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Enthalpy

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

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Enthalpy

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  1. Enthalpy

  2. Enthalpy (H) ~ heat content (q) @ constant pressure DH = thermal (heat) energy change = q Physical process H2O (l) + energy  H2O (g) DHvap = 44 kJ/mole heat of vaporization What happens for the reverse process? DH = - 44 kJ/mole and exothermic

  3. What does the sign of the enthalpy tell you? Endothermic process or reaction DH > 0 or positive Exothermic process or reaction DH < 0 or negative Endothermic or exothermic? H20 (l)  H2O (s)

  4. Standard State Conditions • Temperature - 25oC or 298K • Pressure – 1.00 atm • Element in its stable state O (g) O2 (g) O2 (l) O3 (g) Br2 (g) Br2 (l) Br2 (s) C (diamond) C (graphite)

  5. Chemical process C (graphite) + O2 (g)  CO2 (g) + 394 kJ DHof = -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. DHof = 0 kJ/mole for free elements

  6. Combustion of methanol 2CH3OH (l) + 3O2 (g)  2CO2 (g) + 4H2O (g) DHof: -239 kJ/mol 0 -394 -242 DHrxn = SnDHof,products - SnDHof,reactants where n is the number of moles How about just vaporizing the methanol? CH3OH (l)  CH3OH (g) DHof: -239 kJ/mol -201 kJ/mole

  7. Describe all processes for the following: I2 (s)  2I (g) I2 (s)  I2 (g) DHsubl = 0 62 DHof: I2 (g)  2I (g) DHdiss = DHof: 62 107 Bond energy overall: I2 (s)  2I (g) DHrxn =

  8. Energy Diagram 2I (g) 2I (g)  I2 (s) Exothermic rxn I2 (s)  2I (g) Endothermic rxn I2 (g) I2 (s)

  9. H20 2H + O H+ + OH- H2 + 0.5O2 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 From an energetics point of view, what is happening in each case for the water? Is the reaction endothermic or exothermic? answer

  10. Compare the DH for this reaction calculated by DHf data with that calculated from bond energies. Bond Energy Calculator - an interactive Excel spreadsheet - Click here

  11. Write the reaction used for the measurement of the standard molar enthalpy of formation of benzene. 6C (graphite) + 3H2 (g)  C6H6 (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 DHf for benzene? answer

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