Comparing the Enthalpy Changes of Combustion of Different Alcohols

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Comparing the Enthalpy Changes of Combustion of Different Alcohols

By Steven Duong

Background Information

                   The enthalpy change of the combustion of a fuel depends on two things. First, there is the number of bonds to be broken and made – and that depends on the size of the molecule involved.

The ΔH°c also depends on the type of bonds involved:

e.g. combustion of a) methane and b) methanol

a)  CH4  +  2O2          CO2  +  2H2O

       H               O=O                    O=C=O      H-O-H

                         O=O                                       H-O-H

H     C     H



b)  CH3OH  +  1½O2          CO2  +  2H2O

          H                        O=O                 O=C=O     H-O-H

                                     O                                        H-O-H

  H      C     O - H


                   From the examples shown above, the products are the same, but the key difference is that methanol already has an O-H bond; one of the bonds to oxygen is already made, unlike with methane, where all the new bonds have been made. The energy released during combustion comes from the making of bonds to oxygen. If methanol already has one bond made, it will give out less energy when it burns.

                   The enthalpy change of a fuel is a measure of the energy transferred when one mole of the fuel burns completely. A value for the enthalpy change can be obtained by using the burning fuel to heat water and using the fact that 4.17J of energy are required to raise the temperature of 1g of water by 1ºC.

                   So, the aim of this investigation is to find the enthalpy change of combustion of a number of alcohols so that you can investigate how and why the enthalpy change is affected by the molecular structure of the alcohol.

Preliminary Investigation

                   To give a clear indication of how to conduct the experiment precisely and properly, preliminary tests may be required. The results from these particular tests may give you an indication as to what type of range and temperature difference you may wish to use in your actual experiment.

Combustion of methanol: test 1

  • Mass of water = 60ml
  • Temperature difference = 68ºC
  • Difference in weight of methanol = 2.2g
  • Weight of calorimeter = 65.4g
  • Specific heat capacity (SHC) of water = 4.17 Jg-1K-1
  • SHC of copper = 0.38 Jg-1K-1

                   We have already established that the aim of this experiment is to investigate how and why the enthalpy change is affected by the molecular structure of the alcohol. To do so, you need to burn a certain alcohol under a calorimeter containing water and calculate the energy enthalpy change being discharged. During the experiment, you will notice that heat energy is not only lost as steam, from the water evaporating, but also from the copper calorimeter, which absorbs some of the heat energy.

Using the results above, to calculate the total energy enthalpy change, you add the energy gained by the copper with the energy given when heating the water…

Total enthalpy change = mass x specific heat capacity x difference in temperature

          E                        =    m                    c                                     ΔT

         (J)                             (g)                (Jg-1K-1)                           (ΔK)       

Enthalpy change for calorimeter:                   Enthalpy change for water:

= 65.4 x 0.38 x 68                                           = 60 x 4.17 x 68

= 1689.9 J                                                    = 17013.6 J

So, 1689.9 + 17013.6 = 18703.5 J

Having calculated the total energy, you need to calculate the moles of the alcohol, in order to calculate the enthalpy change per mol.

e.g. methanol (using the results from above)

mol = mass ÷ relative molecular mass

       = 2.2 ÷ 32

     = 0.07 M

So, energy per mol = 18703.5 ÷ 0.07 = 267192.9 Jmol-1

Combustion of methanol: test 2

Total energy = 18703.5 J

Mol of methanol = 0.03 M

Energy per mol = 544101.9 Jmol-1

                   The results obtained from the preliminary experiments indicate that the enthalpy change is fairly high. So, to get a clearer view of the enthalpy changes of combustion of the other alcohols, certain variables may need to be adjusted. For example, in this particular experiment, the temperature difference was 68ºC, there is no need for this to be quite high so a 20ºC temperature difference may be more sensible, thus allowing you to compare the results from the combustion of other alcohols with ease, producing a suitable graph of results.

                   Another reason why the enthalpy change of the combustion of methanol in this experiment is fairly high may be because of what is known as ‘temperature lag’. This occurred mainly because during the experiment, the burner was extinguished as soon as the temperature on the thermometer reached a 68ºC difference; by doing so, you are allowing the temperature to rise a little further, because the water is still being heated and so you do not gain 68ºC worth of the energy, instead you receive more, resulting in inaccuracy of results. To ensure that this does not happen in the real experiments, you need to extinguish the burner before the temperature reaches a difference of 20ºC, thus improving the accuracy of results.


                   We cannot measure the enthalpy of a substance. What we can do is measure the change in enthalpy when a reaction occurs. The enthalpy change in a chemical reaction gives the quantity of energy transferred to or from the surroundings, when the reaction is carried out in an open container. Like most physical and chemical quantities, enthalpy change varies according to the conditions. In particular, enthalpy change is affected by temperature, pressure and concentration of solutions.

                   In this particular experiment, temperature is a key factor and so the measurement of the temperature difference will be 20ºC and this would need to be kept the same throughout each test; failure to do so will result in inaccuracy of results. With that said, the range to control would indeed apply to the five different alcohols, each with a molecular difference of one carbon atom in their atomic structure. The alcohols are: methanol, ethanol, propan-1-ol, butan-1-ol and pentan-1-ol. Obviously, repeats of each test per alcohol would need to be considered. Preferably 3 tests per alcohol, enabling you to average the results, therefore making a suitable comparison, thus improving the reliability of the results obtained.

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All combustion reactions are exothermic, so the enthalpy change of combustion is always negative.

e.g. measuring bond enthalpies of the combustion of methanol (finding the expected value of enthalpy change)

CH3OH  +  1½O2          CO2  +  2H2O


NOTE: when interpreting a combustion equation, you normally take in account of the combustion of 1 mol of the fuel.

Calculating the bond enthalpies before and after the reaction of the combustion of methanol…

Before                                 ...

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