Chemistry 132 – Lecture 11 – Equilibrium Continued
Thermodynamic Equilibrium Constant
We have discussed the equilibrium constants
K
c
and
K
p
.
However, we often run into the
“thermodynamic equilibrium constant”,
K
.
K
is a unit less constant found by using ratios of
partial pressures to a “reference pressure” of 1.0
atm or by using ratios of concentrations to a
“reference concentration” of 1.0 mol/L.
e.g. Consider the reaction
3 NO(g)
N
2
O(g) + NO
2
(g)
Kp = [P
N2O(g)
][P
NO2(g)
]/[P
NO(g)
]
3
which has units
atm
1
K = [P
N2O(g)
/ P
ref
][P
NO2(g)
/ P
ref
]/[P
NO(g)
/ P
ref
]
3
which
has no units
Note
: if you use 1.0 atm as the reference
pressure and 1.0 mol/L as the reference
concentration, the numerical values of
K
and
K
p
will be identical and the numerical values of
K
compared to
K
c
will be identical.
1
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View Full DocumentChemistry 132 – Lecture 11 – Equilibrium Continued
The thermodynamic equilibrium constant (
K
)
has some benefits over
K
c
and
K
p
.
•
It can be calculated from
∆
G
o
(that is, from
H
o
and
S
o
)
so the extent of any
equilibrium reaction can be calculated
using
calorimetric data alone
.
•
K
is always dimensionless.
Heterogeneous Equilibrium
When all reactants and products in an
equilibrium situation are gases, the system is an
example of
homogeneous equilibrium
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 Fall '09
 ProfessorNewstead
 Thermodynamics, Equilibrium, Partial Pressure

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