The smaller-molecule hydrocarbons are more volatile – they form a vapour easily. For example, we can smell petrol (with molecules containing between 5 and 10 carbon atoms) much more easily than we can smell engine oil (with molecules containing between 14 and 20 carbon atoms) because petrol is more volatile. Another difference between the fractions is how easily they burn and how smoky their flames are. size
small
large
ease of burning
ignite easily
difficult to light
smokiness of flame
blue flame almost no smoke
blue
yellow/grey yellow smoky flame
∆∆Fig. 4.6 How different hydrocarbons burn.
QUESTIONS 1. Petroleum is a ‘non-renewable’ fuel. What does this mean? 2. When drilling for oil, there is often excess gas to be burned off. What is this gas? Where does it come from?
3. One of the oil fractions obtained from the fractional distillation of crude oil is light-coloured and runny.
Is this fraction more likely to have a small chain of carbon atoms or a long chain?
4. Another of the oil fractions obtained from the fractional distillation of petroleum burns with a very sooty yellow flame.
Is this fraction more likely to have a small chain of carbon atoms or a long chain?
5. Some fractions obtained from petroleum are very ‘volatile’.
311
Fuels
What does this mean?
92654_P306_364.indd 311
19/06/14 7:10 PM