OXIDATION AND REDUCTION 



to any appreciable extent. On the other hand, leucosafranine readily 

 reduces methylene blue. With this in mind, one can set up a sequence 

 of all reversible oxidation-reduction systems according to their oxida- 

 tive power. For the four systems mentioned, the sequence is: ferri- 

 cyanide, indophenol, methylene blue, safranine. 



A quantitative expression of the oxidizing power is the "oxida- 

 tion-reduction potential" of a dye in mixture with its leuco dye. Vari- 

 ous substances capable of reversible oxidation and reduction can be 

 arranged in a sequence according to their oxidation-reduction, or 

 "redox," potentials. Each membei', when present in the reduced 

 state, can be oxidized by any following member present in its oxidized 

 state; and each member, when present in the oxidized state, can be 

 reduced by any preceding member present in its reduced state. 



Let us now discuss the kinetic aspect of the problem. If we are 

 dealing only with reversible redox systems, as above, establishment 

 of the equilibrium is always so rapid that the reaction may be con- 

 sidered almost instantaneous. But this is not the case, in general, if 

 an irreversible reaction occurs. In the oxidation of alcohol to acetal- 

 dehyde, for example, to oxidize alcohol, a powerful oxidizing agent 

 such as chromic acid must be used; and to reduce acetaldehyde, a 

 powerful reducing agent such as sodium amalgam must be used. 

 Although an excess of oxidative (or reductive) power must be applied 

 in order to make the reaction proceed, the reaction is sluggish. Addi- 

 tional energy is required far beyond the quantity expected on a purely 

 thermodynamic basis because, obviously, an obstacle has to be over- 

 come. Despite the fact that the reaction between alcohol and chromic 

 acid releases energy, energy must first be spent, which is of course 

 eventually released again; and this extra energy is called the activa- 

 tion energy. Although the path of energy is, as a whole, downward, 

 it must first pass over a hill. Generally an activation energy is required 

 for all bimolecular chemical reactions. In reversible reactions the 

 energy of activation is very small, interaction occurring only when the 

 two molecules "collide." The collision is impeded by the fact that 

 molecules of any kind, on approaching each other in the course of 

 thermal motion, will exhibit a mutual repulsion, and only those mole- 

 cules which happen to have enough kinetic energy to overcome the 

 repulsion will really collide and react with others. 



In irreversible o.xidations another, more serious, impediment 



219 



