Nitrogen Oxides

Sources of Nitrogen Oxides

Nitrogen oxides (NOx) are a group of gases made up of nitrogen and oxygen. They mainly come from human activities involving combustion at high temperatures. The main sources include:

• Combustion of fossil fuels: Burning coal, oil, and gas in power stations and factories releases nitrogen oxides.
• Vehicle engines: Cars, lorries, and buses produce nitrogen oxides when their engines burn fuel, especially petrol and diesel.
• Industrial processes: Some manufacturing processes, such as producing cement or steel, release nitrogen oxides as by-products.

These sources contribute significantly to air pollution in urban and industrial areas.

Types of Nitrogen Oxides

There are several nitrogen oxides, but the most important for GCSE chemistry are:

• Nitric oxide (NO): A colourless gas formed during combustion.
• Nitrogen dioxide (NO₂): A brown, toxic gas formed when nitric oxide reacts with oxygen in the air.
• Other oxides such as nitrous oxide (N₂O): Also known as laughing gas, it is less common and mainly produced by agricultural activities (covered in other topics).

NO and NO₂ are the main nitrogen oxides involved in atmospheric pollution.

Formation of Nitrogen Oxides

Nitrogen oxides form when nitrogen and oxygen in the air react at high temperatures. This happens in:

• Internal combustion engines: The heat inside car engines (up to 20006C) causes nitrogen (N₂) and oxygen (O₂) molecules to react, forming nitric oxide (NO).
• Lightning strikes: The extreme heat of lightning causes nitrogen and oxygen in the atmosphere to combine, producing nitrogen oxides naturally.

The initial product is usually nitric oxide (NO), which quickly reacts with oxygen to form nitrogen dioxide (NO₂).

For instance, in car engines, the high temperature causes nitrogen and oxygen to react:

$\mathrm{N_2 + O_2 \rightarrow 2NO}$

Then nitric oxide reacts with oxygen:

$\mathrm{2NO + O_2 \rightarrow 2NO_2}$

Environmental Impact

Nitrogen oxides have several harmful effects on the environment and human health:

• Contributes to acid rain: Nitrogen dioxide dissolves in rainwater to form nitric acid, which falls as acid rain. Acid rain damages buildings, harms plants, and pollutes rivers and lakes.
• Causes respiratory problems: Nitrogen dioxide irritates the lungs and airways, making breathing difficult, especially for people with asthma or other lung conditions.
• Leads to photochemical smog: Nitrogen oxides react with sunlight and other pollutants to form smog, a harmful mixture of gases and particles that reduces air quality and visibility.

For example, in cities with heavy traffic, nitrogen dioxide levels can rise, causing breathing difficulties and contributing to smog formation.

For example, a nitrogen dioxide concentration of 0.05 ppm means 0.05 parts of NO₂ per million parts of air, which is enough to cause health issues in sensitive individuals.