(a) Using Fig. 9.3, what is the pressure of O2 in equilibrium with a mixture of Cr and Cr2O3 at 1200oC?

(b) The result of part (a) applies to low pressures (~ 1 atm). How does the equilibrium O2 pressure change if the total pressure is 1000 atm? The densities of Cr and Cr2O3 are 7.2 and 5.2 g/cm3 respectively. The atomic weight of Cr is 52. Hint: Use Eq (5.15) to determine the effect of pressure on the molar Gibbs free energy of the solids, assuming that their molar volumes are constant.

9.20 One mole of an equimolar CO/CO2 gas mixture is contacted with 2 moles of NiO in a closed container at 2000 K. At equilibrium, what is the mole fraction of CO in the gas phase and how many moles of metallic Ni are formed? At 2000 K, the free energy changes of the relevant reactions are:

2CO + O2 =2CO2 AGo = -216 kJ/mole 2Ni + O2 = 2NiO AGo = -46 kJ/mole

9.21 Can magnesium metal reduce UO2 to uranium metal at 1200oC?

9.22 Two formation reactions for Al2O3 are given in Table 9.8, one with Al(s) and the other with Al(L).

2Al(s) + 1.5O2 = Al2O3(s); AGo(s) = As + BsTlnT + CsT 2Al(L) + 1.5O2 = Al2O3(s); AGo(L) = Al + BLTlnT + ClT

where the values of As, CL are given in Table 9.3. Assume that the specific heats of Al(s)

and Al(L) are equal.

(a) Show the relation between AL and As, BL and Bs, and CL and Cs in terms of the heat of fusion of aluminum, AhM and its melting point TM.

(b) Calculate AL from As, BL from Bs, and CL from Cs using the relations developed in part (a) and the data: AhM = 9.7 kJ/mole and TM = 931 K. Compare the calculated results with the numbers for AL, BL, and CL given in Table 9.3.

9.23 A metal M forms a solid oxide MO2(s) and a gaseous oxide MO3(g). The standard enthalpies of the oxidation reactions are AHo (MO2 (s)) = -350 kJ/mole metal and

AHo (MO,(g)) = -250 kJ/mole metal. The metal is exposed to a gas containing O2 at a pressure of 10-3 atm at 1500K.

(a) Write the two oxidation reactions and estimate the standard entropy changes for each [i.e., ASo (MO2(s)) and ASo (MO(g)) ]

(c) What is the partial pressure of MO3(g) in the gas exposed to the metal?

9.24 A initial mixture containing 3 moles of N2, 1 mole of H2, and 4 moles of NH3 is heated to 1200 K and allowed to come into equilibrium at 1 atm total pressure.

(a) Write the reaction.

(b) Calculate the equilibrium constant for this reaction at 1200 K from the following information:

- the enthalpy of formation of NH3(g) at 298K is AH^S= -136 kJ/mole

- the heat capacities (CP) of H2 and N2 are 3.5R and that of NH3(g) is 5R (R = gas constant)

- The standard entropy of the reaction at 1200 K can be estimated from the "entropy rule-of-thumb"

(c) What is the equilibrium composition of the gas (in terms of mole fractions)?

9.25 The CO/CO2/O2 equilibrium in a closed system at a constant pressure was analyzed in Sect. 9.6.2. In the present problem, the initial charge of CO and O2 is contained in a rigid vessel at an initial pressure of 0.25 atm. The temperature is maintained at 2000 K. What is the equilibrium gas composition for this constant-volume condition?

9.26 The hydrogen needed to operate a H2/O2 fuel cell is produced on site by the reaction:

The reactor operates at 700 K at a total pressure of 2 atm.

(a) Decompose this overall reaction into the formation reactions and show how these are combined to produce the overall reaction.

Thermochemical properties of the gases in this reaction are given in the tables below (in kcal)

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