5i6 ON INCREASE IN SIZE [pt. iii 



and Thomson, a solid theoretical basis. The quantity fx is called the 

 temperature characteristic in distinction from the temperature co- 

 efficient, for it is the calculated "energy of activation" of the active 

 molecule in the reaction governing the slowest process in the complex 

 chain of processes under investigation. It is an index, therefore, of the 

 nature of the catalyst in operation, and should do much more than 

 merely distinguish between chemical and physical processes. Reactions 

 having the same catalyst have the same temperature characteristic. 

 For chemical reactions ju. varies between 4000 and 35,000, and can 

 be represented graphically by the slope of the straight line or lines, if 

 there is a break, relating the log. of the velocity in question to the 

 reciprocal of the absolute temperature. The break indicates that one 

 reaction of the catenary has ceased to be the slowest and another 

 has taken its place. The steeper the slope, the greater the increase 

 of velocity with unit rise, and the higher the value of fju. The log. 

 effect/reciprocal of absolute temperature relation does not always 

 give a straight line, but only if the process that is being measured is 

 irreversible. If an equilibrium effect is functioning, then the relation 

 will be a curve. Finally, fju has nothing to do with the Q^ of the 

 van't Hoff equation, for the former has reference to heat of activation 

 and the latter to heat of reaction. 



The advantages of the Arrhenius equation over the van't Hoff 

 equation are considerable. The former has a much greater range of 

 values dealing with thousands instead of decimal units. The Arrhenius 

 equation also reveals abrupt breaks in the straight lines relating 

 log. effect to the reciprocal of absolute temperature at which one 

 limiting reaction is supposed to take the place of another ; these are 

 masked by the van't Hoff equation. Then Q,io is not a constant 

 while jLt is, at any rate between certain definite temperatures where 

 the breaks occur. But most important of all, Q^^q values cannot be 

 definitely associated with specific types of chemical reaction, such as 

 oxidation-reduction, hydrolysis, and synthesis, so that all one can 

 hope to find out by the van't Hoff equation is whether the process 

 is physical or chemical, while with the Arrhenius equation one may 

 discover, perhaps, of what nature the controlling change is at any 

 given moment. 



Arrhenius himself in 191 5 made many applications of this equation 

 to biological processes, and it is interesting that he gives as an average 

 value for segmenting eggs fx 14,100, but without any reference or 



