Atmospheric Pollution — ICSE Class 9 Chemistry Important Questions

Answer: (b) $SO_2$SO_2 and oxides of nitrogen.

Sulphur dioxide and oxides of nitrogen from burning fossil fuels dissolve in rainwater to form sulphuric and nitric acids, causing acid rain.

Answer: (b) Chlorofluorocarbons (CFCs).

UV light frees chlorine atoms from CFCs, and each chlorine atom destroys many ozone molecules: $Cl + O_3 \rightarrow ClO + O_2$Cl + O_3 → ClO + O_2.

Answer: (b) Infra-red (heat) radiation.

Greenhouse gases such as $CO_2$CO_2 and $CH_4$CH_4 allow sunlight in but trap the infra-red radiation re-emitted by the Earth, warming the atmosphere.

Answer: (b) Absorbs harmful ultraviolet radiation from the Sun.

Stratospheric ozone shields living things from damaging UV rays that cause skin cancer and cataracts; ground-level ozone, by contrast, is a pollutant.

Answer: (a) Marble is calcium carbonate; acid rain reacts with it, e.g. $CaCO_3 + H_2SO_4 \rightarrow CaSO_4 + H_2O + CO_2$CaCO_3 + H_2SO_4 → CaSO_4 + H_2O + CO_2, corroding the stone. R correctly explains A.

Atmospheric pollution is the contamination of air by harmful gases, particles or other substances in amounts that adversely affect living things and the environment.

Two pollutants: sulphur dioxide $(SO_2)$(SO_2) and oxides of nitrogen $(NO_x)$(NO_x) (also carbon monoxide, CFCs).

$SO_2 + H_2O \rightarrow H_2SO_3$SO_2 + H_2O → H_2SO_3 (sulphurous acid)

$2SO_2 + O_2 \rightarrow 2SO_3$2SO_2 + O_2 → 2SO_3, then $SO_3 + H_2O \rightarrow H_2SO_4$SO_3 + H_2O → H_2SO_4 (sulphuric acid)

These acids come down dissolved in rain as acid rain.

1. Damage to buildings and monuments: acids corrode marble and limestone (e.g. the Taj Mahal), $CaCO_3 + H_2SO_4 \rightarrow CaSO_4 + H_2O + CO_2$CaCO_3 + H_2SO_4 → CaSO_4 + H_2O + CO_2.

2. Harm to aquatic life: it lowers the pH of lakes and rivers, killing fish and other organisms.

3. Damage to soil and plants: it makes soil acidic, leaches away nutrients and damages the leaves and roots of trees and crops. (It also corrodes metal structures.)

CFCs are very stable and drift up into the stratosphere. There, ultraviolet light breaks them to release chlorine free radicals:

$CFCl_3 \xrightarrow{UV} \overset{\bullet}{C}l + \overset{\bullet}{C}FCl_2$CFCl_3 → (UV) Cl + CFCl_2

Each chlorine atom then destroys ozone in a chain:

$Cl + O_3 \rightarrow ClO + O_2$Cl + O_3 → ClO + O_2

$ClO + O \rightarrow Cl + O_2$ClO + O → Cl + O_2

The chlorine atom is regenerated, so one atom can destroy thousands of ozone molecules, thinning the layer and creating the 'ozone hole'.

Greenhouse effect: the natural trapping of outgoing infra-red (heat) radiation by atmospheric gases such as $CO_2$CO_2, $CH_4$CH_4 and water vapour, which keeps the Earth warm enough to support life.

Global warming: the gradual rise in the Earth's average temperature caused by an increase in these greenhouse gases from human activity.

Relation: Global warming is the harmful consequence of an enhanced greenhouse effect - burning fossil fuels raises $CO_2$CO_2 levels, more heat is trapped than escapes, and the planet warms.

(a) Acid rain is rain with a pH below about $5.6$5.6, made acidic by dissolved oxides of sulphur and nitrogen. Normal rain is already slightly acidic (pH $\approx5.6$5.6) because atmospheric $CO_2$CO_2 dissolves in it to form weak carbonic acid: $CO_2 + H_2O \rightarrow H_2CO_3$CO_2 + H_2O → H_2CO_3.

(b) Oxides of nitrogen form nitric acid:

$N_2 + O_2 \rightarrow 2NO$N_2 + O_2 → 2NO; $2NO + O_2 \rightarrow 2NO_2$2NO + O_2 → 2NO_2

$3NO_2 + H_2O \rightarrow 2HNO_3 + NO$3NO_2 + H_2O → 2HNO_3 + NO

(c) Control measures:

1. Reduce burning of fossil fuels; use cleaner fuels low in sulphur.
2. Remove $SO_2$SO_2 from factory flue gases by scrubbing (e.g. with lime).
3. Fit catalytic converters in vehicles to cut oxides of nitrogen; use renewable energy.

(a) Greenhouse gases and sources:

1. Carbon dioxide $(CO_2)$(CO_2) - burning of fossil fuels (coal, petrol) and deforestation.
2. Methane $(CH_4)$(CH_4) - paddy fields, cattle and decay of organic waste in landfills.
3. CFCs / water vapour - CFCs from refrigerants and aerosols. (Nitrous oxide from fertilisers is also acceptable.)

(b) Mechanism: Sunlight (short-wave radiation) reaches and warms the Earth, which re-emits energy as long-wave infra-red radiation. Greenhouse gases absorb and re-radiate this heat back to the surface instead of letting it escape into space. As their concentration rises, more heat is trapped, and the Earth's average temperature increases - this is global warming, causing melting of glaciers, rising sea levels and climate change.

(c) Measures:

1. Reduce use of fossil fuels; switch to solar, wind and other renewable energy.
2. Plant more trees (afforestation) to absorb $CO_2$CO_2.
3. Improve energy efficiency and phase out CFCs and other emissions.

(i) The gas is sulphur dioxide $(SO_2)$(SO_2); a major source is the burning of sulphur-containing fossil fuels such as coal in power stations (also smelting of sulphide ores).

(ii) $2SO_2 + O_2 \rightarrow 2SO_3$2SO_2 + O_2 → 2SO_3; $SO_3 + H_2O \rightarrow H_2SO_4$SO_3 + H_2O → H_2SO_4

(iii) Acid rain lowers the pH of lakes and rivers, which kills fish and other aquatic organisms and disrupts the food chain.

(iv) Remove sulphur from fuels or scrub $SO_2$SO_2 from flue gases (e.g. using lime/limestone), or switch to low-sulphur and renewable energy sources.