Page 111 - Modeling of Chemical Kinetics and Reactor Design

Thermodynamics of Chemical Reactions 81

the chemical reaction is exothermic and will require cooling. If ∆H o r
> 0, the reaction is endothermic and heating is therefore required.
o
The correlation of ∆H for the ideal gas at low temperature is based
f
on a series expansion in temperature and is expressed as:
o
∆H = A + BT + CT 2 (2-112)
f
o
where ∆H = heat of formation of ideal gas a low pressure, kcal/gmol
f
A, B, and C = correlation constants
T = temperature, K
The Appendix at the end of this chapter shows heats of formation
of some compounds.

Example 2-1
Calculate the heat of reaction for the synthesis of ammonia from
hydrogen and nitrogen at 1 atm and 155.0°C in N + 3H → 2NH 3
2
2
1. kcal/kmol of N reacted.
2
2. kJ/mol of H reacted.
2
3. The true equilibrium constant (K).
The heats of formation, the standard molar entropies at 298 K, are
given below.

∆S o ∆S o ∆H o
s s f
Component kJ/kmol • K cal/gmol • K kcal/kmol
N (g) 191.9 45.87 0.0
2
H (g) 130.9 31.29 0.0
2
NH (g) 192.9 46.10 11,020
3

The heat capacities, expressed as quadratic function of temperature,
are shown below:

Component C , kcal/kmol • K
p
–3
–6 2
N (g) 6.457 + 1.39 × 10 T – 0.069 × 10 T
2
–3
–6 2
H (g) 6.946 – 0.196 × 10 T + 0.476 × 10 T
2
–6 2
–3
NH (g) 5.92 – 8.963 × 10 T – 1.764 × 10 T
3