Page 36 - Basic physical chemistry for the atmospheric sciences

22 Basic physical chemistry

s
state . am(X) is called the molar standard enthalpy (or heat) o f
f o rmation (or simply the molar heat o f f o rmation) of compound X .
Some values o f am are given in Appendix V . For an endothermic
reaction aH?x is positive, and for an exothermic reaction it is negative.
Similarly, positive values of am indicate that heat is absorbed when a
compound is formed from its constituent elements in their standard
e
stat s , and negative values of am indicate that heat is released. By
convention, a value of zero is assigned to the enthalpies of formation
of the elements in their standard states. The following example should
clarify these points.
Exercise . 2 . Calculate aH?x for the combustion of ethane , C2H6(g):
2
2C2H6(g) + 70z(g)� 4C02(g) + 6H20(1) (2. 1 5 )

Solution. From Appendix V we see that the am values for C 2 H6(g),
.
-
COz(g), and H 2 0(1) are - 84 7 , - 393. 5 , and - 2 85.6 kJ moI 1 •
.
Applying Eq. (2. 1 4 ) to Reaction (2 1 5) yields
aH?x = {4am(C02(g)) + 6am(H20(1 ) )} - 2af/ll(C2H6(g))
= {4( - 393 . 5 1 ) + 6( - 285. 8 5 ) - 2( - 84.68)} kJ
= - 3 1 2 0 kJ

The minus sign indicates that the reaction is exothermic (i. . , it re­
e
leases heat). The enthalpy of formation of 02(g) does not appear in the
i
expression for aH?x because 02(g) s , by definition, the stable form of
elemental oxyg n ; therefore, its enthalpy of formation is zero. Note
e
also that the value of aW,x that we have calculated applies specifically
to Reaction (2. 1 5) in which 2 moles of C2H6(g) react with 7 moles of
Oz(g) to form 4 moles of C02(g) and 6 moles of H20(1). An alternate
way of expressing the result would be to state that the enthalpy of the
reaction is - 3 1 20/2 or - 1 5 60 kJ per mole of ethane burnt.

2.3 Entropy and the second law of thermodynamics
The incremental change in the entropy (ds) of a unit mass of a system
is defined as

dqrev
ds = (2. 1 6 )
T
or, for a finite change from state l to state 2 ,