National Chemistry

 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 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


                                                                                addition reaction


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



6.5 Isomers

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

                                orange                     colourless



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.

Addition polymerisation

ethene produces polythene ;  

                               monomer                                           polymer

                  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


                                  ethene                                          polythene


 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)

repeating unit



National 4 and 5



These notes are for the Scottish National Chemistry course taught in fourth year in most Scottish schools.


The notes available here are concise notes. They are NOT to be considered as material to learn from - they are for revision.


 The notes are arranged under the following topics


National 3          Unit 1

                          Unit 2

                          Unit 3


National 4 and 5  Unit 1            Rates of Reaction

                                                  Atomic Structure

                                                  Bonding and Properties

                                                  Acids and bases


National 4 and 5  Unit 2       Fuels and Homologous Series

                                                Consumer Products



National 4 and 5  Unit 3         Metals



                                                   Nuclear Chemistry

                                                  Chemical analysis