Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

802 | Thermodynamics

Assumptions 1 The equilibrium composition consists of CO 2 , CO, and O 2.

2 The constituents of the mixture are ideal gases.

Analysis The stoichiometric and actual reactions in this case are as follows:

Stoichiometric:

Actual:

products reactants

(leftover)

C balance:

O balance:

Total number of moles:

Pressure:

The closest reaction listed in Table A–28 is CO 2 ∆CO ^1 – 2 O 2 , for which

ln KP2.801 at 2600 K. The reaction we have is the inverse of this, and

thus ln KP2.801, or KP16.461 in our case.

Assuming ideal-gas behavior for all components, the equilibrium constant

relation (Eq. 16–15) becomes

Substituting, we get

Solving for xyields

Then

Therefore, the equilibrium composition of the mixture at 2600 K and 304

kPa is

Discussion In solving this problem, we disregarded the dissociation of O 2

into O according to the reaction O 2 →2O, which is a real possibility at high

temperatures. This is because ln KP7.521 at 2600 K for this reaction,

which indicates that the amount of O 2 that dissociates into O is negligible.

(Besides, we have not learned how to deal with simultaneous reactions yet.

We will do so in the next section.)

1.906CO 2  0.094CO  2.074O 2

z 3 

x

2

2.047

y 2 x0.094

x1.906

16.461

x

12 x 213 x> 221 >^2

a

3

5 x> 2

b

 1 > 2

KP

NCOnCO 22

NCOnCONOnO 22

a

P

Ntotal

b

nCO 2 nCOnO 2

P304 kPa3.0 atm

Ntotalxyz 5 

x

2

8  2 xy 2 z¬or z 3 

x

2

2 xy ¬ or y 2 x

2CO3O 2 ¡xCO 2 yCOzO 2

CO^12 O 2 ∆CO 2 1 thus nCO 2 1, nCO1, and nO 2 ^122

(^1231552553)
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