Announcements

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Announcements
Quote of the day The quality of mercy is not
strain'd, It droppeth as the gentle rain from
heavenUpon the place beneath it is twice
blestIt blesseth him that gives and him that
takes

• If you dont have a clicker with you, sign in
  after class
• Tutoring hours this week
• Monday 1200-200pm
• Tuesday 1000am -1200pm 200-300pm
• 530-830 pm
• Wednesday 200-500pm and 530-800pm

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Lab This Week

• IR Spectroscopy (Room 203)
• Molecular Modeling (Room 202)
• Part 2 of last weeks lab

3
Last time

• Hess Law
• Enthalpies can be added
• If reaction is reversed, sign is reversed
• If reaction is multiplied by integer, enthalpy is
  too
• Constant pressure calorimetry (lab last week)

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Constant Volume Calorimetry (Bomb)
N2H4 3 O2 ? 2 NO2 2 H2O Ereleased
Eabsorbed by water Eabsorbed by
calorimeter Ewater Ecalorimeter Total E
?H energy/moles
0.500 g N2H4 600 g water 420 J/oC
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Calculating Reaction Enthalpies from Known Data

• Use values that have been determined from
  experiment
• Two types of data that can be used
• Standard enthalpies of formation (Chapter 5.6)
• Bond enthalpies (Chapter 8.4, pp. 8-10 to 8.13)

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Standard Enthalpy of Formation

• Standard conditions substance in pure form, 1
  bar pressure, usually at 298K (25C)
• Heat of formation for an element in its most
  stable form is zero
• Heat of formation for a compound the energy
  required to form a compound from its constituent
  elements in their most stable form
• Standard heat of formation is given in Joules per
  mole
• Results in fractional coefficients on occasion

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Table 5-2, p. 195
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Using Standard Enthalpies of Formation

• What is the DHrxn for the detonation of
  nitroglycerin?
• How much energy is released when 10g is detonated?

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Bond Enthalpy

• Remember that bond energy is the amount of energy
  required to break a bond in a gas phase molecule
• Can only use bond enthalpies for reactions in
  which everything is in the gas phase
• When calculating DH using bond enthalpies, assume
  all bonds are broken in the reactants (DH ) and
  formed in the products (DH -)

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Using Bond Enthalpy to Calculate DHrxn