Crude Oil
5 marks · EasyThe oil in underground rocks forms over a long time and is separated into fractions by boiling.
State how crude oil forms from ancient sea plants and animals.
A crude oil fraction boils at 350°C. A second fraction boils at 120°C. Calculate the difference in boiling points.
Suggest what you can conclude about the size of hydrocarbon molecules in these two fractions, based on their boiling points.
Give one reason why smaller hydrocarbons are more useful as fuels than larger molecules.
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(a) Crude oil formed when ancient sea plants and animals were buried by sediments and changed by heat and pressure over millions of years.
(b) 230°C
(c) The fraction with a higher boiling point (350°C) contains larger hydrocarbon molecules than the one with a lower boiling point (120°C). Big molecules need more energy to boil.
(d) Smaller molecules burn more easily/are more volatile/easier to ignite than larger molecules.
Common mistakes: Swaps boiling points in calculation
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States crude oil forms from ancient sea plants and animals buried and converted by heat and pressure
A1 (b) · 1 mark · Calculates boiling point difference as 350°C - 120°C = 230°C
B1 (c) · 1 mark · States higher boiling point means larger molecules
B1 (c) · 1 mark · Explains why (larger molecules need more energy to boil/stronger forces)
B1 (d) · 1 mark · Gives valid reason why smaller hydrocarbons are better fuels (easier to ignite/burn, more volatile)
Worked solution
a) Crude oil formed from the remains of sea plants and animals, buried under sediments, converted by heat and pressure over millions of years.
b) Difference in boiling points = 350°C - 120°C = 230°C
c) The fraction with a higher boiling point has larger hydrocarbon molecules. Higher boiling point means the molecules are bigger and need more energy to separate.
d) Smaller hydrocarbons burn more easily and are more useful as fuels.