Question 21
Alcohols such as methanol (CH3OH), ethanol (CH3CH2OH) and propan-1-ol (CH3CH2CH2OH) are good fuels.
(a) A student carried out an experiment to determine the enthalpy of combustion of methanol.
Methanol was placed in a spirit burner and the mass of the spirit burner measured. The student placed 100 g of water in a copper calorimeter and clamped it above the spirit burner. The burner was lit and allowed to burn for a few minutes. The flame was then extinguished and the new mass of the spirit burner found.
The measured temperature rise was 38.0 °C. The specific heat capacity of water is 4.18 J K−1 g−1.
A diagram of the apparatus is shown alongside a table which shows the measurements the student recorded.
Use the student’s data to calculate an experimental value for the enthalpy of combustion of methanol in kJ mol−1.
(4)
(b) Suggest one reason, other than incomplete combustion or heat transfer to the atmosphere, why the student’s value for the enthalpy of combustion of methanol is different from that in a Data Book.
(1)
(c) The uncertainty in each of the temperature readings from the thermometer in this experiment was ±0.25 °C. This gave an overall uncertainty in the temperature rise of ±0.5 °C.
Calculate the percentage uncertainty for the use of the thermometer in this experiment.
(1)
(d) The student said correctly that using a thermometer with an overall uncertainty for the rise in temperature of ±0.5 °C was adequate for this experiment.
Explain why this thermometer was adequate for this experiment.
(1)
(e) The enthalpy of combustion of ethanol is −1371 kJ mol−1. The density of ethanol is 0.789 g cm−3.
Calculate the heat energy released in kJ when 0.500 dm3 of ethanol is burned.
Give your answer to an appropriate number of significant figures.
(3)
(Total 10 marks)