CHEMICAL EQUILIBRIUM 243 



Now, when the temperature is raised, the action proceeds in the 

 direction of decomposing more of the pentachloride. That is, the 

 equilibrium is displaced in the direction which absorbs heat. 



In Deacon's process, we find that the interaction of hydrogen 

 chloride and oxygen liberates heat: 



4HC1 + 2 <= 2H 2 + 2C1 2 + 28,000 cal. 



and in this action raising the temperature drives the equilibrium 

 backwards, and a lowering in the temperature is required to 

 increase the yield of chlorine. 



The rule is obvious, and applies to all reversible reactions: 

 When the temperature of a system in equilibrium is raised, the 

 equilibrium point is displaced in the direction which absorbs heat. 

 In other words, a rise in temperature favors the interaction of that 

 one of the two sets of materials to which the heat is added (+ sign) 

 in the equation. If the equation happens to be written with a 

 negative heat of reaction (e.g., p. 162), the heat can, of course, be 

 transferred to the other side with its sign changed. This law is 

 known as Van't Hoff's law of mobile equilibrium. 



This law is of practical value. More than once, in chemical 

 factories, much time and money have been spent on trying to 

 arrange machinery to give a better yield of some substance at a 

 high temperature, when a reference to this law would have shown 

 that the chief change necessary was to use a lower temperature. 

 We shall frequently have occasion to refer to this law. 



Application to Physical Equilibria. Van't Hoff's law 

 applies also to physical equilibria. Thus, the vaporization of 

 a liquid absorbs heat, and so an increase in temperature will 

 increase the pressure, and therefore the concentration of its vapor. 

 The special case of a saturated solution may be somewhat more 

 fully considered. 



When we heat a saturated solution, with excess of solid present, 

 more solid will dissolve as the temperature is raised, if solution 

 is attended with absorption of heat. This is the usual case, as is 



