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In this experiment we measure the enthalpy change of combustion of a sequence of alcohols. If we look at the bonds broken and formed when one mole of two consecutive alcohols are burnt we find that there is always the same difference in bonds involved. An extra mole of C-C, two moles of C-H and 1½ moles of O=O bonds are broken, whereas an extra two moles of C=O and two moles of O-H bonds are formed. We should imagine that there should be a fairly fixed difference in the enthalpies of combustion of two consecutive alcohols. Draw out the displayed formulae for all the products and reactants in the following two reactions. Then convince yourself that the difference in the bonds made and broken is that given above. Be careful, students often make the mistake of assuming that C4H9OH will have four C-C bonds. It only has three - this is obvious if you draw it out.
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C4H9OH + 6O2 ⇒ 4CO2 + 5H2O |
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C5H11OH + 7½O2 ⇒ 5CO2 + 6H2O |
In this experiment we shall use propan-1-ol in a small burner to heat up the water in a calorimeter by about 10°C. The burner is extinguished. By weighing it before and after combustion we can work out the mass, and therefore the number of moles, of alcohol which have burnt. We then use the data book value for the enthalpy of combustion of propan-1-ol (2021.0 kJ mol-1) to calibrate the apparatus. We shall work out the heat capacity of the calorimeter - that is the number of joules required to raise its temperature by 1°C.
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In the second part of the experiment we use octan-1-ol to heat up the same apparatus, containing the same water, by a similar temperature rise. The idea is that any heat lost in the first part of the experiment (for example, hot air through the pipe) will also be lost during the second part, and so measured. We can calculate the heat produced from the temperature rise and the heat capacity that we worked out in the first part of the experiment. Knowing the number of joules and the number of moles we can work out the enthalpy change in kilojoules per mole for the octan-1-ol. |
Watch the following video and collect the data that you will require. There is quite a lot to write down - four masses and four temperatures. Try to calculate a value for the enthalpy of combustion of octan-1-ol. Compare your answer with that in the data book and try to explain any difference. Make a list of the errors that could occur in this experiment and try to work out what effect they would have on your final answer. Tabulate from the data book the enthalpies of combustion of the alcohols, methanol to octan-1-ol. Is there a regular difference in enthalpy of combustion? You can check all your answers by clicking on the link below.
Video - measuring enthalpy change of combustion of octan-1-ol
