Chemical Engineering Thermodynamics
Lecturer: Zhenxi Jiang (Ph.D. U.K.)
School of Chemical Engineering
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13.4 Effect of Temperature on the
Equilibrium Constant
Because the standard-state temperature is that
of the equilibrium mixture, the standard
property changes of reaction, such as ΔG° and
ΔH °, vary with the equilibrium temperature.
The dependence of ΔG ° on T is given by Eq.
(13.13), which may be rewritten:
d (G RT ) H

2
dT
RT
o
o
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13.4 Effect of Temperature on the
Equilibrium Constant
Inview of Eq. (13.11b), this becomes:
d ln K H

2
dT
RT
o
(13.14)
Equation (13.14) gives the effect of temperature
on the equilibrium constant, and hence on the
equilibrium conversion. If ΔH ° is negative, i.e.,
if the reaction is exothermic, the equilibrium
constant decreases as the temperature increases.
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13.4 Effect of Temperature on the
Equilibrium Constant
Conversely, K increases with T for an
endothermic reaction.
If ΔH° , the standard enthalpy change (heat)
of reaction, is assumed independent of T,
integration of Eq. (13.14) from a particular
temperature T’ to an arbitrary temperature T
leads to the simple result:
K
H o  1 1 
ln

 
(13.15)

K
R  T T 
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13.4 Effect of Temperature on the
Equilibrium Constant
This approximate equation implies that a plot of
In K vs. the reciprocal of absolute temperature
is a straight line. Figure 13.2, a plot of ln K vs.
1/T for a number of common reactions,
illustrates this near linearity. Thus, Eq. (13.15)
provides a reasonably accurate relation for the
interpolation and extrapolation of equilibriumconstant data.
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13.4 Effect of Temperature on the
Equilibrium Constant
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13.4 Effect of Temperature on the
Equilibrium Constant
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13.4 Effect of Temperature on the
Equilibrium Constant
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13.4 Effect of Temperature on the
Equilibrium Constant
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13.4 Effect of Temperature on the
Equilibrium Constant
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13.4 Effect of Temperature on the
Equilibrium Constant
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13.4 Effect of Temperature on the
Equilibrium Constant
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13.4 Effect of Temperature on the
Equilibrium Constant
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13.4 Effect of Temperature on the
Equilibrium Constant
Questions:
1. What is the key points in this Section?
2. Please explain Figure 13.2
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13.4 Effect of Temperature on the
Equilibrium Constant
Answers:
What is the key points in this Section?
Relationship between equilibrium constant
and temperature.
Please explain Figure 13.2
横坐标代表温度的倒数1/T的10000倍，纵坐
标代表平衡常数的对数；lnK 对（1/T）*104
是直线关系。每一条直线对应一个反应方程
式。
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13.5 Evaluation of Equilibrium Constants
Aim of this section:
To find the equilibrium constants
the approaches
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13.5 Evaluation of Equilibrium Constants
Values of ΔG° for many formation reactions are
tabulated in standard references.4 The reported
values of ΔG°f are not measured
experimentally, but are calculated by Eq.
(13.16). The determination of ΔG°f may be
based on the third law of thermodynamics,
discussed in Sec. 5.10. Combination of values
from Eq. (5.40) for the absolute entropies of the
species taking part in the reaction gives the
value of ΔG°f . Entropies (and heat capacities)
are also commonly determined from statistical
calculations based on spectroscopic data.
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13.5 Evaluation of Equilibrium Constants
Values of ΔG°f for a limited number of
chemical compounds are listed in Table C.4 of
App. C. These are for a temperature of 298.15 K,
as are the values of ΔH° listed in the same
table. Values of ΔG° for other reactions are
calculated from formation-reaction values in
exactly the same way that ΔH°f 298 values for
other reactions are determined from formationreaction values (Sec. 4.4). In more extensive
compilations of data, values of ΔG°f and
ΔH°f are given for a wide range of
temperatures, rather than just at 298.15 K.
Where data are lacking, methods of estimation 18
13.5 Evaluation of Equilibrium Constants
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13.5 Evaluation of Equilibrium Constants
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13.5 Evaluation of Equilibrium Constants
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13.5 Evaluation of Equilibrium Constants
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13.5 Evaluation of Equilibrium Constants
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13.5 Evaluation of Equilibrium Constants
Questions:
To find the equilibrium constants
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13.5 Evaluation of Equilibrium Constants
Questions:
To find the equilibrium constants
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13.5 Evaluation of Equilibrium Constants
Answers:
To find the equilibrium constants
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13.5 Evaluation of Equilibrium Constants
Answers:
To find the equilibrium constants
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13.5 Evaluation of Equilibrium Constants
That is all for today
Homework
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13.5 Evaluation of Equilibrium Constants
Thanks
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