Topic 6 – Hydrocarbons – Learning Outcomes
General Level
o State that the alkanes are a subset of the set of hydrocarbons
o Identify an alkane from the -ane ending explain that the alkane family is an example of an
homologous series
o State the name for each alkane up to octane
o Give examples of uses of alkanes
o For straight chain alkanes, (C1 to C8), work out the names from molecular formulae, shortened
and full structural formulae
o For straight chain alkanes (C1 to C8), construct full and shortened structural formulae and
molecular formulae given the name
o For alkanes, use the general formula to work out the molecular formulae
o State that the alkanes are a subset of the set of hydrocarbons
o Identify an alkene from the – ene ending
o State the name for each alkene up to hexene (isomers are not required, e.g. only butene is
expected not but-1-ene, etc)
o For alkenes, (C1 to C6), work out names from molecular, shortened or full structural formulae
o For alkenes, C1 to C6 construct full and shortened structural formulae and molecular formulae
given the name
o State what is meant by a saturated hydrocarbon
o Explain that the alkanes are saturated hydrocarbons
o State what is meant by an unsaturated hydrocarbon
o Explain that the alkenes are a sub-set of the set of unsaturated hydrocarbons
o State that it is possible to distinguish an unsaturated hydrocarbon from a saturated hydrocarbon
using bromine (solution)
o State the name of the alkane formed by the reaction of an alkene with hydrogen
o State what is meant by an addition reaction
o Explain that the reactions of alkenes with bromine and hydrogen are addition
reactions
o State that fractional distillation of crude oil yields more long chain hydrocarbons than are useful
for present-day industrial purposes
o State that cracking is an industrial method for producing smaller, more useful
molecules
o State that the cracking produces smaller hydrocarbons, some of which are
unsaturated
1
Credit Level
o State what is meant by an homologous series.
o Derive a general formula for alkanes
o Explain in terms of increasing molecular size why the boiling point increases as the number of
carbon atoms in an alkane increases
o State the name of each cycloalkane up to cyclohexane
o Explain that the cycloalkane family is an example of an homologous series
o State what is meant by isomers
o Explain that the alkene family is an example of an homologous series
o For alkenes, us a general formula to work out molecular formulae
o Derive a general formula for alkenes
o Identify from a structural formula the following types of compound: alkanes, cycloalkanes and
o
alkenes.
For simple organic molecules, including alkanes, alkenes and cycloalkanes, construct
appropriate isometric forms (isomers), given a molecular formula
o State that the catalyst allows the reaction to take place at a lower temperature
o Explain why cracking produces a mixture of saturated and unsaturated products.
Hydrocarbons
In topic we learned that hydrocarbons are compounds made
from carbon and hydrogen. There are many types of compounds
made from hydrogen and carbon; we need to know about the
families called alkanes, alkenes and cycloalkanes. Each family of
hydrocarbons is known as a homologous series. A homologous of
hydrocarbons can be recognised from the name or from having
similar structural properties. Compounds in the same
homologous series have the same general formula linking the
number of carbons to the number of hydrogen’s.
2
Prefixes
All hydrocarbons use prefixes to tell chemists how many carbon
atoms they are made from:
Prefix
Meth
Eth
Prop
But
Pent
Hex
Sept
Oct
Number of Carbons
1
2
3
4
5
6
7
8
We can use a saying to help us remember the number of carbon
atoms with the prefix:
“Many elephants prefer buns, poodles have happy owners”.
If you ever forget there is a table in the data book that lists
the boiling point of alkanes in the correct order.
Alkanes
Alkanes are homologous series of hydrocarbons. They can be
recognised from the –ane ending in their name (e.g. methane,
ethane, etc). Alkane molecules all have single carbon-to-carbon
bonds and have the same general formula, CnH2n+2. We need to
know how to draw full structural formula, shortened structural
formula and how to write chemical formula for the first eight
alkanes. Complete the table below:
3
Name
Full Structural
Formula
Shortened
Formula
Chemical
Formula
Methane
CH4
CH4
Ethane
CH3CH3
Propane
CH3CH2CH3
Butane
As we can see going up the hydrocarbon chain, the molecule
size very quickly increases. This is matched with an
increase in the boiling point. We can see a trend that the
larger the size of the molecules the higher the boiling
point.
