Doc Brown's A Level Organic Chemistry exam revision notes Part 1 ALKANES and PETROCHEMICAL INDUSTRY

1.4 Complete and incomplete combustion of alkanes and environmental pollution

 ALL my advanced A level organic chemistry notes

 All my advanced A level ALKANE chemistry notes

 Index of GCSE level Oil - Useful Products Revision Note

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

What are the products of complete combustion of alkanes - How to balance alkane combustion equations - The pattern - trend of enthalpy of combustion of 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 for complete combustion of alkanes

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)

C_nH_2n+2(g/l) + (1.5n + 0.5)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 of 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

Formation of carbon monoxide and soot in incomplete combustion

• If there is not enough oxygen present to completely burn the fuel to carbon dioxide and water other products may form causing pollution and fuel inefficiency.

  • This is referred to as incomplete combustion.

  • Visually, blue flames indicate complete combustion releasing lots of heat energy, but smokey yellow flames indicate incomplete combustion releasing less energy.

• Carbon monoxide is a toxic gas and unfortunately it is colourless and odourless and so not easily detected.

  • Carbon monoxide combines with haemoglobin in the blood, reducing its capacity to carry oxygen.

  • CO molecules will displace O₂ oxygen molecules from haemoglobin depriving cells of the essential oxidant O₂ !

  • Lack of oxygen can lead to feeling weak, fainting and reduction of brain function. If you breath in more than traces over a period of time you can go into a coma and death - CO is pretty toxic and lethal dose isn't that high!

• The most common partially burned products are likely to be soot (mainly carbon particles) and deadly carbon monoxide CO.

  • A simplified word equation covering most 'pollution' possibilities is ...

    • hydrocarbon + oxygen (air) ==> carbon (soot) + carbon monoxide + carbon dioxide + water

  • Carbon-soot, a fine black powder-dust is potentially harmful and readily formed in fires i.e. its classically produced by smoky yellow flames and inefficient motor vehicle engines.

• Incomplete combustion is still exothermic, but there is also less heat released in incomplete combustion compared to complete combustion since not all the carbon atoms of the fuel are fully combined with the maximum amount of oxygen.

  • e.g. for the incomplete combustion of methane possible word and symbol equations are as follows ...

    • (a) methane + oxygen ====> carbon (soot) + water

      • CH_4(g) + O_2(g) ====> C_(s) + 2H₂O_(l)

    • and

    • (b) methane + oxygen ====> carbon monoxide + water

      • CH_4(g) + 1.5O_2(g) ====> CO_(g) + 2H₂O_(l)

    • For larger hydrocarbon molecules all sorts of equations can be written showing the formation of carbon-soot particles and carbon monoxide molecules e.g.

    • A liquid alkane in petrol:

      • (i) C₇H₁₆(l)  +  4O₂(g)  ===>  7C(s)  +  8H₂O(l)

      • (ii) C₇H₁₆(l)  +  7.5O₂(g)  ===>  7CO(g)  +  8H₂O(l)

      • Some of these hydrocarbons can pass through the engine without being oxidised.

      • Unburned hydrocarbons belong to a group of chemicals known as volatile organic compounds (VOCs).

    • Note again that hydrogen is preferentially oxidised

For a more detailed discussion on pollution and harmful effects of hydrocarbon combustion see

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)