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Lecture 2

CHEM 202 Lecture Notes - Lecture 2: Torr, Water Gas, Relative Humidity

Course Code
CHEM 202
Neal Stolowich

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Chapter 34 Summary:
Entropy favors gas phase; intermolecular forces favor solid phases.
Entropy/enthalpy phase differences: solid-liquid small gap; liquid-gas large gap
Chapter 35
Vaporization/condensation-(liquid/gas phase)
oVaporization in open system (like a glass of water, or a puddle) is what we usually
equate to evaporation. As mentioned last time, liquid particles are in constant motion as
they move through the liquid phase. Those on the surface, ie, the liquid gas interface
have an additional option—if they have enough energy to overcome their IF forces, the
particle can leave the liquid phase and enter the gas phase. Note, the particle itself does
not change. The particles in the gas phase could return back to the liquid, but usually
just go off and “evaporate”.
oIn a close system (ie, a glass jar with a lid) an important thing happens however, these
particles of vaporization accumulate, and begin to return back to the liquid phase—this
is condensation.
The rate3 of vaporization remain constant
The rate of condensation initially increases as more gas molecules become
present in the gas phase.
Eventually the rate of condensation will equal that of the rate of vaporization.
The opposing process occur at equal speeds. The concertation of the gas and
liquid phases no longer change. This is equilibrium.
oTwo approaches can describe the balanced associated with the equilibrium:
Kinetically—the rate of evaporation and condensation are equal
Enthalpy/entropy—at equilibrium the system’s enthalpy (EFs of the liquid) and
entropy (disorder of the gas) are balanced as well.
oEquilibrium vapor pressure (EVP): PV=nRTP=(n/V)RT
P=concentration x R x T
The pressure is the vapor pressure the gas molecule exerts in the gas phase. It
reaches its maximum at equilibrium, ie, the EVP. For example, water’s EVP at 20
degree C is 17.5 torr, and via the IGL you can now calculate the amount of water
gas molecules that must be present.
N=PV/RT=.0019 mol H2O
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