126 



INTRODUCTION TO IMMUNOCHEMICAL SPECIFICITY 



Fig. 9-5. Two possible reversible routes from state 1 to state 2. 



H 



E + PV 



(4) 



it is obvious that // is a thermodynamic function, for we have seen 

 that E depends solely on the values of P and V, and the product 

 PV depends only on these variables. Therefore dH is an exact dif- 

 ferential. The quantity H is called the total heat, or enthalpy. 

 Enthalpy is important because, when the pressure remains constant, 

 which it does during most chemical reactions, the change in enthalpy 

 is equal to the heat absorbed or heat given off, as follows : 



±(AH)p = ±(A(2)p (5) 



The subscript P indicates that the variable P (pressure) remains 

 constant. The sign convention, positive for heat absorbed and nega- 

 tive for heat given off, was more natural in the study of heat en- 

 gines than it is in chemistry, but is now firmly established. 



A// is interesting because in many cases a large negative \H 

 for a chemical reaction goes along with a strong tendency for the re- 

 action to go spontaneously. Indeed it was long believed that —(\H)p 

 was the proper measure of the spontaneity of a reaction. It was 

 eventually found that not — (AH)p but the change in another thermo- 

 dynamic function, the free energy, is the proper measure of the 



