Environmental Chemistry Links

Environmental chemistry connects combustion, acid-base reactions, atmospheric chemistry, and water treatment. The exam tests knowledge of specific pollutants, their sources, their effects, and the chemistry behind treatment and prevention. This page maps every connection.

Air composition and pollution

Clean dry air contains approximately:

• 78% nitrogen
• 21% oxygen
• 0.04% carbon dioxide
• 0.9% argon
• Variable amounts of water vapour

Human activities add pollutants that change this composition and cause environmental harm.

Pollutant gases

Carbon monoxide (CO)

Source: Incomplete combustion of carbon-containing fuels (insufficient oxygen).

2C + O2 -> 2CO (incomplete) vs C + O2 -> CO2 (complete)

Problem: CO is toxic. It binds to haemoglobin in red blood cells more strongly than oxygen, reducing the blood’s ability to carry oxygen. This can be fatal in enclosed spaces.

Solution: Catalytic converters in cars convert CO to CO2: 2CO + 2NO -> 2CO2 + N2.

Carbon dioxide (CO2)

Source: Complete combustion of carbon-containing fuels. Also from respiration and thermal decomposition of carbonates.

Problem: CO2 is a greenhouse gas. Increased atmospheric CO2 (from burning fossil fuels and deforestation) enhances the greenhouse effect, leading to global warming and climate change.

Effects of climate change: Rising sea levels, extreme weather events, melting ice caps, habitat loss, shifting agricultural zones.

Mitigation: Use renewable energy sources, reduce fossil fuel consumption, plant trees (photosynthesis absorbs CO2), carbon capture.

Sulfur dioxide (SO2)

Source: Burning sulfur-containing fossil fuels (coal, oil).

S + O2 -> SO2

Problem: SO2 causes acid rain.

In the atmosphere: SO2 + H2O -> H2SO3 (sulfurous acid) Further oxidation: 2SO2 + O2 -> 2SO3, then SO3 + H2O -> H2SO4 (sulfuric acid)

Effects of acid rain:

• Damages limestone buildings and metal structures (acid attacks CaCO3 and metals)
• Kills fish and other organisms in lakes (lowered pH)
• Damages trees and vegetation
• Leaches nutrients from soil

Solutions: Desulfurisation of flue gases (react SO2 with CaO or CaCO3), use low-sulfur fuels, use alternative energy sources.

Nitrogen oxides (NOx)

Source: Formed in car engines and power stations where nitrogen and oxygen from the air react at high temperatures.

N2 + O2 -> 2NO (at high temperature)

Then: 2NO + O2 -> 2NO2

Problem: NOx contributes to acid rain (forms nitric acid, HNO3) and photochemical smog.

Solution: Catalytic converters: 2CO + 2NO -> 2CO2 + N2 (removes both CO and NO).

Particulates (soot/carbon)

Source: Incomplete combustion.

Problem: Cause respiratory problems, reduce visibility, coat surfaces.

The greenhouse effect

The greenhouse effect is a natural process that keeps Earth warm enough to support life. The concern is the enhanced greenhouse effect caused by increased concentrations of greenhouse gases from human activities.

Greenhouse gases: CO2, CH4 (methane), H2O vapour.

How it works:

1. Short-wavelength radiation from the Sun passes through the atmosphere
2. Earth’s surface absorbs it and warms up
3. Earth re-emits longer-wavelength infrared radiation
4. Greenhouse gases absorb this infrared radiation and re-emit it in all directions
5. Some of this re-emitted radiation returns to Earth, warming it further

Methane sources: Rice paddies, cattle (digestive systems), landfill sites, natural gas leaks. Methane is a more potent greenhouse gas per molecule than CO2, though present in smaller quantities.

Water treatment

Water for drinking must be safe. The treatment process:

1. Screening: Large objects removed by grilles
2. Sedimentation: Suspended particles settle in tanks
3. Filtration: Water passes through sand/gravel beds to remove fine particles
4. Chlorination: Chlorine added to kill bacteria and pathogens

Chlorine reacts with water to form a mixture that acts as a disinfectant. This is why chlorine is important beyond just being a halogen.

Distilled vs treated water

Distilled water is pure (only H2O molecules), but distillation is too expensive for treating the large volumes of water needed for public supply. Treated water still contains dissolved salts but is safe to drink.

Rusting and corrosion

Iron rusts when exposed to both oxygen and water:

4Fe + 3O2 + 2xH2O -> 2Fe2O3.xH2O (hydrated iron(III) oxide)

Both oxygen AND water are needed. Iron does not rust in dry air or in water that has been boiled (to remove dissolved oxygen).

Prevention methods are linked to rusting and prevention:

Method	How it works
Painting/coating	Barrier prevents O2 and H2O reaching iron
Oiling/greasing	Barrier method
Galvanising (zinc coating)	Barrier + sacrificial protection
Sacrificial protection	More reactive metal (Zn, Mg) corrodes instead
Electroplating	Coating with a less reactive metal (e.g. chromium)
Alloying (stainless steel)	Chromium in steel forms protective oxide layer

Fertilisers

Fertilisers provide essential elements for plant growth:

• N (nitrogen): for protein and chlorophyll
• P (phosphorus): for roots and energy transfer
• K (potassium): for overall health and disease resistance

NPK fertilisers are manufactured using ammonia (from the Haber process), nitric acid, sulfuric acid, and phosphate rock. See fertilisers.

Environmental problem: Excess fertiliser washes into waterways (runoff), causing eutrophication — excessive algae growth that blocks light, kills aquatic plants, and depletes oxygen as dead matter decomposes.

Common exam mistakes

1. Confusing CO and CO2: CO is toxic to humans (binds to haemoglobin). CO2 is a greenhouse gas. Different problems, different chemistry.
2. Saying “acid rain is caused by CO2”: While CO2 does dissolve to make weak carbonic acid, acid rain in the exam context refers to strong acids from SO2 and NOx.
3. Forgetting that both water and oxygen are needed for rusting: Students often mention only one.
4. Confusing the greenhouse effect with ozone depletion: These are different problems. The greenhouse effect is about heat trapping; ozone depletion (not in 0620 syllabus) is about UV radiation.

Worked exam questions

Explain how burning coal can lead to acid rain. Include equations. [4 marks]

• Coal contains sulfur impurities [1]
• When burned, sulfur reacts with oxygen: S + O2 -> SO2 [1]
• SO2 dissolves in rainwater: SO2 + H2O -> H2SO3 (sulfurous acid) [1]
• This falls as acid rain, which damages buildings, kills fish, and harms vegetation [1]

State two differences between incomplete and complete combustion of methane. [2 marks]

• Complete combustion produces CO2; incomplete combustion produces CO and/or C (soot) [1]
• Complete combustion requires sufficient oxygen; incomplete combustion occurs with limited oxygen [1]