Concise Physical Chemistry

c05 JWBS043-Rogers September 13, 2010 11:25 Printer Name: Yet to Come

74 ENTROPY AND THE SECOND LAW

but this is not the case. There is more to think about in a chemical or physical change

than just minimization ofUorH. There is the question of order and disorder of the

reactant state and the product state.

5.1.3 Disorder

When we look at a chemical reaction

A→B

or an analogous physical change

A(l)→ A(g)

we must look at the driving force that produces the change. Part of that force comes

from the tendency to seek a minimum (water flows downhill), but part of it comes

from the universal tendency of thermodynamic systems to seek maximum disorder.

A familiar example is vaporization of a liquid such as water.

The liquid state, though not perfectly ordered, is held together by strong inter-

molecular forces. These are the very forces that we say are nonexistent or negligible

in the vapor state. The entropy change for many liquids is about 88 J K−^1 mol−^1 ,a

rule known as Trouton’s rule, that has been verified many times over for liquids as

diverse as liquid Cl 2 , HCl, chloroform, and then-alkanes. Liquids that deviate from

this rule do so, not because of any failure of the entropy concept for vaporization, but

because of abnormal forces in the liquid state. An example is water, which deviates

a little due to hydrogen bonding, and hydrogen fluoride HF, which deviates a lot.

5.2 ENTROPY CHANGES

In general, the entropy change for any change of state, including melting and change in

crystalline form in the solid state, is given by the enthalpy changeandthe temperature

appropriate to the change ortransitionconsidered:

S=

Htrans

Ttrans

5.2.1 Heating

The entropy of heating of an ideal gas is positive because thermal agitation makes

the high-temperature state more disordered than the low-temperature state. From the

expression for molar heat capacity, one has

Cp=

(

∂H

∂T

)

p