Page 46 - A Working Method Approach For Introductory Physical Chemistry Calculations

30 Chapter 3
Consider the situation in a lecture theatre, when the lecturer
leaves for 10 minutes. What is the result? Chaos and disorder! The
class will not maintain the same order as that assumed when the
lecturer is present! Such an analogy is basically the Second Law of
Thermodynamics, i.e. the entropy or disorder, S, of the universe
tends to a maximum (i.e. chaos). The first two laws of thermody-
namics (Law of Conservation of Energy and the tendency for the
entropy of the universe to increase to a maximum) can be
summarised as follows:

First and Second Laws of Thermodynamics: The energy of the
universe is constant (First Law) and the entropy, S, of the universe
tends to a maximum (Second Law).

Consider the following three examples:
(a) N203(g) + NO(,) + NOz(,). Here, 1 mole of gas --.) 2 moles of gas.
Hence, the disorder has increased, i.e. AS is + ve.
(b) NH3(,) + HCl,,, + NH4C1,,). Here there is a change of state from
the highly disordered gaseous state to the more ordered crystalline
solid state. Hence, the disorder has decreased, i.e. AS is - ve.
(c) H20(s) + H200). A liquid is more disordered than a solid; hence,
AS is + ve for this reaction.
AS is defined as the change in entropy. AT (at standard state
conditions) can be determined in the same way as AH" is evaluated:
I ASo = C[S"(Products)] - C[S"(Reactants)] I

The Third Law of Thermodynamics
At 0 K, the vibrational motion of a molecule is at a minimum, and in
a pure crystalline solid, with no defects, the entropy or disorder, S, of
the crystal at this temperature is actually zero. This is the Third Law
of Thermodynamics.

Third Law of Thermodynamics: At absolute zero (i.e. 0 K or
-273 "C), the entropy, S, or disorder of a perfect crystalline
substance is zero.

This concept is shown in Figure 3.3.