CHE 303 (Winter 2009)
__________________
LAST NAME, FIRST
Problem set #10
(1)1 The gas phase reaction
A(g) + B(g) = C(g)
was studied at 100 and 200oC and at a pressure of 1 atm. The following equilibrium mol fractions
were obtained:
yC
yA
yB
100oC
0.172
0.414
0.414
200oC
0.642
0.179
0.179
If equimolar quantities of A and B are reacted at 150oC and a pressure of 10 atm, what will be the
equilibrium composition?
(2)1 The chemical species A is known to decompose according to
A(g) = B(g) + C(g)
A rigid container is filled with pure gaseous A at 300oK and 760 mmHg and then heated. The
pressure was observed to be 1114 mmHg at 400oK and 1584 mmHg at 500oK. Estimate the
pressure for a temperature of 600oK. Assume ideal-gas behavior and chemical equilibrium.
(3)1 Equilibrium with respect to the reaction
A(g) + B(g) = C(g)
will be studied by measuring the volume change accompany the reaction. The temperature and
pressure are held constant and the initial volume and the final volume of the reacting system are
recorded. Three tested were made and are summarized in the table. Has equilibrium been
established? If so what is the value of K?
P(mmHg)
500
600
600
yA
0.5
0.333
0
Initial composition
yB
yC
0.5
0
0.667
0
0
1.0
Volume (cm3)
Initial
Final
200
150
300
233
200
293
(4)1 Calculate the partial pressure of monatomic hydrogen gas at 2000oK and 1 atm pressure.
For ½H2(g)  H(g),
o
o
 H 298
= 217,990 J and  S 298
= 49.35 J/K
Assume that the heat capacity of H(g) = 1.5R. The heat capacity of H2 can be assumed to be 31
J/(moloK).
1
Kyle, B.G., Chemical and Process Thermodynamics, Prentice Hall, 1999
(5)1 For the reaction:
Co(s) + ½O2(g) = CoO(s)
o
o
=  59,850 + 19.6T, where Grxn
is in calories and T is in kelvin.
Grxn
(a) Calculate the oxygen equilibrium pressure (atm) over Co and CoO at 1000oC.
o
(b) What is the uncertainty in the value calculated in part a if the error in the H rxn
term is
estimated to be  500 cal?
(6) Calculate the temperature at which silver oxide (Ag2O) begins to decompose into silver and
oxygen upon heating:
(a) in pure oxygen at P = 1 atm.
(b) in air at Ptotal = 1 atm.
Data: hof for Ag2O =  7300 cal/mol
Standard Entropies at 298oK [cal/(molK)]
Ag2O
O2
Ag
29.1
49.0
10.2
Assume that Cp = 0 for the decomposition reaction.
(7)1 One step in the manufacture of specially purified nitrogen is the removal of small amounts of
residual oxygen by passing the gas over copper gauze at approximately 500oC. The following
reaction takes place:
2Cu(s) + ½O2(g)  Cu2O(s)
(a) Assuming that equilibrium is reached in this process, calculate the amount of oxygen present
in the purified nitrogen.
(b) What would be the effect of raising the temperature to 800oC? Or lowering it to 300oC? What
is the reason for using 500oC?
(c) What would be the effect of increasing the gas pressure?
o
For 2Cu(s) + ½O2(g)  Cu2O(s), Grxn
=  39,850 + 15.06T
(8) Pack cementation is a process where a pure element or master alloy is deposited on the
surface of a superalloy to extend its life in corrosive and oxidizing environments at high
temperature. There are four constituents to this process: a filler, a pure element or master alloy,
an activator, and a substrate. The inert or filler provides a medium for vapor transport, e.g.,
aluminum oxide Al2O3. The pure element or master alloy will be deposited on the substrate. The
activator is used to transport the master alloy through the filler to the substrate, which is the
surface of the superalloy. We will consider the case where aluminum with AlF3 activator will be
mixed with aluminum oxide powder in a pack cementation process at 1300oK. A schematic of
the process is shown in Figure 8-1 where the system is maintained at 1 atm in an environment of
Argon gas. The bulk pack is the region where aluminum and activator exist within the filler. In
Depleted
zone
Substrate
Bulk
pack
Coating
the depleted zone, there is no aluminum or activator. For this process aluminum is transferred
from the bulk pack to the substrate in the form of aluminum flouride vapor, under the action of
the thermodynamic activity gradient that exists between the pack and substrate.
Figure 8-1 Schematic of pack aluminizing process.
At the bulk pack the following reactions will occur
AlF3(s) = AlF3(g)
(8-1)
2Al(l) + AlF3(s) = 3AlF(g)
(8-2)
Al(l) + 2AlF3(s) = 3AlF2(g)
(8-3)
2AlF3(g) = Al2F6(g)
(8-4)
Since the melting point of pure aluminum is 933.6oK, aluminum will exist in the bulk pack as a
liquid. The five partial pressures (PAlF, PAlF2, PAlF3, PAl2F6, and PAr) in the bulk pack can be
obtained from the four equilibrium conditions above and the assumption that
PAlF + PAlF2 + PAlF3 + PAl2F6 + PAr = 1 atm
(8-5)
Table 8-1 provides data for the Gibbs energies of formation for the species present in the process.
(T is in K and log is natural log [ln])
o
Species
g rxn
AlF3(c)
dGAlF3c = -364.127-2.148e-3*T*log(T)-.695e-6*T^2+75.825/T+80.828e-3*T;
AlF(g)
dGAlF = -64.681+3.384e-3*T*log(T)-.111e-6*T^2+16.925/T-41.184e-3*T;
AlF2(g)
dGAlF2 = -167.465+3.113e-3*T*log(T)-0.109e-6*T^2+47.250/T-26.530e-3*T;
AlF3(g)
dGAlF3 = -291.345+2.282e-3*T*log(T)-.323e-6*T^2+79.025/T+0.712e-3*T;
Al(c,l)
0
Al2F6(g)
dGAl2F6 = -633.645+0.566e-3*T*log(T)-.608e-6*T^2+242.500/T+68.953e-3*T;
Determine the partial pressures PAlF, PAlF2, PAlF3, PAl2F6, and PAr in the bulk pack at 1300 K.