Page 108 - Modeling of Chemical Kinetics and Reactor Design

78 Modeling of Chemical Kinetics and Reactor Design

Assuming a steady state process (∆E = 0), no kinetic (KE) or
potential energy (PE) changes and mechanical work done (W) by the
system on the surroundings are zero, that is KE, PE, and W = 0, the
general energy balance

∆E = E – E = –∆{(H + KE + PE) } + Q – W (2-96)
1
2
m
where
∆E = ( U + KE + PE) m − ( U + KE + PE) m (2-97)
t2 2 t1 1

reduces to

{
Q = ∆ H = ∆ H + ( ∆ H − H )}
o
o
f
products
{
o
− ∆ H o f + ( ∆ H − H )} (2-98)
reactants
or

Q = ∆ H o + H ( ∆ − ∆ H ) (2-99)
rxnT ref products reactants

Calculating the heat of reaction is a multi-step process. Beginning
with the standard heats of formation at 298 K, first calculate the
standard heat of reaction, and then calculate ∆H for the actual system
temperature and pressure. The heat of reaction at 298 K, ∆H 298 is
usually referred to as the standard heat of reaction. This can be readily
calculated from the standard heats of formation of the reaction com-
ponents. The standard heat of reaction is expressed as:

∆H o = (∑ α • ∆H )
rxnT 298 products products (2-100)

− (∑ α reactants • ∆H reactants )

The heat of reaction at the system temperature is

∆H = ∆H o + ∆H − ∆H (2-101)
rxnT rxnT 298 products reactants