Toxic and Respiratory Pollutants Revision Notes | GCSE AQA Higher Chemistry

1. The Silent Killer - Carbon Monoxide

Imagine a gas so dangerous that it kills without warning - no smell, no colour, no taste. Carbon monoxide is exactly this invisible threat, and understanding why makes the difference between life and death.

Carbon monoxide forms when fuels burn with insufficient oxygen supply. Think of a car engine running in a closed garage - the limited oxygen means incomplete combustion occurs. Instead of producing harmless carbon dioxide ($CO_2$), the reaction produces deadly carbon monoxide ($CO$).

The danger lies in how CO affects your blood. Haemoglobin in red blood cells normally carries oxygen around your body. But carbon monoxide binds to haemoglobin 200 times more strongly than oxygen does. Once CO attaches, that haemoglobin molecule can no longer carry oxygen - leading to carbon monoxide poisoning.

2. Respiratory Attackers - Sulfur Dioxide and Nitrogen Oxides

While carbon monoxide kills silently, sulfur dioxide and nitrogen oxides attack your breathing directly. These gases irritate your respiratory system and create the acid rain that damages our environment.

Sulfur dioxide comes mainly from burning coal and oil that contain sulfur impurities. Power stations are major sources - when they burn coal, any sulfur present reacts with oxygen to form $SO_2$. This gas immediately irritates your throat and lungs if you breathe it in.

Nitrogen oxides form differently. At the high temperatures inside car engines, nitrogen from the air reacts with oxygen to form $NO$. This then reacts further with oxygen to produce $NO_2$, the brown gas you sometimes see in city smog.

3. The Acid Rain Connection

Both sulfur dioxide and nitrogen oxides don't just harm your lungs - they transform in the atmosphere to create acid rain. This process shows how local pollution becomes a regional environmental problem.

When $SO_2$ and nitrogen oxides rise into the atmosphere, they dissolve in water droplets and react with oxygen. Sulfur dioxide forms sulfuric acid ($H_2SO_4$), while nitrogen oxides form nitric acid ($HNO_3$). These acids make rainwater much more acidic than normal.

Worked example: Formation of Acid Rain from Sulfur Dioxide

Show how sulfur dioxide in the atmosphere leads to acid rain formation.

1. $SO_2(g) + H_2O(l) \rightarrow H_2SO_3(aq)$ — Sulfur dioxide dissolves in water droplets to form sulfurous acid
2. $H_2SO_3(aq) + \frac{1}{2}O_2(g) \rightarrow H_2SO_4(aq)$ — Oxygen in the air oxidises sulfurous acid to sulfuric acid
3. Rain pH drops from 5.6 to 4.0 or lower — The sulfuric acid makes rainwater much more acidic than normal

Answer: Sulfur dioxide dissolves and oxidises in atmospheric water to form sulfuric acid, creating acid rain.

Normal rainwater is slightly acidic (pH 5.6) due to dissolved carbon dioxide. But acid rain can have a pH as low as 4.0 - that's ten times more acidic. This damages buildings, kills fish in lakes, and harms forests by leaching nutrients from soil.

4. Health Impact Comparison

Each pollutant attacks your body differently. Understanding these differences helps explain why air quality monitoring focuses on specific gases and why different areas have different pollution problems.

Carbon monoxide is the immediate killer - it prevents oxygen transport in your blood. Symptoms include headaches, dizziness, and eventually unconsciousness. Because it's undetectable, people often don't realise they're being poisoned until it's too late.

Sulfur dioxide and nitrogen oxides cause respiratory problems by irritating your airways. They trigger asthma attacks, cause coughing and wheezing, and make existing lung conditions worse. People with asthma or other respiratory conditions are particularly vulnerable.

5. Sources and Solutions

Knowing where these pollutants come from helps us understand how to reduce them. Each pollutant has different main sources, requiring different solutions.

Carbon monoxide sources include faulty gas appliances, car exhausts, and any incomplete combustion. Prevention focuses on proper ventilation, regular appliance servicing, and carbon monoxide detectors in homes.

Sulfur dioxide comes mainly from power stations burning coal and oil. The solution is removing sulfur from fuels before burning (desulfurisation) or using cleaner energy sources like natural gas, nuclear, or renewables.

Nitrogen oxides form in any high-temperature combustion with air present. Car engines are major sources, but power stations and aircraft also contribute. Solutions include catalytic converters in cars and lower combustion temperatures in power stations.

6. Exam Success Strategy

Exam questions on atmospheric pollutants often test your ability to link sources, effects, and solutions. The key is being specific about which pollutant causes which problem.

Common exam mistakes include confusing which pollutant causes which effect. Carbon monoxide affects blood, not breathing. Sulfur dioxide and nitrogen oxides affect breathing, not blood oxygen transport. Acid rain comes from SO₂ and nitrogen oxides, never from carbon monoxide.

For evaluation questions about reducing pollution, consider both the effectiveness and practicality of solutions. Catalytic converters work well for cars but can't be fitted to power stations. Desulfurisation reduces SO₂ but adds cost to electricity generation.