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doc b oil notesDoc Brown's GCSE/IGCSE KS4 science-CHEMISTRY Revision Notes

Oil, useful products, environmental problems, introduction to organic chemistry

3. ALKANES - saturated hydrocarbons

The alkanes are a series of hydrocarbon molecules (made of carbon and hydrogen atoms). They are referred to as 'saturated' hydrocarbons because they do NOT contain a double bond and other atoms cannot add to them. The physical properties and chemical reactions of alkanes with oxygen (combustion, burning) and chlorine (to form a chloroalkane) are fully described with word and symbol equations.

Index of KS4 Science GCSE/IGCSE Chemistry Oil & Organic Chemistry Pages: 1. Fossil Fuels : 2. Fractional distillation of crude oil & uses of fractions : 3. ALKANES - saturated hydrocarbons and combustion : 4. Pollution, carbon monoxide, nitrogen oxides, what makes a good fuel?, climate change-global warming : 5. Alkenes - unsaturated hydrocarbons : 6. Cracking - a problem of supply and demand, other products : 7. Polymers, plastics, uses and problems : 8. Introduction to Organic Chemistry - Why so many series of organic compounds? : 9. Alcohols - Ethanol - properties, reactions, biofuels : 10. Carboxylic acids and esters : 11. Condensation polymers, Nylon & Terylene, comparing thermoplastics, fibres and thermosets : 12. Natural Molecules - carbohydrates - sugars - starch : 13. Amino acids, proteins, enzymes & chromatography : 14. Oils, fats, margarine and soaps : 15. Vitamins, drugs-analgesic medicines & food additives and aspects of cooking chemistry! : 16. Ozone, CFC's and free radicals : 17. Extra notes, ideas and links on Global Warming and Climate Change : Multiple Choice and Gap-Fill Quizzes: m/c QUIZ on Oil Products (GCSE/IGCSE easier-foundation-level) : m/c QUIZ on Oil Products (GCSE/IGCSE harder-higher-level) : IGCSE/GCSE m/c QUIZ on other Aspects of Organic Chemistry : and 3 Easy linked GCSE/IGCSE Oil Products word-fill worksheets

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3a. The ALKANE series of saturated hydrocarbons

  • Alkanes are a group of hydrocarbon molecules in which all the carbon and hydrogen atoms are only joined by single covalent bonds (e.g. C-H or C-C).

    • Note that the name ends in ...ane eg methane, ethane, propane, butane etc.

  • Carbon forms four bonds (C-C or C-H) and hydrogen just one bond (C-H) in alkane hydrocarbon molecules.

    • Check this out with the structural/displayed formula in the table below.

  • Alkanes are saturated hydrocarbons with the general formula CnH2n+2 where n is the number of carbon atoms in the molecule, so ..

    • when n = 1 you get CH4, n = 2 gives C2H6, n = 3 gives C3H8 and then C4H10, C5H12, C6H14, C7H16, C8H18 etc.

    • As with naming all organic molecule series, eth.. means 2 carbon atoms in the chain, prop... means 3 and but.. means 4 etc. After that the name is directly derived from the number of carbon atoms in the chain eg pentane, hexane, heptane, octane etc. - match with the formulae quoted above.

  • Alkanes are known as saturated molecules because other atoms cannot add to them.

  • Physical properties: The first four in the series are shown below and are all colourless smelly highly flammable gases. The larger alkanes are colourless liquids and the biggest members of the series are white waxy solids.

  • Physical Properties of linear ALKANES (n-alkanes), see the table below for the first 20 members of the series

  • General formula CnH2n+2 where n = number of carbon atoms in the linear chain (na = not applicable)

  • As the molecular mass increases, quite clear trends in physical properties emerge ...

    • ... the melting points and boiling points steadily increase

      • this is because the bigger the molecule, the greater the attractive intermolecular forces between the molecules (intermolecular bonding),

      • this is also exemplified by the fact that at room temperature and pressure, the first four are gases, then they are all liquids until the 18th alkane onwards, when they become white waxy solids.

    • ... the density increases

    • ... chemically they become more flammable e.g. more easily ignited with a spark

      • this is measured by the 'flash point', this is the lowest temperature at which the alkane liquid gives off sufficient vapour to ignite in air (you don't need to know this for GCSE).

