Advanced Level Organic Chemistry: Complete & incomplete combustion of ALKANES
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1.4 Complete and incomplete combustion of alkanes and environmental pollution
Part 1. ALKANES and the PETROCHEMICAL INDUSTRY - Doc Brown's Advanced A Level Organic Chemistry Revision Notes
What are the products of complete combustion of alkanes - How to balance alkane combustion equations - The pattern - trend of enthalpy of combustion of alkanes
All the basics with lots of equations are described on
EXTRA NOTES for Advanced A Level Chemistry students
Complete combustion - enthalpy of combustion trend in alkanes
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)
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 632670kJ 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 mol1.
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 653658 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) (note the use of a 1/2, no problem, think mole ratios!)
for heptane: C7H16(g) + 11O2(g) ==> 7CO2(g) + 8H2O(l)
So the only products are water and carbon dioxide. (note the use 6½ is perfectly legitimate)
Formation of acidic oxides and environmental air pollution
Doc Brown's Chemistry
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