Carbon monoxide forms when fuels burn in little oxygen
Pollutant
A substance released into the environment that harms living things or damages materials.
Incomplete combustion
Burning of a fuel in a limited supply of oxygen, producing carbon monoxide or carbon particles as well as water.
- With plenty of oxygen, a hydrocarbon fuel undergoes complete combustion to form carbon dioxide and water.
- With a limited supply of oxygen, some of the carbon forms carbon monoxide instead: 2CH4+3O2→2CO+4H2O2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O}2CH4+3O2→2CO+4H2O
- The main sources are vehicle engines and faulty gas boilers, fires and heaters with blocked air vents or flues.
- Carbon monoxide is toxic, because it binds to haemoglobin in red blood cells more strongly than oxygen does.
- This reduces the amount of oxygen the blood can carry, causing headaches, drowsiness and, at high levels, death.
- It is colourless and odourless, so homes with gas appliances need carbon monoxide alarms to detect it.
Do not mix up carbon monoxide with carbon dioxide, because carbon monoxide is toxic in small amounts while carbon dioxide is the greenhouse gas linked to climate change.
Particulates come from incomplete combustion and damage the lungs
Particulate
A tiny solid particle or liquid droplet, such as soot, suspended in the air.
- When oxygen is very limited, a fuel burns to form tiny carbon particles, or soot: CH4+O2→C+2H2O\text{CH}_4 + \text{O}_2 \rightarrow \text{C} + 2\text{H}_2\text{O}CH4+O2→C+2H2O
- Diesel engines, coal fires and wood burners are major sources, releasing soot and droplets of unburnt fuel.
- Fine particulates are breathed deep into the lungs, where they cause and worsen breathing problems such as asthma.
- They also increase the risk of heart and lung disease.
- Soot blackens buildings and makes them costly to clean.
- Particulates in the air reflect sunlight back into space, so less sunlight reaches the ground, an effect called global dimming.
A Bunsen burner with its air hole closed burns with a yellow, sooty flame, because incomplete combustion produces carbon particles that glow and then blacken the gauze.
Sulfur dioxide forms when fuels containing sulfur impurities burn
Acid rain
Rain with a lower pH than normal because sulfur dioxide or oxides of nitrogen have dissolved in it to form acids.
- Coal and fuels made from crude oil, such as diesel and heavy fuel oil, contain small amounts of sulfur impurities.
- When the fuel burns, the sulfur reacts with oxygen to form sulfur dioxide: S(s)+O2(g)→SO2(g)\text{S(s)} + \text{O}_2\text{(g)} \rightarrow \text{SO}_2\text{(g)}S(s)+O2(g)→SO2(g)
- The major sources are coal-fired power stations, industrial furnaces and ships, while volcanoes are a natural source.
- Sulfur dioxide dissolves in water droplets in clouds and reacts with oxygen to form sulfuric acid:
2SO2(g)+O2(g)+2H2O(l)→2H2SO4(aq)2\text{SO}_2\text{(g)} + \text{O}_2\text{(g)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{H}_2\text{SO}_4\text{(aq)}2SO2(g)+O2(g)+2H2O(l)→2H2SO4(aq)
- Rain is naturally slightly acidic, at about pH 5.6, so this extra acid gives rain with a lower pH.
- Breathing in sulfur dioxide irritates the airways and makes asthma worse.
The more sulfur dioxide there is in the air, the lower the pH of the rain, because more acid dissolves in the rainwater.
Oxides of nitrogen form at high engine temperatures
- Nitrogen and oxygen in the air do not react at ordinary temperatures.
- Inside vehicle engines and furnaces, the very high temperature makes them react to form nitrogen monoxide:
N2(g)+O2(g)→2NO(g)\text{N}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{NO(g)}N2(g)+O2(g)→2NO(g)
- Nitrogen monoxide then reacts with more oxygen in the air to form nitrogen dioxide: 2NO(g)+O2(g)→2NO2(g)2\text{NO(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{NO}_2\text{(g)}2NO(g)+O2(g)→2NO2(g)
- Nitrogen dioxide dissolves in cloud water to form nitric acid, which adds to acid rain.
- Oxides of nitrogen irritate the lungs, worsen asthma and help to form smog in busy, sunny cities.
Do not say that oxides of nitrogen come from nitrogen in the fuel, because the nitrogen comes from the air drawn into the engine.
Acid rain damages buildings, forests and lakes
- Acid rain reacts with limestone and marble, wearing away buildings and statues: CaCO3+H2SO4→CaSO4+H2O+CO2\text{CaCO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + \text{H}_2\text{O} + \text{CO}_2CaCO3+H2SO4→CaSO4+H2O+CO2
- It corrodes metals, such as the iron and steel in bridges and railings.
- It damages the leaves of trees and washes nutrients out of the soil, so whole forests can die back.
- Lakes and rivers become more acidic, which can kill fish and other aquatic life.
- Winds carry the pollutants for hundreds of kilometres, so acid rain often falls far from the source.
When a question asks for the problem caused by a pollutant, match it to that pollutant: carbon monoxide is toxic, particulates damage the lungs and cause global dimming, and sulfur dioxide and oxides of nitrogen cause acid rain and breathing problems.
- What condition causes a fuel to produce carbon monoxide when it burns?
- Why does carbon monoxide reduce the amount of oxygen carried in the blood?
- Where does the sulfur in sulfur dioxide come from?
- Why do vehicle engines produce oxides of nitrogen?
- Give two problems caused by acid rain.