      • The first four gaseous alkanes are very explosive!

n molecular formula (state at RTP) abbreviated structural formula name of alkane relative molecular mass Mr melting point temperature oC/K boiling point temperature oC/K flash point oC density g/cm3
1 CH4 (g) CH4 methane 16.0 -182/91 -164/109 na 0.466(l)
2 C2H6 (g) CH3CH3 ethane 30.1 -183/90 -88/185 na 0.572(l)
3 C3H8 (g) CH3CH2CH3 propane 44.1 -190/83 -42/231 na 0.585(l)
4 C4H10 (g) CH3CH2CH2CH3 butane 58.1 -138/135 0/273 na 0.601(l)
5 C5H12 (l) CH3(CH2)4CH3 pentane 72.2 -130/143 36/309 -49 0.626
6 C6H14 (l) CH3(CH2)4CH3 hexane 86.2 -95/178 69/342 -22 0.660
7 C7H16 (l) CH3(CH2)5CH3 heptane 100.2 -90/183 99/372 -4 0.684
8 C8H18 (l) CH3(CH2)6CH3 octane 114.2 -57/216 126/399 13 0.703
9 C9H20 (l) CH3(CH2)7CH3 nonane 128.3 -51/222 151/424 31 0.718
10 C10H22 (l) CH3(CH2)8CH3 decane 142.3 -30/243 174/447 46 0.730
11 C11H24 (l) CH3(CH2)9CH3 undecane 156.3 -25/248 196/469 60 0.740
12 C12H26 (l) CH3(CH2)10CH3 dodecane 170.3 -9/264 216/489 71 0.749
13 C13H28 (l) CH3(CH2)11CH3 tridecane 184.4 -5/268 234/507 102 0.756
14 C14H30 (l) CH3(CH2)12CH3 tetradecane 198.4 4/279 250/523 99 0.763
15 C15H32 (l) CH3(CH2)13CH3 pentadecane 212.4 10/283 267/540 132 0.769
16 C16H34 (l) CH3(CH2)14CH3 hexadecane 226.4 18/291 281/554 135 0.773
17 C17H36 (l) CH3(CH2)15CH3 heptadecane 240.5 22/295 302/575 148 0.777
18 C18H38 (s) CH3(CH2)16CH3 octadecane 254.5 28/301 326/599 165 0.777
19 C19H40 (s) CH3(CH2)17CH3 nonadecane 268.5 31/304 330/603 168 0.786
20 C20H42 (s) CH3(CH2)18CH3 eicosane 282.5 37/310 343/616 na 0.789
n molecular formula abbreviated structural formula name of alkane relative molecular mass Mr melting point temperature oC/K boiling point temperature oC/K flash point oC density g/cm3
  **************** ********************** ***************** ************** ******************* **************** ******** *********

STRUCTURE and REACTIONS

They are not very reactive unless burned!

The principal source of alkane hydrocarbons is crude oil - see section 2. (c) doc b Fractional distillation of crude oil & uses of fractions

The molecular structure of ALKANES

(1) is the molecular formula: a summary of the totals of each atom of each element in one molecule.

(2) is a 'shorthand' version of the full structural formula (3).

(3) is called the structural formula or displayed formula: it shows how all the atoms are linked by covalent bonds (the dashes ).

(4) is a '3D' representation of the structural formula (3) showing the spatial arrangement of the atoms.

(5) is a simple 'molecular diagram' showing the atoms as spheres joined together

Table showing the structure of the first four ALKANES name of alkane
(1)doc b oil notes   (3)doc b oil notes       (4)doc b oil notes methane

(main molecule in natural gas)

(1)doc b oil notes   (2)doc b oil notes   (3)doc b oil notes    (5)

(4)doc b oil notes      

ethane
(1)doc b oil notes   (2)doc b oil notes

(3)doc b oil notes       (4)alkanes structure and naming (c) doc b    (5)

propane

in bottled gas

(1)doc b oil notes   (2)doc b oil notes

(3)doc b oil notes    (5)

butane

in bottled gas

C5H12

C6H14

C7H16

pentane, hexane and heptane in petrol
C8H18

C9H20

octane and nonane
C10H22

  C11H24

C12H26

decane, undecane and dodecane

Advanced Level Notes on the Structure and Naming of Alkanes

 
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3b. The reactions of Alkanes

doc b oil notesThe complete combustion of hydrocarbons (excess air)

Introduction via coal combustion

  • The fuel coal consists mainly of carbon, which, if burned/ignited in excess air, combusts to form carbon dioxide.

    • carbon + oxygen ==> carbon dioxide

    • C(s) + O2(g) ==> CO2(g)

    • This is what you expect to happen in an open domestic coal fire.

  • If not enough air/oxygen is available, coal will only 'half' burn to form the deadly odourless, colourless and toxic gas carbon monoxide.

