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

This equation has two important implications. First, it provides a means of

calculating the of a reaction from a knowledge of KP, which is easier to

determine. Second, it shows that exothermic reactions such as

combustion processes are less complete at higher temperatures since KP

decreases with temperature for such reactions (Fig. 16–16).

1 hR 6 02

hR

Chapter 16 | 807

EXAMPLE 16–6 The Enthalpy of Reaction

of a Combustion Process

Estimate the enthalpy of reaction for the combustion process of hydrogen

H 2 + 0.5O 2 →H 2 O at 2000 K, using (a) enthalpy data and (b) KPdata.

Solution The at a specified temperature is to be determined using the

enthalpy and Kpdata.

Assumptions Both the reactants and the products are ideal gases.

Analysis (a) The of the combustion process of H 2 at 2000 K is the amount

of energy released as 1 kmol of H 2 is burned in a steady-flow combustion

chamber at a temperature of 2000 K. It can be determined from Eq. 15–6,

Substituting yields

(b) The value at 2000 K can be estimated by using KPvalues at 1800

and 2200 K (the closest two temperatures to 2000 K for which KPdata are

available) from Table A–28. They are KP 1 18,509 at T 1 1800 K and

KP 2 869.6 at T 2 2200 K. By substituting these values into Eq. 16–18,

the value is determined to be

Discussion Despite the large temperature difference between T 1 and T 2

(400 K), the two results are almost identical. The agreement between the

two results would be even better if a smaller temperature interval were used.

hR
251,698 kJ/kmol

ln

869.6

18,509

hR

8.314 kJ>kmol # K

a

1

1800 K



1

2200 K

b

ln

KP 2

KP 1

hR

Ru

a

1

T 1



1

T 2

b

hR

hR

251,663 kJ/kmol

¬ 1 0.5 kmol O 22310  67,881 86822 kJ>kmol O 24

¬ 1 1 kmol H 22310 61,400 84682 kJ>kmol H 24

hR 1 1 kmol H 2 O 231 241,82082,593 99042 kJ>kmol H 2 O 4

¬NO 21 hf°h2000 Kh298 K (^2) O 2
NH 2 O 1 hf°h2000 Kh298 K (^2) H 2 ONH 21 hf°h2000 Kh298 K (^2) H 2
hRaNp 1 hf°hh° (^2) paNr 1 hf°hh° (^2) r
hR
hR
hR
4000
1000
2000
3000
T, K KP
Reaction: C + O 2 → CO 2
4.78  1020
2.25  1010
7.80  106
1.41  105
FIGURE 16–16
Exothermic reactions are less
complete at higher temperatures.
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