Chemistry 132 – Lecture 12 – Thermodynamic
Description of the Equilibrium State
Thermodynamics views a chemical reaction
as
the
spontaneous “flow” of atoms from
reactants to products
carried out at constant T
and P
and requires that
∆
G<0
to be
spontaneous.
When the reaction is at
equilibrium,
G = 0
. Think of
Free energy
as
available energy
.
The
T
S
term is
the
unavailable energy
discarded in the form of
heat, no matter what.
Only
G is available
(free) to do useful work
.
Equilibrium Expression for Reactions in the
Gas Phase
Recall that if the pressure of
an ideal gas
is
changed from P
1
to P
2
at constant T,
∆
G =
∆
(H – TS) =
∆
H – T
∆
S
(1)
Because the reaction is isothermal,
∆
H = 0.
Therefore,
∆
G = T
∆
S
(2)
1
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View Full DocumentChemistry 132 – Lecture 12 – Thermodynamic
Description of the Equilibrium State
But
∆
S =
nR
ln(P
1
/P
2
) = 
nR
ln(P
2
/P
1
)
(3)
Substitute equation (3) into (2).
∆
G =
nRT
ln(P
2
/P
1
)
(4)
If P
1
is the reference state of 1 atm,
∆
G =
nRT
ln(P
2
/P
ref
) =
RT
ln(P
2
/P
ref
)
n
which is the change in Gibb’s free energy in
taking a gas from the reference to any pressure
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 Fall '09
 ProfessorNewstead
 Thermodynamics, Entropy, equilibrium state, Thermodynamic Description

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