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
All Advanced Organic
Chemistry Notes
Index of GCSE/IGCSE Oil - Useful Products
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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
4.4.2
The enthalpies of combustion of linear aliphatic alcohols
The standard enthalpies of
complete combustion (ΔHθcomb at 298K, 1 atm = 101kPa) are listed below (4
sf)
C. no. |
alcohol |
formula of '1-ol' primary alcohols |
ΔHθcomb
in kJ/mol |
1 |
methanol |
CH3OH |
–726 |
2 |
ethanol |
CH3CH2OH |
–1367 |
3 |
propan–1–ol |
CH3(CH2)2OH |
–2021 |
4 |
butan–1–ol |
CH3(CH2)3OH |
–2676 |
5 |
pentan–1–ol |
CH3(CH2)4OH |
–3329 |
6 |
hexan–1–ol |
CH3(CH2)5OH |
–3984 |
7 |
heptan–1–ol |
CH3(CH2)6OH |
–4638 |
8 |
octan–1–ol |
CH3(CH2)7OH |
–5294 |
Graph
interpretation and comments
For
alkanes and linear primary alcohols, the graph of
ΔHθcomb
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 ΔHc 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
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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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