5 & 6 Fuels and Hydrocarbons
5.1 A fuel is a chemical which burns (combustion) in oxygen (or air - 20% oxygen) to release energy (exothermic) safely.
- oxygen gas relights a glowing splint.
Coal is formed from dead plant life which has been crushed underground at high temperatures and pressure for millions of years.
Oil and gas are formed from dead marine life which has been crushed underground at high temperatures and pressure for millions of years. oiland gas contain hydrocarbons.
Fossil fuels (coal, oil and gas) are a finite resource.
5.2 Hydrocarbons contain the elements carbon and hydrogen only and when they burn in oxygen, the products are carbon dioxide (which turns lime water milky) and hydrogen
oxide (water - which boils at 100°C and freezes at O°C).
CO2 must come from the presence of C; while H2O must come from the presence of H.
Therefore CH4 (with both C and H must produce CO2 and H2O
5.3 When any fuel containing carbon burns in a shortage or insufficient supply of oxygen, then poisonous carbon monoxide is produced by incomplete combustion - this occurs in petrol engines in cars. Nitrogen reacts with oxygen to form oxides of nitrogen.
Cars produce pollution:
(i) carbon monoxide - poisonous - catalytic converter changes to harmless CO2
(ii) oxides of nitrogen - produce acid rain - changes to harmless N2
(iii) lead - poisonous - use unleaded petrol
Air pollution from cars can be reduced by special exhaust systems by increasing the air to fuel ratio- improves efficiency of combustion therefore less CO (more CO changes to harmless CO2) - decreasing pollution
If sulphur is present in a fuel as an impurity, then it will burn in the oxygen to produce sulphur dioxide (SO2) - a pollutant gas causing acid rain.
5.4 Fossil fuels (oil, coal and gas) were formed over millions of years when dead organisms were buried and subjected to high pressure. Coal was formed from dead vegetation while oil and gas were formed from dead marine life. These fuels are a finite resource and will eventually run out.
5.5 Oil is a mixture of hydrocarbons each of which has a different boiling point.
The different boiling points are used by fractional distillation to separate the crude
oil into fractions containing a group of hydrocarbons within a given boiling point range.
fraction number of Boiling pt. % present in % present in % market demand
carbon atoms (oC) North Sea crude Arabian crude .
gas 1-4 below 12 3 1 6
petrol 5- 8 7-127 20 15 28
naphtha 6-11 107-217 13 11 8
kerosene 10-14 177-277 17 11
diesel 14-19 233-377 24 15 20
residue 19+ +377 29 45 27
(i) different sources have different mixes
(ii) size/mass increases so do physical properties (melting & boiling pt., density viscosity, ignition temperature)
(iii) used mainly as fuels but (especially naphtha) also as a source of other chemicals (medicines, dyes, plastics)
(iv) market demand outstrips availability- market favours short hydrocarbons but supply has more of the less useful long chain hydrocarbons.
Take less popular long chain hydrocarbons and break into smaller more useful hydrocarbons by catalytic cracking
fraction use number of carbon atoms
refinery gas fuel 1-4
petrol fuel for cars 5-8
naphtha making chemicals 6-10
kerosene plane fuel 10-14
diesel diesel fuel, heating oil 14-19
residue tar, bitumen 19+
Viscosity - thickness - increases with size/mass
flammability = ease of burning - gases with low ignition temperature are easiest.to set alight
6.1 Alkanes, aIkenes and cycloalkanes are examples of groups of hydrocarbons (compounds of carbon and hydrogen only) - these groups are called Homologous series as they have similar chemical properties and a general formulae
6.2 Hydrocarbon homologous series
ALKANE ALKENE CYCLOALKANE
general formula CnH2n+2 CnH2n CnH2n
example with 3 x C propane propene cyclopropane
molecular formula C3H8 C3H6 C3H6
structural formula (i) (ii) (iii)
saturated unsaturated saturated
single bonds C=C double bonds single bonds
slow reaction fast reaction slow reaction
decolourising bromine water
Remember alkenes by : double 'e'; double bond, double the number of hydrogens CnH2n
6.3 Page 5 of the data book gives the alkanes in increasing number of carbons - use this to remind you of the starting part of each name.
one C atom = METH methane
two C atoms = ETH ethane and ethene
three C atoms = PROP propane and proopene and cyclopropene
4 C atoms = BUT butane and butene and cyclobutane
5 C atoms = PENT
6 C atoms = HEX
7 C atoms = HEPT
8 C atoms = OCT
As the number of carbon atoms increases so does mas and/or /size this causes an increase in
(a) boiling point, (b) melting point point, (c) ignition temperature, (d) viscosity, (e) density,
6.4 Fractional distillation of crude oil produce more long chain hydrocarbons than are required but less short chain hydrocarbons than required so catalytic cracking is used to change long chain hydrocarbons into short chain hydrocarbons which are more useful.
Cracking must produce unsaturated hydrocarbons as there are not sufficient hydrogens to saturate all of the molecules
hexane ----> butane + ethene
C6H14 C4H10 C2H4
saturated saturated unsaturated
Molecules with the same molecular formula but different structural formulae are called isomers
eg propene and cycloprpane have the same molecular formula C3H6 but
propene has a C=C double bond (unsaturated while cyclopropane is saturated;
propene is a straight chain moleule while cyclopropane is a ring structure
isomers of butane
isomers of butene
isomers of hexane
6.6 Addition reactions
Alkenes (double 'e'; double bond, double the number of hydrogens) undergo addition reactions. these reactions occur at the C=C carbon to carbon double bond.
The reacting diatomic molecule simply adds onto the double bond
eg C2H4 + H2 -----------> C2H6
C2H4 + Br2 -----------> C2H4Br2
When orange bromine reacts it becomes colourless. This is used as a test for a C=C.
Every C=C needs two bromine atoms.
Topic 13 Polymers
13.1 Most plastics and synthetic (man-made) fibres are made from oil and are not biodegradable
which can cause environmental problems - they are polymers
Properties - easy clean - hygienic; light; durable; insulator
some plastics burn or smoulder giving off poisonous fumes.
Natural - expensive, wear out, but biodegradable and made from renewable sources
Man-made - made from finite resource - not biodegradable but cheap, mass-produced and long lasting.
13.2 Thermosetting plastics cannot be reshaped by heating and melting - used for items which get hot - plugs, switches, electrical appliances.
Thermoplastic plastics can be melted or reshaped.
13.3 Plastics are polymers - very large molecules made from a small molecule called a monomer-
Many monomers are unsaturated hydrocarbons - aIkenes (with C=C) - produced by cracking longer chain aIkenes. Monomers form polymers by addition polymerisation.
ethene produces polythene ;
H H H H H H H H H H H H H
C=C C=C C=C ------> -C-C- C- C-C-C-C -
H H H H H H H H H H H H H
Alkene monomers react by addition (breaking one of the bonds in the C=C) to polymerise
One bond of the double bond breaks open allowing each unsaturated monomer to make a
connecting bond. (watch)