CHM 102 Sinex Enthalpy. CHM 102 Sinex Enthalpy (H) ~ heat content constant pressure  H = thermal (heat) energy change = q H 2 O (l) + energy 

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Presentation transcript:

CHM 102 Sinex Enthalpy

CHM 102 Sinex Enthalpy (H) ~ heat content constant pressure  H = thermal (heat) energy change = q H 2 O (l) + energy  H 2 O (g) vap  H vap = 44 kJ/mole heat of vaporization Physical process What happens for the reverse process?  H = - 44 kJ/mole and exothermic

CHM 102 Sinex Endothermic process or reaction Exothermic process or reaction  H < 0 or negative  H > 0 or positive H 2 0 (l)  H 2 O (s) Endothermic or exothermic? What does the sign of the enthalpy tell you?

CHM 102 Sinex Temperature - 25 o C or 298K Pressure – 1.00 atm Element in its stable state Standard State Conditions O (g) O 2 (g) O 2 (l) O 3 (g) Br 2 (g) Br 2 (l) Br 2 (s) C (diamond) C (graphite)

CHM 102 Sinex Chemical process C (graphite) + O 2 (g)  CO 2 (g)+ 394 kJ  H o f = -394 kJ/mole This is the standard molar enthalpy of formation for the formation of one mole substance from its elements in their standard states.  H o f = 0 kJ/mole for free elements

CHM 102 Sinex Combustion of methanol 2CH 3 OH (l) + 3O 2 (g)  2CO 2 (g) + 4H 2 O (g) Hof:Hof: kJ/mol0-394  H rxn =  n  H o f,products -  n  H o f,reactants How about just vaporizing the methanol? CH 3 OH (l)  CH 3 OH (g) -239 kJ/mol-201 kJ/mole Hof:Hof: where n is the number of moles

CHM 102 Sinex Describe all processes for the following: I 2 (s)  2I (g) I 2 (s)  I 2 (g) I 2 (g)  2I (g)  H subl =  H diss = Hof:Hof: Hof:Hof: overall: I 2 (s)  2I (g)  H rxn = Bond energy

CHM 102 Sinex I 2 (g) 2I (g) I 2 (s) Energy Diagram 2I (g)  I 2 (s) Exothermic rxn I 2 (s)  2I (g) Endothermic rxn

CHM 102 Sinex H20H20 2H + OH + + OH - H O 2 From an energetics point of view, what is happening in each case for the water? Is the reaction endothermic or exothermic? Breaking both O-H bonds Breaking one O-H bonds Breaking both O-H bonds and forming H-H and O=O bonds (need bond energies) endothermic answer

CHM 102 Sinex Compare the  H for this reaction calculated by  H f data with that calculated from bond energies. Bond Energy Calculator - an interactive Excel spreadsheet - Click hereClick here

CHM 102 Sinex Write the reaction used for the measurement of the standard molar enthalpy of formation of benzene. 6C (graphite) + 3H 2 (g)  C 6 H 6 (l) answer Do you think this reaction is possible? Reacting graphite in the presence of hydrogen gas will not produce benzene. So how do we get the  H f for benzene? answer

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