    • 2C(s) + O2(g) ==> 2CO(g)

    • This can happen if organic material, coal or peat is smouldering underground and is obviously a dangerous situation.

    • If it was formed in a domestic coal fire it will quite happily burn with a pale blue flame to form the 'safe' combustion product carbon dioxide gas.

    • top2CO(s) + O2(g) ==> 2CO2(g)

doc b oil notes

  • The diagram shows how to detect the products of hydrocarbon combustion e.g. burning candle wax.

  • When hydrocarbons are burned in air a fast exothermic reaction occurs releasing heat and forming carbon dioxide and water - their formation is an oxidation reaction.

  • It is an oxidation reaction because of oxygen atom gain by the carbon and hydrogen atoms of the hydrocarbon molecules.

  • The carbon dioxide is chemically detected with limewater - with which it forms a white precipitate (milky appearance) of calcium carbonate.

  • The water is chemically detected either by

    • (i) anhydrous white copper sulphate turning blue

    • or

    • (ii) dried blue cobalt chloride paper turning pink.

  • A physical test for water is to measure its boiling point (should be 100oC).

Equations for the complete combustion of a hydrocarbon

doc b oil notesWhen a hydrocarbon molecule (reactant) burns in an excess of air-oxygen their are only two products of the reaction. The carbon atoms are oxidised on combining with oxygen to form carbon dioxide molecules, and the hydrogen atoms are oxidised to water molecules ('hydrogen oxide'). This section ignores the combustion of the pollutant sulphur.

general word equation: hydrocarbon + oxygen ==> carbon dioxide + water

word equations e.g. methane + oxygen ==> carbon dioxide + water

and the corresponding symbol equation is

CH4(g) + 2O2(g) ==> CO2(g) + 2H2O(l)

Note that one CO2 for every C, and one H2O for every two H's in the hydrocarbon molecule.

doc b oil notes

topIn terms of displayed formula the equation would be written as ...

... in which every individual atom is shown and how it is bonded ('connected') with other atoms in the molecule. All the dashes represent the covalent bonds between the atoms in the molecules.

 

Another example is the combustion of propane ...

propane + oxygen ==> carbon dioxide + water

C3H8(g) + 5O2(g) ==> 3CO2(g) + 4H2O(l)

and in terms of displayed formula and balancing numbers ...

and the above diagrams show how the atoms have rearranged themselves in the reaction after the reactant bonds are broken (C-H, O=O and C-C in ethane etc. below)) and the new bonds formed in the products (C=O and O-H). Note the number of atoms of each element must be the same on each side of the equation (1C, 4H's and 4 O's, Law of Conservation of mass) and the products are different substances with different properties compared to the reactants. See Elements, Compounds and Mixtures page for more on (c) doc b writing and balancing equations

for ethane and butane the more awkward symbol equations are ...

 

ethane + oxygen ==> carbon dioxide + water

2C2H6(g) + 7O2(g) ==> 4CO2(g) + 6H2O(l)

or avoiding the 1/2 !

C2H6(g) + 31/2O2(g) ==> 2CO2(g) + 3H2O(l)

 

butane + oxygen ==> carbon dioxide + water

2C4H10(g) + 13O2(g) ==> 8CO2(g) + 10H2O(l)

or avoiding the 1/2 !

C4H10(g) + 61/2O2(g) ==> 4CO2(g) + 5H2O(l)

 

and for pentane the symbol equations is ...

 

pentane + oxygen ==> carbon dioxide + water

C5H12(l) + 8O2(g) ==> 5CO2(g) + 6H2O(l)

 

(c) doc b More on incomplete combustion is described on the pollution page section 4a

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3c. More on ALKANES - saturated hydrocarbons

