132 UNITY AND DIVERSITY IN BIOCHEMISTRY 



In fact, if the pressure P remains constant, and the increase in volume is 

 A V, then the work which has been done is equal to PA V. Since we have 

 selected an endothermic reaction, AH has a positive value. Its value would 

 have been negative if an exothermic reaction had been chosen. 



At 20° and 1 atmosphere, AH in the case of the combustion of glucose 

 (CgHiaOe + 60, -> 6H2O + 6CO2) has a value of -673,000 calories per 

 mole of glucose (heat of combustion), whilst in the case of a mole of palmitic 

 acid (C16H32O2 + 23O2 -> I6CO2 + I6H2O) its value is -2,380,000 

 calories. 



If we place in a calorimeter, at a temperature of 15° (one calorie = the 

 amount of heat required to raise the temperature of a gram of water from 

 14-5° to 15-5°), zinc, mercurous sulphate and water in the molar proportions 

 in accordance with the following equation : 



Zn + HggSO^ + 7H2O -> ZuSO^.THgO + 2Hg 



we obtain the products indicated and we can measure a AH value of 

 —82,000 calories. 



However, if we assemble the same components into a battery at 15° 

 and 1 atmosphere so that it functions perfectly reversibly, the electrical 

 energy obtained is only equivalent to 66,000 calories. The term free 

 energy (AF) is given to the maximum work which is obtainable from a 

 chemical reaction taking place under completely reversible conditions 

 at constant temperature and pressure. In the case of the reaction 

 between zinc, mercurous sulphate and water, AH — AF = (—82,000) — 

 (-66,000) = -16,000 cal/mole. 



This represents the energy which is lost (in the form of heat) during the 

 reaction. Although the first law of thermodynamics states that there is a 

 definite relation between work and heat, it says nothing about the work 

 which can be obtained from a given amount of heat. It says only that the 

 total amount of energy does not change. 



The second law of thermodynamics states that there are definite re- 

 strictions on the transformation of heat into work. The weight which 

 furnishes a clock with its mechanical energy descends by a spontaneous 

 process. It is possible, at constant temperature, by the introduction of 

 external work, to cause the weight to be raised again, but one cannot re- 

 verse this spontaneous process by supplying heat at constant temperature. 

 In natural spontaneous processes, a part of the energy liberated is not used 

 to perform work at constant temperature and pressure. This fraction, 

 which is not used isothermally, divided by the absolute temperature T, is 

 the increase in entropy (AS). All spontaneous processes are accompanied 

 by an increase in entropy. Such a process is the diffusion of a substance iii 

 solution from a concentrated solution to a less concentrated solution. The 

 increase in entropy which accompanies such a process can be considered 



