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

A double arrow is used in equilibrium equations as an indication that a

chemical reaction does not stop when chemical equilibrium is established;

rather, it proceeds in both directions at the same rate. That is, at equilibrium,

the reactants are depleted at exactly the same rate as they are replenished

from the products by the reverse reaction.

16–3 ■ SOME REMARKS ABOUT THE KP

OF IDEAL-GAS MIXTURES

In the last section we developed three equivalent expressions for the equilib-

rium constant KPof reacting ideal-gas mixtures: Eq. 16–13, which expresses

KPin terms of partial pressures;Eq. 16–14, which expresses KPin terms of

the standard-state Gibbs function change∆G*(T); and Eq. 16–15, which

expresses KPin terms of the number of molesof the components. All three

relations are equivalent, but sometimes one is more convenient to use than

the others. For example, Eq. 16–15 is best suited for determining the equi-

librium composition of a reacting ideal-gas mixture at a specified tem-

perature and pressure. On the basis of these relations, we may draw the

following conclusions about the equilibrium constant KP of ideal-gas

mixtures:

1.The KPof a reaction depends on temperature only.It is independent of

the pressure of the equilibrium mixture and is not affected by the presence

of inert gases. This is because KPdepends on ∆G*(T), which depends on

Chapter 16 | 799

Analysis This is a dissociation process that is significant at very high tem-

peratures only. For simplicity we consider 1 kmol of H 2 , as shown in Fig.

16–8. The stoichiometric and actual reactions in this case are as follows:

Stoichiometric:

Actual:

reactants products

(leftover)

A double-headed arrow is used for the stoichiometric reaction to differentiate

it from the actual reaction. This reaction involves one reactant (H 2 ) and one

product (H). The equilibrium composition consists of 0.9 kmol of H 2 (the

leftover reactant) and 0.2 kmol of H (the newly formed product). Therefore,

NH 2 0.9 and NH0.2 and the equilibrium constant KPis determined

from Eq. 16–15 to be

From Table A–28, the temperature corresponding to this KPvalue is

Discussion We conclude that 10 percent of H 2 dissociates into H when the

temperature is raised to 3535 K. If the temperature is increased further, the

percentage of H 2 that dissociates into H will also increase.

T3535 K

KP

NHnH

NHnH 22

a

P

Ntotal

b

nHnH 2



1 0.2 22

0.9

a

10

0.90.2

b

2  1

0.404

H 2 ¡ 0.9H 2 0.2H

H 2 ¬∆¬2H¬ 1 thus nH 2 1 and nH 22

1 kmol H 2

10 atm

Initial

composition

0.9H 2

0.2H

Equilibrium

composition

FIGURE 16–8

Schematic for Example 16–2.

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