Science with Tara

What are Oxides of Nitrogen?

The two main oxides of nitrogen are:

• Nitrogen monoxide (NO) – also known as nitric oxide. Colourless, odourless, neutral, and insoluble in water.
  
• Nitrogen dioxide (NO₂) – Reddish-brown, pungent gas. Soluble in water to form nitric acid.

Sources of oxides of Nitrogen

Natural Sources:

• Lightning (provides nitrogen for plant growth)

Man-made Sources:

• High-temperature combustion in internal combustion engines
• Coal-burning power stations

Formation of Oxides of Nitrogen

Oxides of nitrogen are formed at high temperature.

Nitrogen gas and oxygen gas react to form nitrogen monoxide.

Equation 1: N₂(g) + O₂(g) → 2NO(g)

Nitrogen monoxide then reacts with more oxygen to form nitrogen dioxide.

Equation 2: 2NO(g) + O₂(g) → 2NO₂(g)

Harmful Effects of Oxides of Nitrogen

Health Effects

• Irritates the lungs and causes breathing difficulties.
• In severe cases, can lead to lung inflammation.

Environmental Effects

1. Formation of Ground-Level Ozone

In the presence of Sun’s UV light, nitrogen dioxide reacts with oxygen gas to produce oxygen atoms.
NO₂ (g) + O₂ (g) ⇌ NO (g) + O (g) + O₂ (g)

Oxygen atoms then react with oxygen molecules to form ozone.
O(g) + O₂ (g) ⇌ O₃ (g)

Ozone, at high altitude, protects us by absorbing harmful UV radiation from the Sun.

However, at ground level, it is a pollutant that irritates eyes, chests and throats. It also leads to asthma attacks.

2. Acid Rain

Nitrogen dioxide gives rise to acid rain. Nitrogen dioxide is an acidic gas. It reacts with oxygen and rain water to form nitric acid.

4NO₂(g) + 2H₂O(l) + O₂(g) → 4HNO₃(aq)

Unpolluted rainwater has a pH of around 5.6 due to carbon dioxide gas.

Carbon dioxide is an acidic gas, which dissolves in rainwater to form weak carbonic acid. This slight acidic is natural and harmless.

However, with the presence of man-made nitrogen dioxide and sulfur dioxide, the pH of rainwater will drop to around 4.

Harmful effects of acid rain

• Acid rain reacts with metals and carbonates. It corrodes limestone structures and buildings. It speeds up corrosion of metal structure like bridges and motor vehicles.
• Acid rain leaches nutrients from the soil needed by the plants for healthy growth. Plants wither and die. Acid rain react with aluminium hydroxide in the soil to produce Al³⁺ ions, which are toxic to plants.
• Acid rain reduces the pH value of lakes and streams, making it too acidic for aquatic life to survive.

Control of Oxides of Nitrogen

Fit vehicles with catalytic converters to reduce nitrogen oxides into harmless nitrogen gas. Ensure more complete combustion of fuels.