Page 35 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch03 Elementary Physical Chemistry

Chapter 3

The Second Law of Thermodynamics

The First Law enables one to determine energy changes that accompany
given processes. The First Law does not say whether such processes occur
(spontaneously!) or do not occur. Such and many other experimental facts
are not covered by the First Law.
For example,

1) When two bodies in thermal contact are brought together, heat will flow
from the hotter to the colder body. The reverse process (heat flowing
from the colder to the hotter body) does not even occur. There would
be no violation of the First Law if heat would flow from the colder to
the hotter body as long as there is no net gain or loss of energy.
2) Expansion of a gas into a vacuum occurs spontaneously, but the reverse
does not occur.
3) Some chemical reactions occur spontaneously, others do not. It was once
thought that if ∆U is negative (energy is lowered), the change will occur
spontaneously. This is frequently true but not always. For example, the
reaction H 2 O(s, 10 C) → H 2O(l, 10 C) occurs spontaneously although
◦
◦
∆U is positive (∼ +1.5kcal mol −1 ).
4) It was once thought also that a negative ∆H is the proper criterion
for spontaneity. This is generally the case but not always. For example,
∆H for the reaction Ag(s)+1/2Hg Cl 2 (s) → AgCl(s)+Hg(l) is positive
2
(+1.28 kcal mol −1 ) but occurs spontaneously.
5) Some solvents (for example, benzene and toluene) mix completely
although heat has to be supplied and the enthalpy change is positive.

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