Properties of Liquids at the Boiling-Point. 527 



which on vaporization suffers a partial chemical dissociation 

 into amylene and hydriodic acid. 



Suppose that the process of vaporization takes place in 

 two stages, viz. : — (1) The vaporization of the associated 

 molecules without dissociation ; (2) the dissociation of the 

 associated molecules of vapour. Let the heat absorbed per 

 associated molecule in the first stage be HMn, where H is 

 the specific heat absorbed, M is the molecular weight, and 

 n is the association factor. This will be equal to K VYn. 

 For the second stage let the heat absorbed per associated 

 molecule be XMrc, where X is the specific heat absorbed. 

 Therefore in the complete vaporization of one associated 

 molecule the total heat absorbed (apart from heat required 

 to do external work) may be written 



HMra + XMn = K V Vn + XMn. 



And if \ represents the heat required to do external work 

 during the second stage of the vaporization, the true latent 

 heat L x of vaporization becomes 



L,= ~{KV77i-hXMn}-\, 

 _ K VV _ 



Whereas the calculated latent heat not considering molecular 

 association is 



M 



Therefore we may say that for liquids where molecular 

 dissociation occurs on vaporization, the latent heat calculated 



according to the equation LM=1583 VY may be either 

 greater or smaller than the observed latent heat according to 

 the magnitude of the heat of dissociation. 



It may therefore happen that an associated liquid might 



not show any deviation from the equation LM = K i/V. An 

 example is formic acid for which, as it will be noticed from 

 the Table, the calculated value of the latent heat is very close 

 to the observed result. Water and the alcohols give calculated 

 values lower than the observed, whilst some of the acids 

 give higher and some lower calculated values. 



On the whole it may be said that with the exception of 

 the aliphatic hydrocarbons and ethers, the relation holds 

 for normal liquids with an exactitude which quite corre- 

 sponds to the exactitude of the latent heat determinations.. 



