Page 115 - Modeling of Chemical Kinetics and Reactor Design

Thermodynamics of Chemical Reactions 85

o
The true equilibrium constant at 428 K is ∆G = –RT ln K = ∆H o
– T∆S, that is

o
lnK = 1   ∆ S − ∆ H  
o
R  T 

where

cal cal
o
,
∆H =− 23 326 and R =1 987.
molN mol K
•
2
lnK = 1  −51 .14 −  −23326   = . 1 691
 

. 1 987   428 
and

K = 5.42

o
The value of K is very sensitive to small errors in ∆H and ∆S o
r
and the equilibrium constant belongs to the reaction equation N + 3H 2
2
→ 2NH . Table 2-1 shows the calculated results of the heats of reaction
3
at standard state (298 K) and at system temperature of 428 K using the
software ENTHALP.
Example 2-2
Calculate the heat that must be removed or provided for the gas
phase reaction proceeding

CO (g) + 4H (g) → 2H O(g) + CH (g)
4
2
2
2
with 100% conversion and the gas entering at 450°C. The heat of
formation at standard conditions and the specific heat capacities are
shown below:
Component Heat of formation ∆H (J/gmol)
f
CO (g) –393,513
2
H (g) 0.0
2
H O(g) –241,826
2
CH (g) –74,848
4