Question 25
The alcohol 2-methylpropan-2-ol, (CH3)3COH, reacts to form esters that are used as flavourings by the food industry. The alcohol can be oxidised to produce carbon dioxide and water.
A student carried out an experiment on a pure sample of 2-methylpropan-2-ol to determine its enthalpy of combustion. A sample of the alcohol was placed into a spirit burner and positioned under a beaker containing 50 cm3 of water. The spirit burner was ignited and allowed to burn for several minutes before it was extinguished.
The results for the experiment are shown in Table 1.
(a) Use the results from Table 1 to calculate a value for the heat energy released from the combustion of this sample of 2-methylpropan-2-ol.
The specific heat capacity of water is 4.18 J K–1 g–1.
Show your working.
(2)
(b) Calculate the amount, in moles, of 2-methylpropan-2-ol burned in the experiment.
Hence calculate a value, in kJ mol–1, for the enthalpy of combustion of
2-methylpropan-2-ol.
Show your working.
(If you were unable to calculate an answer to part (a), you should assume that the heat energy released was 5580 J. This is not the correct value.)
(3)
(c) An equation for the combustion of 2-methylpropan-2-ol is
(CH3)3COH(I) + 6O2(g) 4CO2(g) + 5H2O(I)
Table 2 contains some standard enthalpy of formation data.
Use the data from Table 2 to calculate a value for the standard enthalpy of combustion of 2-methylpropan-2-ol. Show your working.
(3)
(d) An accurate value for the enthalpy of combustion of 2-methylpropan-2-ol in which water is formed as a gas is –2422 kJ mol–1.
Use this value and your answer from part (b) to calculate the overall percentage error in the student’s experimental value for the enthalpy of combustion of 2-methylpropan-2-ol.
(1)
(e) Suggest one improvement that would reduce errors due to heat loss in the student’s experiment.
(1)
(f) Suggest one other source of error in the student’s experiment. Do not include heat loss, apparatus error or student error.
(1)
(Total 11 marks)