Gibbs energy and spontaneous change

1
Lecture 7
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• Gibbs energy and - spontaneous change
• - equilibrium
• - non-volume work

2
Reaction at given P,T
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Gibbs energy G tells us whether reaction
A B C ?G
will occur spontaneously or not, without looking
at the surroundings.
3
Gibbs energy G
?
T constant
Reaction
heat qP
Ssur changes due to qP
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Define ?Gsys ?Hsys - T?Ssys
lt 0
gt 0
favourable ?Ssur gt 0
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Thus ?Gsys lt 0 ?Stot gt 0 , for
given P,T
equivalent with
4
Gibbs energy determines for given P,T the
spontaneous direction of a reaction
?
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A B C ?G
?G lt 0 ?Stot gt 0 spontaneous ?G 0 ?Stot
0 equilibrium ?G gt 0 ?Stot lt 0 not allowed
by Second Law
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?G is the basis of all equilibrium chemistry
?G ?H T?S P, T constant
5
How to maximise electrical work we (and
minimise q) ?
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First Law ?U q w
-
we electrical work 0 when ..
-
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Electro- chemical reaction
q
?G 0 when . Conclusion ?
we
-

A ? B G
q
H2O
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Gibbs energy G and reversible, non-volume work
we
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Definition
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P,T constant
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Equilibrium wrev,e 0 Spontaneous wrev,e
lt 0
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Nb. dH is obtained as follows
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Definition H U PV dH dU PdV
VdP First Law dU dq dw dqrev
dwrev dqrev PdV wrev,e Second
Law
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8
Lecture 8
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• Summary of the First and Second Law
• Entropy determines direction of heat flow
• Maximal work is reversible work
• The Claudius inequality

9
Alternative Formulation Second Law
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• The Clausius Inequality

10
Reversibility vs. irreversibility
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?U qirr wirr qrev wrev wirr gt
wrev Example -10 kJ gt -100 kJ
we get more work from the system
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An irreversible spontaneous direction of a
reaction increases the total entropy
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reversible
irreversible
q
q


12
Reversibility vs. irreversibility
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Spontaneous, irreversible
Clausius Inequality