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

See also all the basics with lots of equations are described on

ALKANES - saturated hydrocarbons - basic introduction - complete combustion

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

Air pollution - effects of sulfur oxides and nitrogen oxides including a local acid rain project!

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 ΔH^θ_comb (at 298K, 1 atm = 101kPa) are listed below (4 sf)

General formula of these homologous series: Alkanes C_nH_2n+2

Graph interpretation and comments

The graph of ΔH_comb 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 ΔH_c 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 ΔH_vap(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

Thermochemistry – Hess's Law calculations, enthalpies of reaction, combustion, formation etc.

Bond Enthalpy Calculations

See also Calorimeter methods of determining energy changes - burning fuels

and Experimental methods for determining enthalpy changes and treatment of results

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

alkanes: C_nH_2n+2(g/l) + (1¹/₂n + ¹/₂)O_2(g) ===>  nCO_2(g) + (n + 1)H₂O_(l)

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

 methane + oxygen ==> carbon dioxide + water

CH_4(g) + 2O_2(g) ==> CO_2(g) + 2H₂O_(l)

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

(handy equation style for solving bond enthalpy calculations)

for propane: C₃H_8(g) + 5O_2(g) ==> 3CO_2(g) + 4H₂O_(l)

for butane: C₄H_10(g) + 6¹/₂O_2(g) ==> 4CO_2(g) + 5H₂O_(l)  

 (note the use of a ¹/₂, no problem, think mole ratios!)

for heptane: C₇H_16(g) + 11O_2(g) ==> 7CO_2(g) + 8H₂O_(l)

So the only products are water and carbon dioxide. (note the use 6½ is perfectly legitimate)

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

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)