in about 12 to 15 times more air by mass than food each day.
2. So the statement that air consumption is nearly 15 times that of food is the best choice.

Teacher's Note:
1. Air pollution therefore affects health strongly.
2. Know the approximate ratio from NCERT.

Answer: (a) X = [O], Y = NO, A = O2, B = O3

Explanation:
1. NO2 splits in sunlight: NO2 gives NO + [O].
2. The oxygen atom reacts with O2 to give ozone: [O] + O2 gives O3.
3. Ozone reacts with NO to regenerate NO2: O3 + NO gives NO2 + O2.
4. So X = [O], Y = NO, A = O2 and B = O3.

Teacher's Note:
1. This cycle produces ozone in photochemical smog.
2. Ozone, PAN and aldehydes are the harmful products.

Answer: (d) A only

Explanation:
1. A is correct because ammonium salts form fine aerosol particles that cause haze.
2. B is wrong because chlorine radicals destroy ozone, they do not make it.
3. C is wrong because polychlorinated biphenyls are persistent pollutants, though used as industrial fluids, and are not cleansing solvents.
4. D is wrong because blue baby syndrome is caused by excess nitrate, not sulphate.
5. Only A is correct.

Teacher's Note:
1. Blue baby syndrome is methemoglobinemia caused by nitrate.
2. Sulphate excess has a laxative effect.

Answer: (c) Photochemical smog has a high concentration of oxidising agents

Explanation:
1. Classical smog is reducing smog, formed from smoke, fog and SO2 in cool humid weather, and it has no NO2 as a feature.
2. Photochemical smog forms in warm sunny weather from NOx and hydrocarbons and contains oxidising agents such as O3 and PAN.
3. So the correct statement is (c).

Teacher's Note:
1. Classical smog is reducing and photochemical smog is oxidising.
2. Classical smog forms in winter and photochemical smog in summer.

Answer: (a) X = 6, Y = 5

Explanation:
1. Fish need dissolved oxygen above 6 ppm to grow well.
2. Clean water has BOD below 5 ppm, while highly polluted water has BOD of 17 ppm or more.
3. So X = 6 and Y = 5.

Teacher's Note:
1. Higher BOD means more organic pollution.
2. DO below 6 ppm stresses aquatic life.

Answer: (d) NO, O and O3

Explanation:
1. NO2 splits in light into NO and an oxygen atom, so A and B are NO and O in some order.
2. The oxygen atom reacts with O2 to give ozone, so B = O and C = O3.
3. Then A = NO, and NO + O3 gives NO2 + O2, which fits (iii).
4. A, B, C = NO, O, O3.

Teacher's Note:
1. Reaction (iii) is the reverse of ozone formation.
2. Atomic oxygen is very reactive.

Answer: (c) very clean water

Explanation:
1. Clean water has BOD below 5 ppm, and highly polluted water has BOD of 17 ppm or more.
2. A BOD of 4 ppm is below 5 ppm, so the pond water is very clean.

Teacher's Note:
1. BOD is the oxygen required by bacteria to break down organic matter.
2. Lower BOD means cleaner water.

Answer: (c) X = ClONO2, Y = HOCl, Z = HNO3

Explanation:
1. ClO + NO2 gives chlorine nitrate, ClONO2, which acts as a reservoir species in the stratosphere.
2. Hydrolysis: ClONO2 + H2O gives HOCl + HNO3.
3. So X = ClONO2, Y = HOCl and Z = HNO3.

Teacher's Note:
1. Chlorine nitrate temporarily holds chlorine and so slows ozone destruction.
2. Its hydrolysis on polar stratospheric clouds releases active chlorine.

Answer: (a) CO2 + H2O gives H2CO3

Explanation:
1. Atmospheric CO2 dissolves in rain to give carbonic acid, which ionises weakly and lowers the pH to about 5.6.
2. The oxides of nitrogen and sulphur in (b), (c) and (d) cause acid rain with a pH below 5.6.

Teacher's Note:
1. Acid rain has pH below 5.6.
2. Normal rain is acidic only due to dissolved CO2.

Answer: (c) By using catalytic converters in automobiles and industry

Explanation:
1. Photochemical smog forms from unburnt hydrocarbons and nitrogen oxides in vehicle exhaust.
2. Catalytic converters oxidise hydrocarbons and CO and reduce NOx, which removes the smog precursors.
3. Tall chimneys only move pollutants and "a catalyst" is not specific.

Teacher's Note:
1. Control the primary pollutants to prevent secondary pollutants like ozone.
2. Planting trees such as Pinus also helps to remove NOx.

Answer: (d) industrial production of urea

Explanation:
1. Cement, steel and coal-fired power plants release large amounts of CO2.
2. Urea manufacture also uses CO2 as a raw material (NH3 + CO2 gives urea), so it consumes CO2, making it the least responsible.

Teacher's Note:
1. Urea synthesis uses the CO2 produced in making ammonia.
2. CO2 is the main greenhouse gas from industry.