  • These are obtained directly from crude oil by fractional distillation (see oil notes section 2).
  • The saturated hydrocarbons form an homologous series called alkanes with a general formula CnH2n+2
  • Saturated means the molecule has no C=C double bonds, only carbon-carbon single bonds, and so has combined with the maximum number of atoms i.e. no atoms can add to it. The alkanes don't really have a functional group and have quite a limited chemistry BUT they are still a clearly defined homologous series.
  • Alkane examples: The gases (names and molecular formula): methane CH4, ethane C2H6, propane C3H8, butane C4H10, liquids: pentane C5H12, hexane C6H14 etc.
  • The first four alkane structures are shown above and the names end in ...ane
  • Carbon always forms 4 bonds with other atoms and hydrogen 1 bond with other atoms e.g. Propane: molecular formula C3H8, structural and displayed formula styles include ...
    • or or
  • Isomerism occurs when two or more compounds have the same chemical formula but have different structures. e.g. for the molecular formula C4H10 there are two possibilities - one 'linear' and one with carbon chain 'branching', both are shown in three ways ...
    • butane:
    • or or
    • and its isomer methylpropane:
    • or or
  • topCan you work out the structures of the 3 isomers of C5H12 ? (you will find enough to work out the answers on the Advanced Level page ALKANES)
  • Isomers show variation in physical properties which depend upon the strength of the intermolecular forces. Intermolecular forces are due to weak electrical attractive forces that exist between all molecules.
  • (a) For a homologous series the strength of intermolecular forces increases as the carbon chain length increases
  • (b) For isomers (same C number, molecular formula), the forces decrease as the amount of chain branching increases.
  • This is because the attractive forces are a function of the potential surface-surface contact i.e. the compactness of the molecules.
    • (a) as the chain length increases the surface-surface contact must increase per molecule,
    • (b) for isomers, with more branching, the chain length decreases and the molecule is more 'compact' reducing the surface-surface contact per molecule.
  • For example in the series ...
    • (a) from methane ... ethane ... propane ... petrol ... oils ... grease ... waxes etc. the melting point/boiling point rises and so does the viscosity (stickiness! less runny, more sticky) as the carbon chain length increases.
    • This trend also indicated by the change from gases to liquids to solids ...
      • ... illustrated above by the boiling points of hydrocarbons obtained from crude oil.
      • See 2. Uses of Oil Products page for more details - the use of alkanes is very strongly linked to their physical properties.
    • (b) 'linear' butane has a higher boiling point than the 'branched' methylpropane (diagrams above).
  • Alkanes and alkenes undergo combustion reactions (see above).
  • CHLOROALKANES (halogenoalkanes): Alkanes are usually not very reactive unless burned! BUT they will react with reactive chemicals like chlorine when heated or subjected to uv light to form chlorinated hydrocarbons.
    • Despite the reactivity of chlorine you still need something extra to initiate the reaction.
    • A substitution reaction occurs and a chloro-alkane is formed e.g.
    • a hydrogen is swapped for a chlorine and the hydrogen combines with a chlorine atom e.g.
      • methane + chlorine ==> chloromethane + hydrogen chloride
        • CH4 + Cl2 ==> CH3Cl + HCl
        • alkanes structure and naming (c) doc b + Cl2 ==> (c) doc b + HCl
      • ethane + chlorine ==> chloroethane + hydrogen chloride
        • C2H6 + Cl2 ==> C2H5Cl + HCl
        • + Cl2 ==> + HCl
    • Chloromethane and chloroethane are gases at room temperature, but bigger chloro-alkane molecules are useful solvents in the laboratory or industry but they are still quite volatile and chlorohydrocarbon vapours can be harmful if breathed in.

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Multiple Choice Quizzes and Worksheets

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (easier-foundation-level)

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (harder-higher-level)

KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry

and (c) doc b 3 linked easy Oil Products gap-fill quiz worksheets

ALSO gap-fill ('word-fill') exercises originally written for ...

... AQA GCSE Science (c) doc b Useful products from crude oil AND (c) doc b Oil, Hydrocarbons & Cracking etc.

... OCR 21st C GCSE Science (c) doc b Worksheet gap-fill C1.1c Air pollutants etc ...

... Edexcel 360 GCSE Science Crude Oil and its Fractional distillation etc ...

... each set are interlinked, so clicking on one of the above leads to a sequence of several quizzes

Advanced Level Organic Chemistry revision notes


Revise KS4 Science GCSE/IGCSE/O level Chemistry Revision-Information Study Notes for revising for AQA GCSE Science, Edexcel GCSE Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science WJEC/CBAC GCSE science-chemistry CCEA/CEA GCSE science-chemistry (and courses equal to US grades 8, 9, 10)


equation keywords: O2 CH4 + 2O2 ==> CO2 + 2H2O H2O * C3H8 + 5O2 ==> 3CO2 + 4H2O * 2C2H6 + 7O2 ==> 4CO2 + 6H2O * C2H6 + Cl2 ==> C2H5Cl + HCl * CH4 + 2O2 ==> CO2 + 2H2O * C3H8 + 5O2 ==> 3CO2 + 4H2O * 2C2H6 + 7O2 ==> 4CO2 + 6H2O * C2H6 + Cl2 ==> C2H5Cl + HCl *


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