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Discovering chemistry
Discovering chemistry

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1 Petrol engine emissions

In a world where electric and hybrid cars are becoming commonplace, your focus will be on the ‘old fashioned’ petrol engine and one reaction in particular, which is in part responsible for the move towards this new generation of ‘cleaner’ vehicles. This is a chemical process which leads to gaseous emissions thought to be detrimental to the environment, and human health.

Table 1 shows typical percentages of the main constituents of the exhaust gas that emerges from a modern car engine.

Table 1 The percentage by volume of the different gases in a typical car exhaust stream
GasVolume per cent
nitrogen and argon71.00
carbon dioxide13.50
water vapour12.50
carbon monoxide00.68
nitric oxide00.11

Looking at Table 1, the main concern stems from the release of carbon monoxide, CO, and nitric oxide (strictly known as nitrogen monoxide), NO into the environment. Both are very poisonous gases.

For example, when nitric oxide emerges from the exhaust into the open air and cools down, it reacts with oxygen to form nitrogen dioxide, NO2. This causes respiratory problems even at very low concentrations .You may recall (from Session 3) that NO2 is a product of the reaction of copper with concentrated nitric acid (Figure 1).

Described image
Figure 1 Brown nitrogen dioxide gas being produced, in this case, by the reaction of copper with concentrated nitric acid

However these dangers apart, there is a reaction that could be very beneficial:

2NO(g) + 2CO(g) = N2(g) + 2CO2(g)
Equation label: (1)

If NO and CO reacted like this, then the nitric oxide in the exhaust would disappear, and take a substantial amount of poisonous carbon monoxide with it. Unfortunately, the reaction does not seem to happen in practice. To understand why, you need to be introduced to a concept fundamental to an appreciation of chemical reactions - chemical equilibrium.