Question 23
A value for the enthalpy of combustion of an alcohol can be determined using the apparatus shown in the diagram. The calorimeter is held in position by a clamp.
This experiment can be repeated by using a different volume of water that would result in a more accurate value for the enthalpy of combustion because there would be a reduction in the heat lost.
State a change in the volume of water that would cause a reduction in heat loss and explain your answer.
(Total 2 marks)
Question 24
Ethanol is an important fuel.
(a) A dilute aqueous solution of ethanol can be produced by the fermentation of an aqueous solution of glucose.
It is claimed that the ethanol obtained from this solution is a carbon-neutral biofuel.
Write an equation for this fermentation reaction.
Give two other essential conditions for this reaction to produce a good yield of ethanol.
Name a process used to produce a much more concentrated solution of ethanol from a dilute aqueous solution.
State the meaning of the term carbon-neutral in the context of this biofuel.
(5)
(b) A student carried out a laboratory experiment to determine the enthalpy change when a sample of ethanol was burned. The heat produced was used to warm some water in a copper calorimeter. The student found that the temperature of 75.0 g of water increased by 5.50 °C when 2.40 × 10–3 mol of pure ethanol was burned in air.
Use the student’s results to calculate a value, in kJ mol–1, for the enthalpy change when one mole of ethanol is burned.
(The specific heat capacity of water is 4.18 J K–1 g–1)
Deduce two reasons why the student’s value for the standard enthalpy of combustion of ethanol is different from a Data Book value of –1279 kJ mol–1.
(5)
(c) Mean bond enthalpies can be used to calculate enthalpies of reaction.
(i) Give the meaning of the term mean bond enthalpy.
(2)
(ii) Consider the mean bond enthalpy data in the following table.
Use the data in the table above and the equation shown to calculate a value for the bond enthalpy for the O=O double bond in an oxygen molecule.
CH3CH2OH(g) + 3O2(g) 2CO2(g) + 3H2O(g) ΔH = –1279 kJ mol–1
(3)
(Total 15 marks)