Oxides of nitrogen and pollution
Oxides of nitrogen and pollution
- Oxides of nitrogen ($\text{NO}$ and $\text{NO}_2$, together NOₓ) pollute the air.
- They come from lightning and from engines.
- They cause smog and acid rain — but we can remove some.
Where NOₓ comes from
- natural: lightning gives enough energy for $\text{N}_2$ and $\text{O}_2$ in the air to combine.
- man-made: the high temperature in a car engine makes them react: $\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$.
Practice
Oxides of nitrogen (NOₓ) are produced by:
Lightning (natural) and the heat of internal combustion engines both make N₂ and O₂ combine into NO.
Catalytic converters
- A catalytic converter cleans the exhaust by letting harmful gases react to harmless ones:
$$2\text{NO} + 2\text{CO} \rightarrow \text{N}_2 + 2\text{CO}_2$$
Practice
In a catalytic converter, NO and CO react to form:
2NO + 2CO → N₂ + 2CO₂; the harmful gases become harmless ones.
Smog and acid rain
- In sunlight, NOₓ reacts with unburned hydrocarbons to form PAN, part of brown photochemical smog.
- NOₓ makes acid rain two ways: $\text{NO}_2$ forms nitric acid directly, and it also catalyses the oxidation of $\text{SO}_2$ to $\text{SO}_3$ (→ sulfuric acid).
Practice
How does NO₂ contribute to acid rain?
NO₂ dissolves to form nitric acid, and it also speeds up SO₂ → SO₃, which forms sulfuric acid.
Practice
Photochemical smog forms when NOₓ reacts in sunlight with unburned hydrocarbons to make:
In sunlight NOₓ and hydrocarbons form PAN, a harmful part of the brown photochemical smog.
You've got it
Key idea
- NOₓ comes from lightning (natural) and the high temperature in engines ($\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$)
- a catalytic converter: $2\text{NO} + 2\text{CO} \rightarrow \text{N}_2 + 2\text{CO}_2$
- NOₓ → photochemical smog (PAN) and acid rain (nitric acid + catalysing $\text{SO}_2 \rightarrow \text{SO}_3$)