Page 466 - Elements of Chemical Reaction Engineering Ebook

Sec. 8.2 The Energy Balance 437
Example 8-3 Heat of Reaction

Calculate the heat of reaction for the synthesis of ammonia from hydrogen and
nitrogen at 150°C in kcal/mol of N2 reacted and in kJ/mol of H2 reacted.
Solzition

2NH3
N2 + 3H2 4
Calculate the heat of reaction at the reference temperature using the heats of forma-
3
tion of the reactiug species obtained from Perry's Handbook or the Handbook of
Chemistry and Physics.
= 2H~3(TR)-3H;;2(TR)-H~2(TR) (~a,-3.i)

The heats of formation of the elements (H,, N2) are zero at 25°C.
AH;Z,(TR) = 2H&$TR)-3(0)-0 2Hk3
= 2(-11,020) -
Cal
mol N,
= -22,040 cal/mol N, reacted
or
AHix(298 K) = -22.04 kcal/mol N, reacted
= -92.22 W/mol N, reacted
Exothermic reaction The minus sign indicates the reaction is exothemzic. If the heat capacities are con-
stant or if the mean heat capacities over the range 25 to 150°C are readily available,
the determination of aHRx at 150°C is quite simple.

CPHZ = 6.992 cal/mol H,. K
CPN2 = 6.984 cal/mol N, K

8.92
tpNH, cal/mol NH, K
=
ACp = 2CpNH - 3CpH2 - CPN2 (E8-3.2)
= 2(8.92) - 3(6.992) - 6.984
= - 10.12 cal/g mol N, reacted-K

AHR,(T) = AH~x(~R)+A~,(~-~R) (8.-27)
AH~,(423 K) = -22,040+ (-10.12)(423-298)
= -23,310 cal/mol N, = -23.31 kcallmol N,
= -97,530 J/mol N,
(Recall: 1 cal = 4.18.4 J)

R. H. Perry, D. 'W. Green, and J. 0. Maloney, eds., Perry's Chemical Engine,ers'
Handbook, 6th ed. (New York: McGraw-Hill, 1984), pp. 3-147.