Department of Chemistry of Texas at Austin University Name Thermodynamics Unit RAQ Consider the following CHEMICAL CHANGE Acetylene C2H2 combusts in oxygen to form carbon dioxide and water 1 Estimate the enthalpy of combustion of acetylene using bond energies data First write the balanced equation where the coefficient of acetylene is 1 C2H2 g 5 2 O2 g 2CO2 g H2O l Then draw the structures for each species Reactants Two C H bonds one C C bond and one O O bond Products Two C O bonds and two O H bond H nBE react nBE prod 5 H 1 2x413 1 1x835 1x498 2 2x799 1 2x463 2 kJ H 1216 mol All combustion reactions are exothermic and so have negative H values A more thorough understanding can be gained by thinking through how energy is absorbed and released through the breaking and formation of chemical bonds Breaking Bonds Requires Energy Forming Bonds Releases Energy All tabular values for bond energies are positive but you can imagine that for the bonds that are breaking reactants energy is put into the system and for bonds that are forming products energy is released out of the system So H Bond Energy of Reactants Bond Energy of Products Revised DVB 12 03 13 LaBrake Vanden Bout 2013 Department of Chemistry of Texas at Austin University Name 2 Calculate the enthalpy of combustion of one mole of C2H2 using heats of formation data found on the course website using your personal wireless device Values found on course website First write the balanced equation were the coefficient of acetylene is 1 C2H2 g 5 2 O2 g 2CO2 g H2O l H n f H prod n f H react 5 H 2 f H CO2 g 1 f H H 2O l 1 f H C 2 H 2 g f H O2 g 2 5 H 2 393 5 1 286 1 227 0 2 kJ 1molrxn kJ H 1300 1300 molrxn 1molC2 H 2 molC2 H 2 3 Calculate the change in entropy for this reaction using standard molar entropy data found on the course website Values found on course website First write the balanced equation were the coefficient of acetylene is 1 C2H2 g 5 2 O2 g 2CO2 g H2O l S rxn nS prod nS react 5 S rxn 2S CO2 g 1S H 2O l 1S C 2 H 2 g S O2 g 2 5 S rxn 2 214 1 70 1 201 205 2 J S rxn 215 5 K molrxn Revised DVB 12 03 13 LaBrake Vanden Bout 2013 Department of Chemistry of Texas at Austin University Name 4 Calculate the change in Gibbs free energy for this reaction Is there ever a temperature where this reaction would be non spontaneous If so what is that temperature If not why Work G H T S J J G 1300000 298K 215 5 mol mol J kJ G 1235781 1236 mol mol Temp where non spont G H T S 0 H T S J H mol 6032 K T J S 215 5 mol 5 Imagine this reaction was run at constant pressure and temperature what is the work for this process combustion of 4 g C2H2 C2H2 g 5 2 O2 g 2CO2 g H2O l w Pext V ngasRT 7 J w 2mol prod molreact 8 314 298K 2 molK J w 3716 molrxn For 4g 1300000 w 4 g 3716 J 1molrxn 1molC2 H 2 4gC2 H 2 molrxn 1molC2 H 2 26gC2 H 2 w 4 g 571 7J Positive work indicates that work was done ON the system The moles of gas in the reaction chamber decreased as the reaction progressed forward meaning the gas was compressed by the external pressure work done ON the system 6 At constant pressure use the change in enthalpy and the work to find the change in internal energy for this process combustion of 4 g C2H2 U q w Revised DVB 12 03 13 LaBrake Vanden Bout 2013 Department of Chemistry of Texas at Austin University Name At constant pressure H qp U H w H 4g H rxn nC2 H 2 kJ 1molrxn 1molC2 H 2 4gC2 H 2 molrxn 1molC2 H 2 26gC2 H 2 H 4g 200kJ H 4g 1300 U q w U 4g 200 000J 571 7J 199 4kJ 7 4 g of acetylene was combusted in a bomb calorimeter that had a heat capacity of 3 51 kJ C for the device and contained 2000 g of water C 4 184 J g C to absorb the heat as well What is the expected temperature change in such a calorimeter given the complete combustion of the 4 g of the fuel In a bomb calorimeter the heat measured is equal to the change in internal energy U q w 200kJ 0 572 kJ 199 4 kJ This is the internal energy change for this combustion reaction in general In a bomb calorimeter there would be no expansion or compression work So the internal energy change would all be experienced by a heat exchange between the system and the calorimeter water surroundings U qsys qsurroundings qcal qwater U Ccal T mwaterCwater T U T Ccal mwaterCwater 199 4 kJ T 3 51kJ C 1 2000g 0 004184 kJg 1 C 1 T 199 4 kJ 11 88 kJ C 1 T 16 8 C Consider the following PHYSICAL CHANGE N 2 liq 77K N 2 gas 298K and the following THERMODYNAMIC DATA for N2 0 H vaporization 5 56 kJ mol 1 C N 2gas 29 1 J K 1mol 1 Tb 77 K Tsurr 298 K Revised DVB 12 03 13 LaBrake Vanden Bout 2013 Department of Chemistry of Texas at Austin University Name 1 How much heat is absorbed during this change given 4 moles of N2 Work q n H vap nCgas T q 4mol 5 56 kJ kJ 4mol 0 0291 298K 77K mol mol q 47 9644 kJ 2 What is the work for this process assuming the initial volume of the liquid is zero w ngasRT 4mol 0mol 8 314Jmol 1K 1 298K 9910 29 J 9 91 kJ 3 What is the change in internal energy for this process U q w 47 96 kJ 9 91 kJ 38 05 kJ 4 What is the change in enthalpy for this process H q because pressure is constant H 47 9644 kJ 5 What is the change in entropy of the system for this process Work Ssys Svap Swarm Ssys n H vap T nCgas ln Tf Ti J J 298K mol 4mol 29 1 Ssys ln 77K molK 77K J Ssys 446 3 K 6 What is the change in entropy of the surrounding for this process Work q q Ssurr surr sys T T 47 9644kJ Ssurr 298K kJ J Ssurr 0 16095 160 95 K K 7 What is the total change in entropy change in entropy of universe for this process Suni Ssys Ssurr 446 3 JK 1 160 95 JK 1 285 35 JK 1 4mol 5560 Revised DVB 12 03 13 LaBrake Vanden Bout …
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