11.01.1 Alkanes
Syllabus
Crude oil is a finite resource found in rocks.
Crude oil is the remains of an ancient biomass consisting mainly of plankton that was buried in mud.
Crude oil is a mixture of a very large number of compounds.
Most of the compounds in crude oil are Hydrocarbons, which are molecules made up of Hydrogen and Carbon atoms only.
Most of the Hydrocarbons in crude oil are Hydrocarbons called Alkanes.
The general formula for the homologous series of Alkanes is CnH2n+2
The first four members of the Alkanes are Methane, Ethane, Propane and Butane.
Alkane molecules can be represented in the following forms: C3H8 or as right
Students should be able to recognise substances as Alkanes given their formulae in these forms.
Students do not need to know the names of specific Alkanes except Methane, Ethane, Propane & Butane.
What does this mean?
Finite resources will one day run out - oil is finite because it is forming much more slowly than we are using it.
Because it won't quickly come back we often say that oil is non-renewable.
Where does oil come from?
In Lower School you may have seen pictures like this...
... which suggest that oil formed from the remains of large animals like fish and even crocodiles.
It is true that oil comes from once living material, but almost all of that was plankton - microscopic plants and animals.
When the material died it sank to the sea-floor and was unable to rot due to lack of Oxygen.
Eventually it became buried by metres of sand.
As the sand above built up into sandstone, the heat and pressure increases.
And the plankton slowly became Hydrocarbons - molecules containing only Carbon and Hydrogen
These float up in the space between sand grains until they get trapped by a later of impermeable rock - usually shale (mudstone).
The diagram below looks like the oil is in some sort of underground cave, but actually it is between the sand grains inside the sandstone
Since the oil came from plankton which lived millions of years ago, we can call oil the remains of an ancient biomass.
What are Alkanes?
We said Hydrocarbons only contain Hydrogen and Carbon.
The simplest Hydrocarbons are called Alkanes - these contain only single bonds.
You need to be able to name and draw the first four Alkanes - Methane to Butane.
The easiest way to know how many Carbon and Hydrogen atoms to draw is to learn the general formula CnH2n+2
This means that the number of Hydrogen atoms in an Alkane is always twice the number of the Carbon atoms plus two.
Displayed formulae of the first 4 Alkanes - each is -CH2-longer than its predecessor.
We only need to be able to name the first 4 Alkanes.
They will all end with -ane to signify that they are Alkanes.
They begin with a prefix that tells you how many Carbons are in the chain.
C1 = Meth-, C2 = Eth-, C3 = Prop-, C4 = But-, C5 = Pent-
After 4 the prefixes are the same as for Polygons in Maths (Pent -, Hex-, Hept-, Oct-, Non-, Dec- etc)
We can remember the first 5 as: My Evil Plane Bombed Paris
Types of formula
The examiner may ask you for the molecular formula of an Alkane with, for example, three Carbon atoms.
In which case, you apply the general formula (CnH2n+2 ) and get
But you may also be asked to draw a displayed formula.
In which case you link the three Carbon atoms with single bonds, then add Hydrogen atoms until every Carbon atom is surrounded by four bonds.
There's no reason why you couldn't be asked to do the same for an Alkane with 6 or 8 Carbon atoms.
But you can't be asked to name these.
Videos
Past Paper Questions
2021
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