Page 106 - Modeling of Chemical Kinetics and Reactor Design

76 Modeling of Chemical Kinetics and Reactor Design

depends on the conditions under which the reaction is carried out. The
standard heat of reaction is the enthalpy variation when the reaction
is carried out under standard conditions using pure components in their
most stable state or allotropic form, at standard pressure (1 atm) and
temperature (usually, but not necessarily 298 K).
The values for standard heats of reaction may be found in the
literature or calculated by thermodynamic methods. The physical state
of the reactants and products (e.g. gas, liquid, or solid) must also be
specified, if the reaction conditions are such that different states may
coexist. For example,

Hg ( ) + 1 O g ( ) → H O g ( ) ∆ H 298 =− 241 6 . kJ (2-90)
2
2
2
2
Hg ( ) + 1 O g ( ) → H O l ( ) ∆ H 298 =− 285 6 . kJ (2-91)
2
2
2
2
In process design calculations, it is usually more convenient to
express the heat of reaction in terms of the enthalpy per mole of
product formed or reactant consumed. Since enthalpy is a state function,
standard heats of reaction can be used to estimate the ∆H at different
temperatures by making a heat balance over a hypothetical process:

∆H rxnT = ∆H rnxT o + ∆H products − ∆H reactants (2-92)

where ∆H rxnT = heat of reaction at temperature, T
∆H = heat of reaction at a known standard temperature
rnxT o
∆H reactants = enthalpy change to bring reactants from temperature
T to standard temperature
∆H products = enthalpy change to bring products from the standard
temperature back to reaction temperature T

The specific heats, C are usually expressed as a quadratic or a
p
polynomial function of temperature and expressed as:

2
C = A + ( B •10 −2 • T) +( C •10 −5 • T ) (2-93)
p i i i i
or

2
3
C = A + ( B •10 −2 • T) + ( C •10 −5 • T ) + ( D •10 −9 • T ) (2-94)
p i i i i i