Calorimetry Heats of Reactions and Hess s Law Catherine Lozier 03 12 2013 Chem 1310 Section C02 TA Ryan Bucher Lab Partners Chelsey Arnold Sunya Morin Tran G Honor Pledge I did not copy this work from any others student s current students in lab or old lab reports Signature DATA AND OBSERVATIONS Part A Trial 1 Trial 2 Trial 3 Temperature of the cold water and calorimeter before mixing Temperature of the hot water before mixing Final temperature of the mixed water Heat capacity J C Temperature of the cold water and calorimeter before mixing Temperature of the hot water before mixing Final temperature of the mixed water Heat capacity J C Temperature of the cold water and calorimeter before mixing Temperature of the hot water before mixing Final temperature of the mixed water Heat capacity J C Average Standard Deviation 20 5 C 80 0 C 50 0 C 3 5 J C 20 5 C 75 0 C 45 5 C 37 66 J C 21 0 C 85 0 C 53 C 0 J C 13 72 J C 20 38 J C Part B 1 Balanced equation Mg HCl Mg s 2HCl aq MgCl2 aq H2 g Concentration HCl 1 M Volume HCl 100 mL Moles HCl 0 1 moles Mass of Mg g Moles Mg Initial Temp C Final Temp C Temp C Time s 20 40 60 80 100 120 2 0 130 0 138 0 006 21 5 C 28 0 C 7 5 C 21 0 C 22 0 C 1 0 C Temperature C 21 5 22 0 22 0 22 0 22 0 22 0 Time 20 40 60 80 100 120 140 160 180 Temperature C 23 0 24 0 25 0 26 0 27 0 28 0 28 0 28 0 28 0 Balanced equation MgO HCl MgO s 2HCl aq MgCl2 aq H2O g Concentration HCl 1 M Volume HCl 100 mL Moles HCl 0 1 moles Mass of MgO g Moles MgO Initial Temp C Final Temp C Temp C 0 699 21 0 C 27 0 C 6 0 C Time s 20 40 60 80 100 Temperature C 27 0 27 0 27 0 27 0 27 0 CALCULATIONS Part A qhot qcold qcal cwater mhot Thot cwater mcold Tcold ccal Tcal 4 184J C 50g 29 5 C 4 184J C 50g 25 C ccal 25 C Ccal 37 656 J C Average 13 72 J C Standard Deviation 20 38 J C Part B 1 Enthalpy Mg HCl H qrxn qcal nlimiting qrxn mc T 0 138g 100g 4 184 J C 1 C 419 0 J qcal c T 13 72 J C 1 C 13 72 J nlimiting 0 006 moles Mg H 419 0 J 13 72 J 0 006 moles H 67 5 kJ mol 2 Enthalpy MgO HCl H qrxn qcal nlimiting qrxn mc T 0 699g 100g 4 184 J C 6 C 2528 0 J qcal c T 13 72 J C 6 C 82 32 J nlimiting 0 017 moles MgO H 2528 0 82 32 0 017 H 143 9 kJ mol 3 Enthalpy of Mg O2 H 67 5 kJ C 143 9 kJ C 285 84 kJ C 362 24 kJ C 4 Percent Error Error 100 Observed Actual Actual Error 100 326 24 602 602 Error 39 8 DISCUSSION Part A The purpose of Part A was to calculate the heat capacity of the calorimeter used The average calculated C was 13 72 J C with a standard deviation of 20 38 J C This is consistent with the expected value which was between 0 and 100 J C The high standard deviation could be due to the error in this experiment which consisted primarily of the human error of only using one Styrofoam cup instead of 2 Because one less cup was used the water was not insulated as well and more heat was lost to the surroundings Part B The purpose of Part B was to use Hess Law to calculate the enthalpy of the reaction Mg O2 The enthalpy of the Mg HCl reaction was calculated to be 67 5 kJ C This was unexpected as the reaction was supposed to be endothermic therefore the enthalpy should be negative This result is likely due to human error as only one Styrofoam cup was used for the calorimeter It is possible that heat from the hand holding the cup artificially increased the temperature of the reaction The enthalpy of the MgO HCl reaction was calculated to be 143 9 kJ C which is lower than expected but at least indicates an exothermic reaction as expected Using these experimental values for enthalpy and the given value of 285 84 kJ C for the H2 O2 reaction the value of the enthalpy of the Mg HCl reaction was calculated to be 362 24 kJ C This value has a 39 8 error compared to the expected value of 602 kJ C This discrepancy can likely be attributed to the errors of the Mg HCl reaction REFERENCES Lab Manual Chemistry for Engineering Students Second Edition Lawrence S Brown Thomas A Holme