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Summary of the Properties and Uses of Hydrocarbons

Name Molecular Formula Molecular Mass Melting Point (oC) Boiling Point (oC) State (25oC, 101.3kPa) Density (liquid g cm-3, 20oC) Flashpoint (oC) Enthalpy of Combustion (kJ mol-1) Uses
methane CH4 16 -182 -162 gas     -889 major component of natural gas (fuel)

ethane C2H6 30 -183 -88.6 gas     <-1557 component of natural gas (fuel)

propane C3H8 44 -188 -42.1 gas     -2217 component of liquefied petroleum gas (LPG), bottled gas (fuel)

butane C4H10 58 -138 -0.5 gas     -2874 component of liquefied petroleum gas (LPG), cigarette lighters (fuel)

pentane C5H12 72 -130 36.1 liquid 0.626 -49 -3536 component of petrol (fuel)

hexane C6H14 86 -95.3 68.7 liquid 0.659 -22 -4190 component of petrol (fuel)

heptane C7H16 100 -90.6 98.4 liquid   -4 -4847 component of petrol (fuel)

octane C8H18 114 -56.8 126 liquid     -5506 major component of petrol (fuel)

nonane C9H20 128             component of petrol (fuel)

decane C10H22 142 -30 174 liquid 0.730     component of petrol (fuel)

hexadecane C16H34 226 18.5 288 liquid 0.775     component of diesel fuel & heating oil

eicosane C20H42 282 36 343 solid        
 

Trends

  • Alkanes are colourless

    methane to butane are colourless gases
    (propane and butane are easily condensed under pressure & are commonly sold as liquids)
    alkanes containing 5 carbons up to about 19 are colourless liquids
    (petrol & kerosene are mixtures of liquid alkanes, dye is added to the fluids for safety reasons)
    alkanes with more than about 20 carbon atoms are colourless, waxy solids
    (paraffin wax is a mixture of solid alkanes)

  • Alkanes are less dense than water (alkanes will float on top of water)

    density increases with increasing molecular mass

  • Simple alkanes have low melting and boiling points.

    Alkanes are non-polar so only weak intermolecular forces act between the alkane molecules
    (Van der Waal's Forces/London Forces/Dispersion Forces/Weak Intermolecular Forces)
    Melting and Boiling Points increase as the molecular mass increases

  • Alkanes are insoluble in polar solvents like water

  • Alkanes are relatively unreactive

    (they will combust: commonly used as fuels since large amounts of energy are released, the longer the chain, the more bonds are broken, the greater the energy released)
    (will undergo halogenation by substitution reaction in the presence of ultra-violet light)

  • Alkanes with *flashpoints below room temperature (the components of petrol for example) should be stored in strong metal containers with narrow mouths & tightly sealed lids to prevent the vapour from escaping & to prevent a naked flame or spark from igniting the vapour/air mixture.


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*Flashpoint: the minimum temperature at which the vapour pressure of a liquid is high enough for an explosive mixture to be formed with air. Safety precautions for handling & storing fuels are determined by the flashpoint.

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