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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