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 (CcalC_{\text{cal}}Ccal) of the calorimeter is calculated using a fuel for which the heat change is known. In an experiment to calculate CcalC_{\text{cal}}Ccal, 1.50 g1.50\text{ g}1.50 g of pentane (Mr=72.0M_r = 72.0Mr=72.0) is ignited. A temperature change (ΔT\Delta TΔT) of 11.8 ∘C11.8\text{ }^\circ\text{C}11.8 ∘C is recorded. Under the conditions of the experiment, 1.00 mol1.00\text{ mol}1.00 mol of pentane releases 3509 kJ3509\text{ kJ}3509 kJ of energy when combusted.
The heat energy released in the calorimeter, q=CcalΔTq = C_{\text{cal}}\Delta Tq=CcalΔT. Calculate the heat capacity (CcalC_{\text{cal}}Ccal) in kJ K−1\text{kJ K}^{-1}kJ K−1.
When the experiment is repeated with 1.80 g1.80\text{ g}1.80 g of heptane (Mr=100.0M_r = 100.0Mr=100.0) the temperature change recorded is 14.5 ∘C14.5\text{ }^\circ\text{C}14.5 ∘C. Calculate the heat change, in kJ mol−1\text{kJ mol}^{-1}kJ mol−1, for heptane in this combustion reaction. If you were unable to calculate a value for CcalC_{\text{cal}}Ccal in Question 1, use 5.50 kJ K−15.50\text{ kJ K}^{-1}5.50 kJ K−1 (this is not the correct value).
State why the heat change calculated from the bomb calorimeter experiment is not an enthalpy change.
The thermometer used to measure the temperature change of 14.5 ∘C14.5\text{ }^\circ\text{C}14.5 ∘C in Question 2 has an uncertainty of ±0.15 ∘C\pm 0.15\text{ }^\circ\text{C}±0.15 ∘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.
34 exam-style questions on AQA A Level Chemistry Energetics, covering Enthalpy change, Calorimetry, Applications of Hess's law, and Bond enthalpies. Each one has a worked solution and a mark scheme showing where the marks go.