Sulfur impurities give sulfur dioxide
- Crude oil contains small amounts of sulfur mixed in with the hydrocarbons.
- That sulfur passes into the fuels made from the oil.
- Burning the fuel oxidises the sulfur: S+O2→SO2\text{S} + \text{O}_2 \rightarrow \text{SO}_2S+O2→SO2
- Sulfur dioxide therefore leaves the chimney or exhaust with the other waste gases.
- Removing the sulfur from the fuel before it is burned prevents this.
Key Idea
The sulfur comes from an impurity, not from the hydrocarbon itself.
Sulfur dioxide dissolves to give acid rain
Definition
Acid rain
Rain made more acidic than normal by dissolved gases such as sulfur dioxide.
- Sulfur dioxide rises into the atmosphere and dissolves in water droplets.
- The solution formed is acidic, so the rain that falls has a lower pH than normal.
- Acid rain can fall a long way from where the gas was released.
- Rain is always slightly acidic from dissolved carbon dioxide, so acid rain is more acidic than that.
- The damage it causes depends on how much the pH has fallen.
Note
The pollution crosses borders, which is why acid rain is dealt with by international agreement.
The damage acid rain causes
- Acid rain makes lakes and rivers more acidic, which kills fish and other water life.
- It damages trees, both directly and by harming the soil they grow in.
- It reacts with limestone and marble, eating away buildings and statues.
- It corrodes metal structures such as bridges and railings.
- Soil chemistry is altered, which affects the crops that can be grown.
Example
- Limestone building: the surface is worn away where rain runs over it.
- Upland lake: fish disappear as the water becomes too acidic for their eggs.
Oxides of nitrogen form in hot engines
- Air is about 78%78\%78% nitrogen and 21%21\%21% oxygen, and both are drawn into an engine.
- Nitrogen is normally unreactive, so the two pass through unchanged at ordinary temperatures.
- Inside a running engine the temperature is high enough for them to react.
- The two elements combine directly: N2+O2→2NO\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}N2+O2→2NO
- The oxides of nitrogen formed are pollutants and contribute to acid rain as well.
Common Mistake
These oxides come from the air drawn into the engine, not from anything in the fuel.
Self review
- Where does the sulfur in a hydrocarbon fuel come from?
- Write the equation for sulfur burning in oxygen.
- How does sulfur dioxide lead to acid rain?
- Give two kinds of damage caused by acid rain.
- Why do oxides of nitrogen form inside an engine but not in ordinary air?
