The Foundations of Chemistry

When
Hand
Shave opposite signs (classes 1 and 4), they act in the same direction,
so the direction of spontaneous change does not depend on temperature. When
Hand

Shave the same signs (classes 2 and 3), their effects oppose one another, so changes in
temperature can cause one factor or the other to dominate, and spontaneity depends on
temperature. For class 2, decreasing the temperature decreases the importance of the unfa-
vorable T
Sterm, so the reaction becomes product-favored at lower temperatures. For
class 3, increasing the temperature increases the importance of the favorable T
Sterm,
so the reaction becomes product-favored at higher temperatures.
We can estimate the temperature range over which a chemical reaction in class 2 or 3
is spontaneous by evaluating
H^0 rxnand
S^0 rxnfrom tabulated data. The temperature at
which
G^0 rxn0 is the temperature limit of spontaneity. The sign of
S^0 rxntells us whether
the reaction is spontaneous belowor abovethis limit (Table 15-7).

Figure 15-15 A graphical

representation of the dependence of


Gand spontaneity on temperature

for each of the four classes of

reactions listed in the text and in

Table 15-7.

TABLE 15-7 Thermodynamic Classes of Reactions

H
S Temperature Range
Class Examples (kJ/mol) (J/molK) of Spontaneity

1

2H 2 O 2 (
)88n2H 2 O(
)O 2 (g)  196  126 All temperatures

H 2 (g)Br 2 (
)88n2HBr(g) 72.8  114 All temperatures

2

NH 3 (g)HCl(g)88nNH 4 Cl(s)  176  285 Lower temperatures ( 619 K)

2H 2 S(g)SO 2 (g)88n3S(s)2H 2 O(
)  233  424 Lower temperatures ( 550 K)

3

NH 4 Cl(s)88nNH 3 (g)HCl(g)  176  285 Higher temperatures ( 619 K)

CCl 4 (
)88nC(graphite)2Cl 2 (g)  135  235 Higher temperatures ( 517 K)

4

2H 2 O(
)O 2 (g)88n2H 2 O 2 (
)  196  126 Nonspontaneous, all temperatures

3O 2 (g)88n2O 3 (g)  285  137 Nonspontaneous, all temperatures

           

NOT SPONTANEOUS

SPONTANEOUS

(∆H > 0)

∆H

= +,

∆S = – (Class 4)

∆H

(^) = +,
∆S
= +
(Class 3)
∆H
= –,
∆S
= –
(Class 2)
(∆H < 0)
∆H
= –,
∆S = + (Class 1)
∆G
0

T