Atmospheric Gas Pollutants
This topic covers the main atmospheric pollutants resulting from human activity, focusing on their chemical origins and their detrimental effects on health and the environment.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Carbon Monoxide (CO) is a toxic gas produced by the incomplete combustion of carbon-based fuels, such as in poorly ventilated heaters or car engines with insufficient oxygen.
- Carbon Dioxide (CO₂) is a product of complete combustion of fossil fuels and biomass, as well as respiration. Its primary environmental effect is as a greenhouse gas, contributing to global warming.
- Sulfur Dioxide (SO₂) originates from the combustion of fossil fuels, particularly coal, that contain sulfur impurities. It dissolves in atmospheric water to form acid rain, which damages buildings, forests, and aquatic ecosystems.
- Nitrogen Oxides (NOx) are formed when the high temperatures in vehicle engines or power plants cause atmospheric nitrogen and oxygen to react. They contribute to both acid rain and the formation of photochemical smog.
- Carbon monoxide is dangerous because it binds irreversibly to haemoglobin in red blood cells, preventing them from transporting oxygen around the body.
- Acid rain is primarily caused by sulfuric acid (from SO₂) and nitric acid (from NOx), which lower the pH of rainwater.
Formulae
2C + O2 → 2CO To represent the formation of carbon monoxide from incomplete combustion of a carbon fuel.
S + O2 → SO2 To show the formation of sulfur dioxide from the combustion of sulfur impurities found in fossil fuels.
N2 + O2 → 2NO To show the initial formation of a nitrogen oxide from atmospheric gases under high temperature and pressure, for example inside an internal combustion engine.
Definitions
- Incomplete Combustion
- A combustion reaction that occurs when the supply of oxygen is limited. It produces carbon monoxide and/or soot (carbon) instead of or in addition to carbon dioxide.
- Acid Rain
- Rain or any other form of precipitation that is unusually acidic (low pH). It is caused by atmospheric pollution, mainly from sulfur dioxide and nitrogen oxides.
- NOx
- A generic term for a mixture of nitrogen oxides, primarily nitric oxide (NO) and nitrogen dioxide (NO₂), which are significant air pollutants.
Worked example
A car with a poorly maintained engine is left running in a sealed garage. Which combination of pollutant and effect is the most immediate and significant threat to a person inside the garage? A) CO₂ causing global warming B) SO₂ causing acid rain C) NOx causing photochemical smog D) CO causing oxygen deprivation
- 1
Identify the scenario:
incomplete combustion in a confined space.
A poorly maintained car engine is a classic source of incomplete combustion.
- 2
Recall the products of incomplete combustion.
The primary toxic product is carbon monoxide (CO).
- 3
Evaluate the effects of the pollutants listed.
CO is a toxic gas that impairs oxygen transport in the blood.
- 4
Consider the context:
an immediate threat in a sealed room.
Global warming, acid rain, and smog are large-scale, long-term environmental effects, not immediate personal threats in this scenario.
- 5
Conclude that the direct toxic effect of CO, leading to oxygen deprivation, is the most immediate and significant danger.
- 6
Match this conclusion to the options.
Option D correctly links the pollutant (CO) with its direct physiological effect (oxygen deprivation).
Answer: D
Common mistakes
- ×Confusing the effects of carbon monoxide (CO) and carbon dioxide (CO₂). CO is directly toxic to humans, while CO₂ is a greenhouse gas.
- ×Incorrectly identifying CO₂ as a cause of acid rain. While it forms weak carbonic acid, the term 'acid rain' in an environmental context refers to the much stronger acids formed from SO₂ and NOx.
- ×Forgetting that the formation of NOx requires the extreme conditions (high temperature and pressure) found in engines or lightning, as atmospheric nitrogen (N₂) is very unreactive.
No-calculator tips
- ✓Use associations to remember sources: 'S' in SO₂ comes from 'Sulfur' in fuel. 'N' in NOx comes from 'Nitrogen' in the air reacting in hot eNgines.
- ✓Link the number of oxygens to the type of combustion for carbon fuels: CO (one Oxygen) is from 'inCOmplete' combustion, while CO₂ (two Oxygens) is from 'COmplete' combustion.