A bomb calorimeter is used to determine the heat change during the combustion of various alcohols. The calorimeter has a constant volume, meaning it can withstand high pressure changes during the reaction. The calorimeter's heat capacity (CcalC_{\text{cal}}Ccal) must first be calibrated using a compound with a known heat of combustion.
In a calibration experiment, 1.38 g1.38\text{ g}1.38 g of ethanol (Mr=46.0M_r = 46.0Mr=46.0) is completely combusted in excess oxygen inside the calorimeter. A temperature rise (ΔT\Delta TΔT) of 7.6 ∘C7.6\text{ }^\circ\text{C}7.6 ∘C is recorded. Under these experimental conditions, the combustion of 1.00 mol1.00\text{ mol}1.00 mol of ethanol releases 1367 kJ1367\text{ kJ}1367 kJ of energy.
The heat energy released in the calorimeter is given by q=CcalΔTq = C_{\text{cal}}\Delta Tq=CcalΔT. Calculate the heat capacity (CcalC_{\text{cal}}Ccal) of this calorimeter in kJ K−1\text{kJ K}^{-1}kJ K−1. Give your answer to 3 significant figures.
The experiment is repeated under identical conditions using 1.85 g1.85\text{ g}1.85 g of butan-1-ol (Mr=74.0M_r = 74.0Mr=74.0). A temperature rise of 12.2 ∘C12.2\text{ }^\circ\text{C}12.2 ∘C is recorded. Calculate the heat change, in kJ mol−1\text{kJ mol}^{-1}kJ mol−1, for the combustion of butan-1-ol. If you were unable to calculate a value for CcalC_{\text{cal}}Ccal in Question 1, use 4.80 kJ K−14.80\text{ kJ K}^{-1}4.80 kJ K−1 (note that this is not the correct value). Give your answer to 3 significant figures.
State why the heat change measured in a bomb calorimeter is not defined as an enthalpy change (ΔH\Delta HΔH).
The thermometer used to measure the temperature change of 12.2 ∘C12.2\text{ }^\circ\text{C}12.2 ∘C in Question 2 has an uncertainty of ±0.12 ∘C\pm 0.12\text{ }^\circ\text{C}±0.12 ∘C for each individual reading. Calculate the percentage uncertainty in this temperature change measurement. Suggest one adjustment to the experiment that would decrease this percentage uncertainty without changing the 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.