Gibb's free energy

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GIBBS FREE ENERGY ?G
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most abstract of thermodynamic state functions
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w1 ?w reversible
work w2
P
V
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Definition G w PV w reversible work PV
pressure/volume work isothermal, reversible path
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?G ?w P?V V?P at constant P ?P 0 so
V?P 0 ?G ?w P?V
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?G ?w P?V V?P at constant P ?G ?w
P?V at constant V ?V 0 so P?V 0 ?G ?w
useful work
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G cannot be measured

• must measure ?G over a process

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ZERO REFERENCE ?G 0 for elements in stable form
under Standard Thermodynamic Conditions T
25oC P 1 atm
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?Gf0 standard Free Energy of Formation from the
elements Appendix L, text
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?G follows Hess Law ?G0 (rxn) ?n?Gf0(p) -
?n?Gf0(r)
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Summary of Laws of Thermodynamics Zeroth
Law Heat Gain Heat Loss
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Summary of Laws of Thermodynamics First Law Law
of Conservation of Energy
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Summary of Laws of Thermodynamics Second
Law Defines Entropy
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Summary of Laws of Thermodynamics Third
Law Defines Absolute Zero
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GIBBS HELMHOLTZ EQUATION
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combine
?G
?H
T
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?G -aT ?H ?G, ?H are state functions, thus
a must be a state function ?G -?S(T) ?H
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Gibbs Helmholtz Equation ?G ?H - T?S
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Units on the State Functions
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thus, a process is spontaneous if and only if ?G
is negative
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Spontaneity controlled by enthalpy (minimum
energy)
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Sponaneity controlled by enthalpy entropy (maximu
m disorder)
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Sponaneity controlled by enthalpy entropy both
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Predict Spontaneity IF ?H(-) and ?S() ?G -?H
- T(?S) ?G lt 0, gt spontaneous
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Predict Spontaneity IF ?H() and ?S(-) ?G ?H
- T(-?S) ?G gt 0, gt NOT spontan
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Summary of Spontaneity ?H ?S ?G
Spont. - - yes
- no or -
? - - or - ?
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Uses of the Gibbs Helmholtz Equation
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1. Find the molar entropy of formation
for ammonia.
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2. Elemental boron, in thin fibers, can be
made from a boron halide BCl3(g) 3/2
H2(g) -----gt B(s) 3 HCl(g)
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Calculate ?H0, ?S0 and ?G0.

• Spontaneous?
• Driving force?

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3. Using thermodynamic information, determine
the boiling point of bromine.