164 



A CONCEPTUAL INTRODUCTION TO BIOENERGETICS 



from one state to another, rather than absolute quantities in any state, the 

 absolute quantities disclosed via the Third Law permit easy evaluation of 

 the changes. 



More Detailed Consideration of the First Law. Enthalpy or Heat Content 



The internal energy of a body is defined as the sum total of all the kinetic 

 and potential energy contained within the body. When expressed per gram 

 molecular weight it is given the symbol U cal/mole, and is a "state vari- 

 able," that is, one whose value depends only upon the temperature, pres- 

 sure, and composition, irrespective of how it arrived at this condition. Heat 

 energy, (that contained in the motion of the molecules), potential energy of 

 the electron cloud of the atom, and the binding energy of the nucleus all 

 contribute to the internal energy. 



If a transformation takes place in one molecular weight of a substance, 

 two things in general can occur: energy can be taken in by the substance, 

 and work can be done. If an amount of energy, q, is taken in, and an amount 

 of work, w, is done, the difference, q — w, must be the increase in energy of 

 the substance during the process; this difference must be stored as internal 

 energy, and hence the change in internal energy is: 



AU = q - w 



where 



AU 



U 2 - U x or 



^final ' ' ^initial 



Now AU = q — w is the concise, algebraic statement of the First Law. 

 The concepts are illustrated in Figure 7-2. 



>> 



a 

 UJ 



(a) 



environment 



final 



AU 



(b) 



initial 



State 



Figure 7-2. The First Law of Thermodynamics: (a) a state diagram showing 

 internal energy change, A il, during a process; (b) the process: heat taken in, q, 

 and work done, w. 



