UNION OF ANTIBODY WITH ANTIGEN 129 



The discovery that the integral of dQ/T* defines a new thermo- 

 dynamic function constitutes also a discovery of the second law of 

 thermodynamics. It is probahly the best way of introducing the con- 

 cept of entropy, which is not, like the concepts of temperature, pres- 

 sure, heat content, etc., an obvious generalization of ideas already 

 more or less familiar to the non-scientist but a subtle and powerful 

 new concept. The best attempt to explain the concept in words is to 

 say that it is a measure of the disorder of a system, or of the extent 

 of the loss of availability of energy. 



The second law of thermodynamics can be stated in words in a 

 number of other ways, though none of them adequately suggests the 

 significance and applicability of the principle. For example, we may 

 say that no heat engine can produce work by taking a quantity of heat 

 from the environment at a certain temperature and returning the 

 unused heat to the environment at the same temperature. Such an 

 engine would be a perpetual motion machine of the "second type," 

 and the second law of thermodynamics asserts that no such machine 

 can ever be constructed. 



The significant thing about the second law for chemists is that it 

 provides a valid measure of the tendency of a process to take place, 

 when the change in entropy 



(the subscripts meaning volume and energy are constant) is large 

 and positive, the process will tend to take place spontaneously, and 

 this tendency is greater the larger a5"f,£;. 



Free Energy 



Although fine for the processes that take place in heat engines, 

 as a measure of the spontaneity of a chemical reaction ^.Sv.e has its 

 drawbacks. In chemical reactions, the volume of the system often 

 does not remain the same and the energy practically never does. 

 Pressure and temperature are usually constant but volume and 

 entropy vary. Consequently, we want a new thermodynamic function. 



* The dQ in the definition of entropy must be the heat absorbed in a re- 

 versible process and is sometimes written explicitly rfQrev. 



