46 SECTIONAL ADDRESSES. 



absorption bauds of the inactive materials lie in the ultra-violet, and 

 those of the activated substances lie in the visible region. 



In the second place it follows that the same super-activated state is 

 produced when the inactive molecule absorbs the quantum Avg at its 

 characteristic frequency, and when the chemically reactive molecule 

 absorbs the quantum /iv., at its characteristic frequency. In this way a 

 possible connection with the radiation hypothesis is indicated. W. C. 

 McC. Lewis developed a relation whereby the observed heat of a reaction 

 may be calculated from the critical increments of activation. As stated 

 on page 36 he obtained the expression 



d log A;/RT=N;iv,/RT2 



where hv^ is the critical increment of activation and k is the velocity 

 constant. If the reaction be monomolecular and reversible then 



d log F/RT=N/iv„/RT^ 



where hv,j is the critical increment of the resultant of the forward reaction 

 and k'- ia the velocity constalnt of the reverse reaction. It follows that 



d log Z/(iT=N7i(v,-v„)/RT2 



where K is the equilibrium constant. Comparing the last expression with 

 the van't HofE isochore 



(llogK;OT=:-Q,/RT^ 



Lewis concluded that the heat absorbed per stoichiometric quantity of 

 the reactant transformed is given by 



-Q=N/i(v,-vJ, 



that is to say, the heat involved in the reaction is equal to the critical 

 increment of the resultant minus the critical increment of the reactant. 



In this argument there is involved the view that in a reversible mono- 

 molecular reaction the activated reactant and activated resultant molecules 

 are indistinguishable from one another. Applying this to a mono- 

 molecular photochemical reaction which is reversible it follows from 

 what has gone before that the photochemical quanta may be substituted 

 for the critical increments in Lewis' expression. The observed heat of 

 the reaction will be given by 



Q=N/Kv,.-Vo) 



where v,. and v^, are the characteristic ultra-violet frequencies of the 

 resultant and reactant molecules, respectively. A near approximation 

 to a monomolecular photochemical reaction is afforded by the conversion 

 of oxygen into ozone, which is reversible. The central wave-lengths of 

 the characteristic ultra-violet absorption bands of these two substances 

 are very near to 185 [xjjl and 2.50 [i.[j,, respectively, the corresponding 

 frequencies being V(,=l-622xlC and v,=l-2 xlO'\ The observed heat 

 of reaction will be 



—Mx4-22xW ergs or —36,400 calories. 



This is very near to the accepted heat of formation of ozone. 



It may be concluded from the foregoing that a definite position has 

 been reached which is of some interest. The radiation hypothesis states 

 that the first stage of a chemical reaction is the activation of each moelcule 



I 



