Part 4. The chemistry of ALCOHOLS

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

Part 4.4 The Combustion of alcohols - products, equations, enthalpies of combustion and use as fuels

INDEX of notes on ALCOHOLS chemistry

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4.4.1 The combustion of alcohols and calorimetry

• When burned, ethanol, like any alcohol, on complete combustion forms carbon dioxide and water
  • (i)  ethanol + oxygen ==> carbon dioxide + water
    • CH₃CH₂OH_(l) + 3O_2(g) ====> 2CO_2(g) + 3H₂O_(l)
    • As mentioned in section 9b Fuels Survey, ethanol can be blended with petrol to fuel road vehicles.
  • Similarly, but the symbol equations are more awkward to balance  ...
    • (ii)  methanol + oxygen ====> carbon dioxide + water
      • 2CH₃OH_(l) + 3O_2(g) ====> 2CO_2(g) + 4H₂O_(l)
    • (iii)  propan-1-ol + oxygen ====> carbon dioxide + water
      • 2CH₃CH₂CH₂OH_(l) + 9O_2(g) ====> 6CO_2(g) + 8H₂O_(l)
    • (iv)  butan-1-ol + oxygen ====> carbon dioxide + water
      • CH₃CH₂CH₂CH₂OH_(l) + 6O_2(g) ====> 4CO_2(g) + 5H₂O_(l)
    • See also a Fuels Survey, ethanol can be blended with petrol to fuel road vehicles.
    • As already mentioned, ethanol is used in sprit burners where it burns much more cleanly with a blue flame - using a hydrocarbon in the same situation is more smelly and gives a more yellow smokey flame - less efficient combustion.
  • Measuring the enthalpy of combustion of alcohols
    • This can be determined using the simple copper calorimeter (diagram on the right).
    • You can compare the heat energy released by different alcohols e.g. methanol, ethanol, propanol and butanol.
    • The alcohol is poured into a little spirit burner which is then weighed.
    • The burner is placed under the copper calorimeter, which is filled with a known mass of water at a known start temperature.
    • After burning for e.g. 5 minutes, the flame is blown out and the final temperature noted.
    • The burner reweighed and the mass decrease is equal to the mass of alcohol burned.
    • From the mass of water, the heat capacity of water and the temperature change you can work out the heat energy released.
    • You can then work out the heat released per gram or per mole of alcohol.
    • Or more simply, you might just compare the mass of fuel burned to give the same temperature rise.
    • The results should show that the efficiency of an alcohol fuel increases with increase in carbon chain length of the molecule i.e. pentanol > butanol > propanol > ethanol > methanol.
    • For lots more details on the method and calculations see
    • methods of measuring heat energy transfers in chemical reactions

  • Determining the enthalpy of combustion of an alcohol

    • 100 cm³ of water (100g) was measured into the calorimeter.

    • The spirit burner contained the fuel ethanol CH₃CH₂OH ('alcohol') and weighed 18.62g at the start.

    • After burning it weighed 17.14g and the temperature of the water rose from 18 to 89^oC.

    • The temperature rise = 89 – 18 = 71^oC (exothermic, heat energy given out).

    • Mass of fuel burned = 18.62–17.14 = 1.48g.

    • Heat given out to the water = mass of water x SHC_water x temperature change

      • = 100 x 4.18 x 71 = 29678 J (for 1.48g)

    • M_r(ethanol) = 46 (H=1, C=12, O=16)

    • Therefore 1.48g ethanol = 1.48/46 = 0.03217 mol

    • So, scaling up to 1 mole of ethanol burned gives 29678 x 1 / 0.03217 = 922536 J

    • Enthalpy of combustion of ethanol = ΔH_c(ethanol) = –923 kJmol^–1  (only accurate to 3 sf at best)

    • for the reaction: CH₃CH₂OH_(l) + 3O_2(g) ===> 2CO_2(g) + 3H₂O_(l)

    • The data book value for the heat of combustion of ethanol is –1367 kJmol^–1, showing lots of heat loss in the experiment!

    • It is possible to get more accurate values by calibrating the calorimeter with a substance whose energy release on combustion is known.

     

4.4.2 The enthalpies of combustion of linear aliphatic alcohols

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

Graph interpretation and comments

For alkanes and linear primary alcohols, the graph of ΔH^θ_comb versus the number of carbon atoms shows an almost linear relationship as the combustion of each extra –CH₂– unit in the carbon chain 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.

In the case of the first 8 alcohols, all liquids at 298K 101kPa, apart from the incremental rise of 641 kJ from methanol to ethanol, all the other incremental rises up this homologous series are 653–656 kJ and these are completely consistent with incremental rises you see for alkanes.

For the same carbon number (n) the values for alcohols 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.

4.4.3 The use of alcohols as fuels

For the industrial production of ethanol see

Alcohols - manufacture of ethanol

For a discussion on the use of ethanol as a biofuel from fermentation (biosynthetic route) or blending ethanol from the petrochemical industry with petrol see:

Biofuels & alternative fuels, hydrogen, biogas, biodiesel

Doc Brown's Advanced Level Chemistry Revision Notes

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INDEX of notes on ALCOHOLS chemistry

 All Advanced Organic Chemistry Notes

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

 

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