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Chemistry of the Environment: Question 8

Syllabus 10.3

Structured Core 6 marks

A haulage company runs a fleet of older diesel delivery vans. Mechanics notice that one poorly maintained van produces thick black smoke containing particulates (soot) from its exhaust, while a newer, well-maintained van in the same fleet, burning the same diesel fuel, produces almost no visible smoke.

(a) State the source, in terms of the type of combustion taking place, of the particulates in the poorly maintained van's exhaust smoke. [1]

(b) State two adverse effects on human health of breathing in particulates such as those in this exhaust smoke. [2]

(c) The newer van's engine is tuned so that its diesel fuel undergoes complete combustion instead of incomplete combustion. Identify the main carbon-containing gas produced by complete combustion, and explain why switching to complete combustion greatly reduces the particulates (and carbon monoxide) released, even though a different environmental concern remains. [3]

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Worked solution

Part (a): Source of the particulates

Particulates (soot) are produced by the incomplete combustion of carbon-containing fuels such as diesel. Combustion that takes place when there is not enough oxygen available to fully burn the fuel.

Part (b): Health effects of particulates

Breathing in particulates in exhaust smoke:

  • Increases the risk of respiratory problems.
  • Increases the risk of cancer.

Part (c): Switching to complete combustion

With a good supply of oxygen, the well-maintained van’s engine burns the diesel fuel via complete combustion. The main carbon-containing product of complete combustion is carbon dioxide, CO2\text{CO}_2, rather than carbon monoxide or particulates.

This happens because carbon monoxide and particulates form specifically when there is not enough oxygen to fully oxidise the carbon in the fuel. Once the engine supplies enough oxygen for complete combustion, the carbon is instead fully oxidised straight to carbon dioxide, so far less carbon monoxide and far fewer particulates are released.

However, this does not remove all environmental concern: carbon dioxide is a greenhouse gas, and higher levels of carbon dioxide in the atmosphere lead to increased global warming, which in turn leads to climate change. So while the immediate toxic (carbon monoxide) and respiratory/cancer (particulate) hazards are greatly reduced, the exhaust still contributes to a different, longer-term environmental problem.

Final answers

  • (a) Incomplete combustion of carbon-containing fuel (insufficient oxygen).
  • (b) Increased risk of respiratory problems and increased risk of cancer.
  • (c) Complete combustion produces carbon dioxide instead; this greatly reduces carbon monoxide/particulate emissions, but carbon dioxide remains a greenhouse gas, contributing to global warming and climate change.