Alkanes are said to be saturated molecules as they contain
all single bonds, this means there is no space for anything
to join on to the molecule.
There is a link between the number of carbons and
hydrogen’s’ in an alkane molecule. The number of hydrogen’s
is double the number of carbons plus 2. We call this the
general formula for alkanes and it is written as the
following:
CnH2n+2
4
Alkenes
Alkenes are homologous series of hydrocarbons. They can be
recognised from the –ene ending in their name (e.g. ethene,
propene, etc). Alkene molecules all have a carbon-to-carbon
double bond and have the same general formula, CnH2n. We need
to know how to draw full structural formula, shortened
structural formula and how to write chemical formula for the
first seven alkenes (it is not possible to get methene as this only
has one carbon so we can not form a carbon to carbon double
bond). Complete the table below:
Name
Full Structural
Formula
Ethene
Shortened
Formula
CH2CH2
Propene
CH2CHCH3
Chemical
Formula
Butene
There is a link between the number of carbons and
hydrogen’s’ in an alkene molecule. The number of hydrogen’s
is double the number of carbons. We call this the general
formula for alkenes and it is written as the following:
CnH2n
Alkenes can be identified by reacting them with bromine
solution. Bromine solution is brown in colour and when this
5
is reacted with an alkene the brown colour disappears. This
is because the bromine can add on across the double bond,
it is known as an addition reaction.
As alkenes have this double bond they are an unsaturated
molecule. This means they have space for things to add on
to the molecule.
Cycloalkanes
Cycloalkanes are homologous series of hydrocarbons. They can
be recognised from the cyclo- start in their name (e.g.
cyclopropane, cyclobutane, etc). Cycloalkane molecules all have
carbon-to-carbon single bonds and have the same general
formula, CnH2n. We need to know how to draw full structural
formula, shortened structural formula and how to write chemical
formula for the first six cycloalkanes (it is not possible to get
cyclomethane or cycloethane as we need at least 3 carbons to
make a ring). Complete the table below:
Name
Full Structural
Formula
Cyclopropane
Shortened
Formula
Chemical
Formula
Cyclobutane
There is a link between the number of carbons and
hydrogen’s’ in a cycloalkane molecule. The number of
hydrogen’s is double the number of carbons. We call this
6
the general formula for cycloalkanes and it is written as the
following:
CnH2n
Cycloalkanes and alkenes have the same general formula and
can be identified by reacting them with bromine solution.
Bromine solution is brown in colour and when this is reacted
with an alkene the brown colour disappears. In and
cycloalkane the brown colour would remain.
As cycloalkanes have all single bonds they are known as
saturated molecules.
Isomers
Longer chain hydrocarbons can join together in many
different ways, not just in straight lines. Compounds with
the same formula but with different structures are called
isomers. An example is butane. This can have the following
two structures:
7
Cracking
In fractional distillation of crude oil we produce lots of long
chain hydrocarbons but not enough of the shorter chain
hydrocarbons. Fractions like petrol are in high demand but
we produce too much bitumen. Chemists can do a cracking
reaction that breaks the longer chain hydrocarbons into the
smaller hydrocarbons. An example is:
C10H22  C4H8 + C6H14
We cannot make any new atoms in this reaction so the total
number of hydrogen’s and carbons in the products has to be
the same as for the reactants.
As we cannot bring any more hydrogen’s into the reaction
the products are a mixture of saturated alkanes and
unsaturated alkenes.
This process happens with a catalyst. This speeds up the
reaction but is not used up.
Past Paper Questions
07 General: 3, 9
07 Credit: 4, 11
08 General: 4, 10b, 19c, 19d
08 Credit: 3, 19
09 General: 4a, 13, 19,
09 Credit: 5, 19a, 19b, 19d
8