Page 163 - Physical Chemistry

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Chapter 5 of a gas is the hypothetical ideal gas at 1 bar, whereas only real gases exist at 1 bar.
Standard Thermodynamic The procedure for this calculation is given at the end of this section.
Functions of Reaction
2. We measure H for mixing the pure elements at T and 1 bar.
2
2
3. We use H C dT (V TVa) dP [Eq. (4.63)] to find H for bringing
1
P
1
the mixture from T and 1 bar to the conditions under which we plan to carry out
∆ f H° 298
the reaction to form substance i. (For example, in the combustion of an element
with oxygen, we might want the initial pressure to be 30 atm.)
4. We use a calorimeter (see after Example 5.1) to measure H for the reaction in
which the compound is formed from the mixed elements.
5. We use (4.63) to find H for bringing the compound from the state in which it is
formed in step 4 to T and 1 bar.
6. If compound i is a gas, we calculate H for the hypothetical transformation of i
from a real gas to an ideal gas at T and 1 bar.
The net result of these six steps is the conversion of standard-state elements at T
to standard-state i at T. The standard enthalpy of formation H° is the sum of these
T,i
f
six H’s. The main contribution by far comes from step 4, but in precise work one in-
cludes all the steps.
Once H° has been found at one temperature, its value at any other temperature
f
i
can be calculated using C data for i and its elements (see Sec. 5.5). Nearly all ther-
P
modynamics tables list H° at 298.15 K (25°C). Some tables list H° at other tem-
f
f
peratures. Some values of H° are plotted in Fig. 5.2. A table of H° is given in
298
f
f
298
the Appendix. Once we have built up such a table, we can use Eq. (5.6) to find H° 298
for any reaction whose species are listed.
EXAMPLE 5.1 Calculation of H°from H° data
f
Find H° for the combustion of one mole of the simplest amino acid, glycine,
298
NH CH COOH, according to
2
2
9 5 1
NH CH COOH1s2 O 1g2 S 2CO 1g2 H O1l2 N 1g2 (5.7)
2
2
2
2
2
2
2
4
2
Substitution of Appendix H° values into H° n H° 298,i [Eq. (5.6)]
f
i i
298
298
f
gives H° as
298
1
5
9
3 102 1 285.8302 21 393.5092 1 528.102 1024 kJ>mol
2
4
2
Figure 5.2
973.49 kJ>mol
H° values. The scales are
f
298
logarithmic. Exercise
Use Appendix data to find H° 298 for the combustion of one mole of sucrose,
C H O (s), to CO (g) and H O(l). (Answer: 5644.5 kJ/mol.)
11
22
12
2
2
Calorimetry
To carry out step 4 of the preceding procedure to find H° of a compound, we must
f
measure H for the chemical reaction that forms the compound from its elements. For
certain compounds, this can be done in a calorimeter. We shall consider measurement
of H for chemical reactions in general, not just for formation reactions.
The most common type of reaction studied calorimetrically is combustion. One
also measures heats of hydrogenation, halogenation, neutralization, solution, dilution,
mixing, phase transitions, etc. Heat capacities are also determined in a calorimeter.
Reactions where some of the species are gases (for example, combustion reactions)