HYDROCARBONS AND COMBUSTION
CH211: Describe what a hydrocarbon is
A hydrocarbon is a compound containing hydrogen and carbon atoms only.
In the three chemicals below, A and C are hydrocarbons because they ONLY contain hydrogen and carbon.
B is not a hydrocarbon because it also contains oxygen.
A
HYDROCARBON
B
NOT A HYDROCARBON
C
HYDROCARBON
CH212: Describe what crude oil is made up of
Crude oil is a mixture of hydrocarbons (See CH211).
Carbon atoms have a valency of 4, meaning they can make 4 covalent bonds.
This high valency leads to many hydrocarbons being formed, ranging from 1 carbon to hundreds of them – which can be in the form of chains and rings.
CH213: Describe what a homologous series is
A homologous series is any group of chemicals that have:
- similar chemical properties,
- a gradual change in physical properties (boiling points and melting points),
- the same general formula,
- Each member of the series goes up by a CH2 each time.
These are called homologues series because they have the same general formula which in this case is CnH2n+2.
- For example, when n=2, the molecule is C2H(2x2)+2 = C2H6.
- This formula works by adding in a Carbon and 2 Hydrogens (CH2) each time.
CH214: Describe what a finite resource is
Crude oil is a finite resource which is the term given to something that is either not being made any more or it is being made very slowly.
This makes crude oil non-renewable (cannot be replaced when it is used up).
CH215: Explain how crude oil is separated into fractions
Crude oil contains many different fuels, with different uses. We cannot just put crude oil into our cars, for example – it needs to be separated into the different fractions.
This is done using fractional distillation – which means separating into their fractions using their boiling points – in the following way:
- The crude oil is vapourised and added to the column
- The fractionating column is hottest at the bottom and coolest at the top.
- The vapours rise throughout the column until they fall below their boiling point – they then condense.
- The liquid falls into the tray and is collected.
The rest of the hydrocarbons will move up the column, and the process continues until you are left with just the gases at the top.