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Pre-university Advanced A Level Organic Chemistry: The enthalpy of combustion of alcohols

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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 a comparison of the combustion of isomeric ethers and alcohols equations and standard enthalpies of combustion

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

(including a few comments on the combustion of ethers and accompanying equations)

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Sub-index for this alcohol chemistry page 4.4

4.4.1 The combustion of alcohols and calorimetry

4.4.2 The enthalpies of combustion of linear aliphatic alcohols

4.4.3 The manufacture and use of alcohols as fuels

4.4.4 Comparison of the complete combustion of alcohols and their isomeric ethers


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) + 3O2(g) ====> 2CO2(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) + 3O2(g) ====> 2CO2(g) + 4H2O(l)
      • (iii)  propan-1-ol + oxygen ====> carbon dioxide + water
        • 2CH3CH2CH2OH(l) + 9O2(g) ====> 6CO2(g) + 8H2O(l)
      • (iv)  butan-1-ol + oxygen ====> carbon dioxide + water
        • CH3CH2CH2CH2OH(l) + 6O2(g) ====> 4CO2(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.
    • (c) doc bMeasuring 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 89oC.

      • The temperature rise = 89 – 18 = 71oC (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 SHCwater x temperature change

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

      • Mr(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 = ΔHc(ethanol) = –923 kJmol–1  (only accurate to 3 sf at best)

      • for the reaction: CH3CH2OH(l) + 3O2(g) ===> 2CO2(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.


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4.4.2 The enthalpies of combustion of linear aliphatic alcohols

The standard enthalpies of complete combustion (ΔHθcomb at 298K, 1 atm = 101kPa) from NIST 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 of enthalpy of combustion of alcohols explaining trend advanced organic chemistry revision notes doc brown

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.

I don't think this is particularly important, but it may due to the highest H/C ratio or the fact that the first molecule in each series doesn't have a C-C bond, whereas the rest have a carbon chain of >1 C atoms.

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.

 


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4.4.3 The manufacture and use of alcohols as fuels

For the industrial production of ethanol see

Alcohols - manufacture of ethanol (basic notes)

The laboratory synthesis and manufacture of alcohols (extra advanced level notes)

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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4.4.4 Comparison of the complete combustion of alcohols and their isomeric ethers

(l) or (g) indicate the physical state of the reactants and products.

The enthalpy values are the standard enthalpy of combustion ΔHθcomb (ΔHθc) in kJ/mol at 298K and 101 kPa/1 atm. pressure.

The ΔHθcomb values for isomeric alcohols are quite similar.

The ΔHθcomb values for isomeric ethers are quite similar.

However, between the two groups of isomers, for a given molecular formula, the enthalpy of combustion of ethers tends to be higher

This is partly accounted for by the one difference in bonding.

Bond enthalpies in kJ/mol:  C-O 360  and  O-H 463, difference 103 kJ/mol.

Compared to ethers, alcohols have a strong O-H bond instead of a 2nd weaker C-O bond.

Ethers have no strong O-H bond, but two weaker C-O bonds.

The difference in the C-O and O-H bond enthalpies means that alcohols start of at a lower potential energy (enthalpy H) than ethers and so less energy will be released on combustion of alcohols compared to ethers.

Therefore you might expect the ether enthalpies of combustion to be ~100 kJ/mol higher.

This is born out by the ΔHθcomb enthalpy values listed below alongside the combustion equation.

In fact for C2 to C4 isomers, the ΔHθc difference ranges from 73 to 102 kJ/mol higher for ethers.

2 isomers of molecular formula C2H6O

ethanol : CH3CH2OH(l) + 3O2(g) ===> 2CO2(g) + 3H2O(l)                 (ΔHθc = -1367 kJ/mol)

methoxymethane : CH3OCH3(g) + 3O2(g) ===> 2CO2(g) + 3H2O(l)   (ΔHθc = -1460 kJ/mol)

The ether ΔHθc value is 93 kJ/mol higher.

In this case, the ether value is higher for a 2nd reason, it is already a gas and so no energy is needed to vapourise it, unlike in the case of liquid ethanol.

3 isomers of molecular formula C3H8O

propan-1-ol : CH3CH2CH2OH(l) + 4½O2(g) ===> 3CO2(g) + 4H2O(l)         (ΔHθc = -2021 kJ/mol)

propan-2-ol : CH3CH(OH)CH3(l) + 4½O2(g) ===> 3CO2(g) + 4H2O(l)         (ΔHθc = -2005 kJ/mol)

methoxymethane : CH3CH2OCH3(g) + 4½O2(g) ===> 3CO2(g) + 4H2O(l)   (ΔHθc = -2107 kJ/mol)

The ether ΔHθc value is 86-102 kJ/mol higher.

Again, the ether value is higher for a 2nd reason, it is already a gas and so no energy is needed to vapourise it, unlike in the case of the liquid propanols.

7 isomers of molecular formula C4H10O

butan-1-ol : CH3CH2CH2CH2OH(l) + 6O2(g) ===> 4CO2(g) + 5H2O(l)            (ΔHθc = -2676 kJ/mol)

butan-2-ol : CH3CH2CH(OH)CH3(l) + 6O2(g) ===> 4CO2(g) + 5H2O(l)            (ΔHθc = -2670 kJ/mol)

2-methylpropan-1-ol : (CH3)2CHCH2OH)(l) + 6O2(g) ===> 4CO2(g) + 5H2O(l)  (ΔHθc = -2669 kJ/mol)

2-methylpropan-2-ol : (CH3)3COH)(l) + 6O2(g) ===> 4CO2(g) + 5H2O(l)            (ΔHθc = -2644 kJ/mol)

ethoxyethane : CH3CH2OCH2CH3(l) +  6O2(g) ===> 4CO2(g) + 5H2O(l)           (ΔHθc = -2727 kJ/mol)

1-methoxypropane : CH3CH2CH2OCH3(l) +  6O2(g) ===> 4CO2(g) + 5H2O(l)   (ΔHθc = -2737 kJ/mol)

2-methoxypropane : (CH3)2CHOCH3(l) +  6O2(g) ===> 4CO2(g) + 5H2O(l)         (ΔHθc = -2750 kJ/mol)

The average alcohol ΔHθc value is ~ 2665 kJ/mol

The average ether ΔHθc values is ~2738 kJ/mol

The ether ΔHθc values are on average ~73 kJ/mol higher.


See also other thermochemistry pages

Advanced Introduction to enthalpy changes – enthalpies of reaction, formation, combustion

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

Bond Enthalpy Calculations

Experimental methods for determining enthalpy changes and treatment of results

Enthalpy data patterns - combustion of alkanes linear aliphatic alcohols, bond enthalpies and bond Length

Enthalpies of neutralisation, enthalpies of hydrogenation and evidence of aromatic ring structure in benzene

Extra enthalpy calculations question page A set of practice enthalpy calculations with worked out answers


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