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Unformatted text preview: rder in a system
or
Extent to which energy is distributed
among the various motions of the
molecules in the system.
State Fnc: )S = Sfinal ! Sinitial )S > 0 Sf > Si inc. in disorder
)S < 0 Sf < Si dec. in disorder 24 A) Isothermal Process
Constant temp. process qrev
)S = T
qrev = heat which would be
transferred if process
were reversible
S is a state function
 Thus this eqn. can be used to
calc. )S for any isothermal
process (whether reversible
or irreversible).
25 1) )S and Phase Changes
Phase changes occur at constant T
qrev = )Hphase change )Hphase change
)Sphase change = T
a) Ex: boiling water at 100 °C )Hvap = 44.01 kJ/mol
)Hvap
44.01 kJ/mol
)Svap =  = T
373.15 K
= 0.1178 kJ/molCK
= + 117.8 J/molCK
26 nd B) 2 Law of Thermodynamics and S
Irreversible Process
Results in inc. in Stotal ()S > 0)
Reversible Process
Results in no change in Stotal ()S = 0)
2nd Law of Thermodynamics
Total entropy of a system &
its surroundings always inc.
for a spont. process
Irreversible (Spont.) Process )Suniv = )Ssys + )Ssurr > 0
Reversible Process )Suniv = )Ssys + )Ssurr = 0
27 1) Ex: 1 mole of Liquid water is left
outside where it’s 10.0 °C and it
freezes. Calc. )Ssys & )Ssurr.
Will it spontaneously freeze?
)Hfus = + 6.01 kJ/mol
)Hfreezing = ! )Hfus = ! 6.01 kJ/mol
)Hfrz (1 mol)( 6.01 kJ/mol)
)Ssys =  = T
273 K
= ! 22.0 J/molCK )Hsurr (1 mol)(+ 6.01 kJ/mol)
)Ssurr =  = T
263 K
= + 22.9 J/molCK )Suniv = )Ssys + )Ssurr
= (! 22.0 J/molCK) + (+ 22.9 J/molCK)
= + 0.9 J/molCK Spont.
28 This shows that even though the entropy
of the system DEC. the process was still
spont. because the entropy of the
surroundings increased more. )Suniv > 0
Spont. processes occur w.
an overall INC. in )Suniv
Practically speaking we can’t
always easily determine )Suniv.
Want to focus on the system.
Will see how to determine
spontaneity based on )Ssys.
Simplify notation and refer
to )Ssys simply as )S
29 III) Molecular Inerpretation of Entropy
A) Expansion of Gas at Molecular Level
Have the following system: Open stopcock:
We know gas expands spont.
Why?
Look at possible arrangements
of the particles
30 Probability that blue molecule is in left
flask is ½. Same for the red particle.
Probability that both molecules
remain in the left flask is:
(1/2)2 = 1/4
For 3 particles it would be
3 (1/2) = 1/8
31 Consider a mole of gas:
Prob. all the molecules are in the
left flask at the same time is:
N (1/2) where N = 6.02 x 10 23 Infinitesimally small!!!
Essentially ZERO prob.
of all molecules in left
flask at same time.
 Gas SPONT. expands
to fill both flasks
 Does NOT spont. go
back to left flask
32 Most probable arrangements are
those with essentially equal numbers
of particles in both flasks.
Gas spreads out and the arrangement
of gas particles is more disordered
than when confined to one flask
Greater entropy Just seen disorder on
the molecular...
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 Fall '13
 Zellmer
 Chemistry, Equilibrium, pH, Kinetics

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