CHEM 1211 Lecture 13 Outline of Last Lecture I Acid Concentrations 1 Example One 2 Example Two 3 Example Three CQ 1 4 Example Four 5 Example Five II Chemical Thermodynamics A Heat Transfer 1 CQ 2 2 CQ 3 3 Example One B Internal Energy NEW THERMODYNAMIC FXN 1 Example One Outline of Current Lecture I Table One II Enthalpy III Calorimetry IV Hess s Law Current Lecture When Zn reacts with HCl hydrogen gas There is a change in enthalpy U qp P V or qp Wp I Table One Energy Transferred Sign Convention Heat to system endo q 0 Heat from system exo q 0 Work done on system W 0 Work done by system W 0 II Enthalpy H KJ mol rxn Effect on Usystem 1 H2 g O2 g 1 H2O g H 241 8 KJ mol rxn If the fuel tank hold 234 265 lbs of hydrogen how much heat is released when it all reacts with oxygen in the tank 1 lb 454g mm H2 2 020 g mol III q H mol rxn 234 265 lb 454g H2 1 mol H2 1 mol rxn 52 652 638 61 mol rxn lb 2 020g 1 mol H2 10 q 241 8 x 52 652 638 61 1 27 x 10 KJ exothermic rxn Calorimetry 1 Ammonium Chloride is very soluble in H2O When 4 50g NH4Cl is dissolved in 53 00g of H2O the temperature of the solution decreases from 20 40 C to 15 72 C Calculate the enthalpy of dissolution of NH4Cl in KJ mol rxn Assume that the specific heat of solution is 4 18 J g C and that calorimeter constant is 26 71 J C mm NH4Cl 53 490 g mol NH4Cl s NH4 aq Cl aq The system at constant pressure qsystem qcal qsoln work o only exists when a gas is formed T 0 process is endothermic T 15 72 C 20 40 C 4 68 M 53g H2O 4 50g NH4Cl 57 50 g soln Qsoln mC T 57 50g x 4 18 J mol C x 4 68 C 1124 8 J Qcal Ccal T 26 71 J C x 4 68 C 125 0J Qsystem qcal qsoln 1249 8 J Mol rxn 4 50g NH4Cl 1 mol NH4Cl 1mol rxn 0 084127 mol rxn 53 49g NH4Cl 1 mol NH4Cl IV Hdiss 1249 8 14 856 11 J 0 084127 mol rxn Hess s Law Law of heat summation Enthalpy state function 14 9 KJ Endothermic mol rxn H H a H b H c etc The enthalpy change for a rxn is the same whether it occurs by one step or by any series of steps
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