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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 blest;It blesseth him that gives and him that takes:
Lab This WeekLab This Week
IR Spectroscopy (Room 203)
Molecular Modeling (Room 202)
Part 2 of last week’s lab:
Last time…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)
Constant Volume Calorimetry (“Bomb”)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
Calculating Reaction Enthalpies from Calculating Reaction Enthalpies from Known DataKnown 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)
Standard Enthalpy of FormationStandard Enthalpy of Formation
Standard conditions: substance in pure form, 1 bar pressure, usually at 298K (25°C)
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
Hrxn H f
products
H f
reactants
Table 5-2, p. 195
Using Standard Enthalpies of Using Standard Enthalpies of FormationFormation
What is the Hrxn for the detonation of nitroglycerin?
How much energy is released when 10g is detonated?
2C3H5(NO3)3(l) 3N2(g)1
2O2(g) 6CO2(g) 5H2O(g)
Bond EnthalpyBond 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 H using bond enthalpies, assume all bonds are broken in the reactants (H= +) and formed in the products (H= -)
Hrxn broken formed
Using Bond Enthalpy to Calculate Using Bond Enthalpy to Calculate HHrxnrxn
CH4 (g) 4Cl2(g) CCl4 (g) 4HCl(g)
Hrxn broken formed