CHAPTER I 

 GENERAL PRINCIPLES OF BIOCHEMICAL ENERGETICS 



I. FREE ENERGY 



A. Free Energy and Work 



The first law of thermodynamics states that the total amount of energy in a 

 system does not change when the different forms of this energy (chemical, 

 mechanical, thermal, electrical, etc.) are converted from one to the other. 

 If a system receives a certain quantity of energy AE, from outside, then the 

 outside loses the same amount AE' and in all the systems together taking 

 part in the exchange, the total energy change is zero : 



AE= - AE' 



Let us consider, for example, a system in the gaseous state. On receiving 

 from its surroundings an amount of heat Q, it will expand and in doing so 

 will perform work W, contributing to the energy of the surroundings : 



AE=Q-W 



But if the system we are considering is unable to perform external work, 

 as would be the case in a container of constant volume in which an endo- 

 thermic reaction is occurring whose energy is supplied by the surroundings in 

 the form of heat, then, we have 



AE=Q 



The reactions studied in the laboratory are not of the type just described. 

 They generally take place at atmospheric pressure, that is, with a change of 

 volume. Let us consider again the above reaction, only this time allowing 

 the pressure to remain constant, which implies that the volume changes and 

 work is done on the surroundings. The system will receive the heat Q 

 from the outside and whilst the endothermic reaction is taking place the 

 reaction mixture will undergo a change in volume performing positive or 

 negative work. The change in heat content of the whole system under 

 consideration is measured by the amount of heat Q, which is divided into 

 that part which brings about a change in the energy of the reacting system 

 and another part equal to the work which has been performed. 



AH=Q = AE+ PAV 

 (variation in heat content) 



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