Question 8
A bomb calorimeter can be used for accurate determination of the heat change during combustion of a fuel.
A bomb calorimeter is a container of fixed volume that withstands the change in pressure during the reaction.
The fuel is mixed with pure oxygen in the calorimeter, ignited and the temperature change is recorded.
The total heat capacity (Ccal) of the calorimeter is calculated using a fuel for which the heat change is known.
In an experiment to calculate Ccal, 2.00 g of hexane (Mr = 86.0) is ignited. A temperature change (∆T) of 12.4 °C is recorded.
Under the conditions of the experiment, 1.00 mol of hexane releases 4154 kJ of energy when combusted.
(a) The heat energy released in the calorimeter, q = Ccal∆T
Calculate the heat capacity (Ccal) in kJ K−1
(3)
(b) When the experiment is repeated with 2.00 g of octane (Mr = 114.0) the temperature change recorded is 12.2 °C
Calculate the heat change, in kJ mol−1, for octane in this combustion reaction.
If you were unable to calculate a value for Ccal in part (a), use 6.52 kJ K−1 (this is not the correct value).
(2)
(c) State why the heat change calculated from the bomb calorimeter experiment is not an enthalpy change.
(1)
(d) The thermometer used to measure the temperature change of 12.2 °C in part (b) has an uncertainty of ± 0.1 °C in each reading.
Calculate the percentage uncertainty in this use of the thermometer.
Suggest one change to this experiment that decreases the percentage uncertainty while using the same thermometer.
(2)
(Total 8 marks)