Page 187 - Modelling in Transport Phenomena A Conceptual Approach

11 ~~1 1 mole of compound in its
6.3. CONSERVATION OF ENERGY 167

Elements in their
states
standard
V, am)
(T, 1 am) G; standard state
1
Figure 6.4 Calculation of the standard heat of formation, AI?:.

The standard state is usually taken as the stable form of the element or com-
pound at the temperature of interest, T, and under 1 atm (1.013 bar). Therefore,
the word standard refers not to any particular temperature, but to unit pressure
of latm. The elements in their standard states are taken as the reference state
and are assigned an enthalpy of zero. The standard heat of formation of many
compounds are usually tabulated at 25 "C and can readily be found in the Perry's
Chemical Engineers' Handbook (1997) and thermodynamics textbooks. For exam-
ple, the standard heat of formation of ethyl benzene, CsHlo, in the gaseous state is
29,790 J/ mol at 298 K. Consider the formation of ethyl benzene from its elements
by the reaction
8C(s) + 5H&) = CSHlOk)
The standard heat of formation is given by
-
(AI?~)c~H,~ fi,2sHlo 8 fi,2 - 5 fig2 = 29,790 J/ mol
=
Since Hz = H& = 0, it follows that

=
(A~~:)c~H~~ fi&H1O 29,790 J/ mol
=
It is possible to generalize this result in the form

(6.3-23)

The standard heat of formation of a substance is just the standard heat of reaction
in which one mole of it is formed from elementary species. Therefore, the standard
heat of reaction, AH&,, is the difference between the total enthalpy of the pure
product mixture and that of the pure reactant mixture at standard conditions as
shown in Figure 6.5.

Reactans in their Products in their
standard states standard states
Cr, 1 am)

Figure 6.5 Calculation of the standard heat of reaction, AH&,