Page 161 - Physical Chemistry

lev38627_ch05.qxd 3/3/08 9:33 AM Page 142

142
Chapter 5 allow us to measure only changes in enthalpies, internal energies, and entropies ( H,
Standard Thermodynamic U, and S). Therefore, thermodynamics does not provide absolute values of U, H,
Functions of Reaction
and S, but only relative values, and we cannot tabulate absolute enthalpies of sub-
stances. Instead, we tabulate standard enthalpies of formation. The next section de-
fines the standard enthalpy of formation H° of substance i and shows that H° of
f T,i T
Eq. (5.3) is given by H° n H° .
T i i f T,i
Phase Abbreviations
The letters s, l, and g stand for solid, liquid, and gas. Solids that have an ordered struc-
ture at the molecular level are called crystalline (abbreviated cr), whereas solids with
a disordered structure are called amorphous (abbreviated am); see Sec. 23.1. The term
condensed phase (abbreviated cd) means either a solid or a liquid; fluid phase
(abbreviated fl) means either a liquid or a gas.

5.3 STANDARD ENTHALPY OF FORMATION

The standard enthalpy of formation (or standard heat of formation) H° of a
f T
pure substance at temperature T is H° for the process in which one mole of the sub-
stance in its standard state at T is formed from the corresponding separated elements
at T, each element being in its reference form. The reference form (or reference
phase) of an element at temperature T is usually taken as the form of the element that
is most stable at T and 1-bar pressure.
For example, the standard enthalpy of formation of gaseous formaldehyde
H CO(g) at 307 K, symbolized by H° , is the standard enthalpy change
2 f 307,H 2 CO(g)
H° for the process
307
1
C1graphite, 307 K, P°2 H 1ideal gas, 307 K, P°2 O 1ideal gas, 307 K, P°2 S
2 2 2
H CO1ideal gas, 307 K, P°2
2
The gases on the left are in their standard states, which means they are unmixed, each
in its pure state at standard pressure P° 1 bar and 307 K. At 307 K and 1 bar, the sta-
ble forms of hydrogen and oxygen are H (g) and O (g), so H (g) and O (g) are taken
2
2
2
2
as the reference forms of hydrogen and oxygen. At 307 K and 1 bar, the most stable
form of carbon is graphite, not diamond, so graphite appears in the formation reaction.
Consider H° of HBr(g). At 1 bar, Br boils at 331.5 K. Therefore, H° of
f
2
f
330
HBr(g) involves liquid Br at 330 K and 1 bar reacting with standard-state H (g),
2
2
whereas H° of HBr(g) involves gaseous standard-state Br reacting.
335
2
f
Since H°values are changes in enthalpies, they can be found from experimen-
f
tal data and thermodynamics equations; for details, see Sec. 5.4.
Foran element in its reference form, H° is zero. Forexample, H° of graphite
f
f
T
307
is, by definition, H°of the reaction C(graphite, 307 K, P°) → C(graphite, 307 K, P°).
Nothing happens in this “process,” so its H°is zero. For diamond, H° is not zero,
f
307
but is H°of C(graphite, 307 K, P°) → C(diamond, 307 K, P°), which experiment
gives as 1.9 kJ/mol.
Even though a particular form of a substance may not be stable at temperature T
and 1 bar, one can still use experimental data and thermodynamics equations to find
H° of that form. For example, H O(g) is not stable at 25°C and 1 bar, but we can
f
2
T
use the measured heat of vaporization of liquid water at 25°C to find H° 298 of
f
H O(g); see Sec. 5.10 for details.
2
We now prove that the standard enthalpy change H° for a chemical reaction is
T
given by
¢H° a n ¢ H° T,i (5.6)*
i
f
T
i