5. Measuring heats of combustion and calculating ... - Pearson

Answers:LEVEL: ASCHEMISTRY – Energetics5. Measuring heats of combustion and calculating enthalpy changes(15 minutes)A student attempted to measure the enthalpy change of combustion of ethanol using theapparatus shown below.(a) Suggest four improvements that could be made to the apparatus to get a more accurateresult.(4 marks)Use a more accurate (narrower temperature range) thermometer. Clamp the calorimeter directly overthe burner and discard the gauze and tripod. Shield the apparatus from air draughts. Stir the water(carefully using the thermometer).(b) The student's results were as follows:Mass of empty calorimeter120 gMass of water in calorimeter100 gMass of ethanol, wick and container before experiment43.56 gMass of ethanol, wick and container after experiment41.36 gInitial temperature of water in calorimeter 20°CFinal temperature of water 75°CSpecific heat capacity of copper 0.387 J g −1 K −1Specific heat capacity of water 4.18 J g −1 K −1Molar mass of ethanol 46.1 g mol −1(i) Use the above results to calculate a value for the molar enthalpy change of combustion ofethanol.(4 marks)Heat capacity of apparatus is 120 × 0.387 + 100 × 4.18 = 464.4 J K −1Rise in temperature is 75 − 20 = 55 KHeat evolved is 464.4 × 55 = 25542 JMass of ethanol burnt is 43.56 − 41.36 = 2.20 gAmount of ethanol burnt is 2.20/46.1 = 0.0477 molMolar enthalpy change is 25542/0.0477 = −535 000 J = −535 kJCopyright © Pearson Education Limited 20011

Answers:LEVEL: ASCHEMISTRY – Energetics(ii) The data book value for the standard molar enthalpy change of combustion of ethanol is–1367 kJ mol –1 . Suggest reasons why the value calculated from the student's results differsfrom the value in the data book.(4 marks)The student's value shows not enough heat has been transferred to the caloimeter and its contentspossibly because heat was lost to the gauze, tripod and surrounding air, combustion was incomplete,some ethanol was lost by evaporation, the reactants and products were not measured under standardtemperature and pressure conditions.(c) Explain why a bomb calorimeter is used for accurate determinations of enthalpy changesand why the heat change measured is not an enthalpy change.(3 marks)Complete combustion occurs when the sample is burnt in pure oxygen under high pressure but thereaction takes place at constant volume, so the heat change measured is an internal energy change, ∆U.(Total 15 marks)Copyright © Pearson Education Limited 20012