Part 1. ALKANES and the PETROCHEMICAL INDUSTRY

Doc Brown's Advanced A Level Organic Chemistry Revision Notes

1.4 Complete and incomplete combustion of alkanes and environmental pollution

What are the products of complete combustion of alkanes.

How to balance alkane combustion equations.

The pattern - trend of enthalpy of combustion of alkanes


Alkanes and Petrochemical Industry INDEX

All Advanced Organic Chemistry Notes

Index of GCSE/IGCSE Oil - Useful Products Chemistry Revision Notes

A basic introduction to the chemistry of alkanes


Complete combustion

The combustion of linear alkanes and linear aliphatic alcohols

The standard enthalpies of complete combustion ΔHθcomb (at 298K, 1 atm = 101kPa) are listed below (4 sf)

Carbon number name alkane ΔHθcomb
1 methane CH4 –890
2 ethane C2H6 –1560
3 propane C3H8 –2219
4 butane C4H10 –2877
5 pentane C5H12 –3509
6 hexane C6H14 –4163
7 heptane C7H16 –4817
8 octane C8H18 –5470

General formula of these homologous series: Alkanes CnH2n+2

Graph interpretation and comments

The graph of ΔHcomb versus the number of carbon atoms shows an almost linear relationship as the combustion of each extra –CH2– unit usually contributes an extra 632–670kJ to the molar enthalpy of combustion. The first incremental rise in ΔHc from C1 to C2 is slightly anomalous in both homologous series compared to the general trend.

For the first 8 alkanes, this incremental rise ranges from 632 kJ to 670 kJ. For methane ==> ethane the incremental rise is 670 kJ. The increment for butane ==> pentane is 632 kJ and this lesser incremental rise corresponds to a the first change in state involved i.e. some of the energy released on burning pentane must be used to vapourise it and evaporation is an endothermic process. In fact ΔHvap(C5H12) is +36 kJ mol–1. This absorbed energy is not required by methane ==> butane which are already in the gaseous state. Apart from these two small anomalies all the other incremental rises are 653–658 kJ.

You get a similar trend for linear aliphatic alcohols, but the values for alcohols for the same carbon number are slightly smaller than those for alkanes because the alcohols are already partially oxidised i.e. the presence of a single oxygen atom in each alcohol molecule.

For more on enthalpy changes see Energetics-Thermochemistry-Thermodynamics Notes INDEX

 

The general equations for complete combustion can be represented as ... (n = 1, 2, 3 etc.)

alkanes: CnH2n+2(g/l) + (11/2n + 1/2)O2(g) ===>  nCO2(g) + (n + 1)H2O(l)

Complete combustion means complete oxidation (===> carbon +4, hydrogen +1, oxygen -2)

 methane + oxygen ==> carbon dioxide + water

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

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

  (handy equation style for solving bond enthalpy calculations)

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

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

So the only products are water and carbon dioxide.

ALKANES - saturated hydrocarbons - section on complete combustion with equations

Greenhouse effect, global warming, climate change, carbon footprint from fossil fuel burning

 


Incomplete combustion

Fossil fuel air pollution - incomplete combustion, carbon monoxide & soot particulates

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Formation of acidic oxides and environmental air pollution

Fossil fuel air pollution - effects of sulfur oxides and nitrogen oxides

(includes the chemistry of catalytic converters and gas desulfurization of power station flue gases)

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