40 CHEMICAL TRANSFORMATIONS 



the total chemical energies of the substances reacting, as shown 

 by combustion, are negligibly small. For example, the heat of 

 combustion of ethyl alcohol is 3405 K. 



The chemical energy required in the combination of hexoses 

 to form disaccharides, a reaction which has been shown above 

 to be reversible by enzymes, or in the formation of starches 

 and fats from their proximate constituents, such as occur in the 

 body cells, is also so small as to be incapable of measurement. 

 But if we neglect small differences in heats of solution of the carbo- 

 hydrates, we can obtain some idea of how small the heats of 

 reaction are by taking the differences in heats of combustion of 

 the substances on the two sides of the equation. 



In the following equations for formation of the three best 

 known examples of disaccharides from their constituent hexoses, 

 the heat of combustion of each sugar is written below its formula, 

 and the difference is given as the approximate heat of reaction 

 in rational calories : 



-31 K. 



(Glucose) (Fructose) (Cane-sugar) 



6737 K + 6759 K = 13,527 K - 31 K. 



2. C 6 H 12 5 + C 6 H 12 6 = C 12 H 22 O n + H 2 - 33 K. 

 (Glucose) (Glucose) (Maltose) 



6737 K + 6737 K = 13,507 K - 33 K. 



3. C 6 H 12 6 + C 6 H 12 6 = C 12 H 22 O n + H 2 0-81 K. 

 (Glucose) (Galactose) (Lactose) 



6737 K + 6696 K = 13,514 K - 81 K. 



It is hence obvious how small the change in chemical energy, 

 as shown approximately by heat of reaction, is in practically 

 reversible reactions as compared with those reactions which run 

 practically to completion. 



The effect of change in temperature is on the whole opposed 

 to that of change of chemical energy ; a rise in temperature 

 having the same effect as a drop in chemical energy and tending 

 to increase the reversibility of the reaction. 



This is seen from the fact that the part of the constant K, which 

 changes with alterations in chemical energy C arid temperature T, 



c 

 is the expression e RT , in which it is clear that similar changes 



