RICHARDS AND WELLS. — TRANSITION TEMPERATURE. 



437 



In this case the reaction is a transition from the dihydrate (NaBr. 2 2H 2 0) 

 to the anhydrous salt (NaBr) and its saturated solution ; the result is to 

 be represented approximately by the following statement : — 



l03NaBr2H 2 O = GlNaBr + 42NaBr, 206H 2 O. 



This statement is based upon the knowu fact that a saturated solution of 

 sodic bromide at the transition temperature contains about 11 G. 8 parts 

 of anhydrous salt to 100 of water ; in other words, almost exactly 42 

 grams of anhydrous salt dissolve in the two gram-molecules (36 grams) 

 of water set free from one gram-molecule of salt. The transition is rep- 

 resented upon the accompanying diagram of solubility curves. 



130 



120 



UO 



100 



00 



bU 



7T~" -_-__-- — 1 — ---------- 



0' 20' 40° 00° 80° 100' 



Figure 1. Solubility of Sodic Bromide in Water. 



The heat of this reaction may easily be computed approximately, 

 although we have not the data for exact computation. Obviously, ex- 

 cept for the effect of the possible change of heat capacity of the reacting 

 system during the reaction, the method of calculation is simply to sub- 

 tract from one another the heats of solution of the initial and final sys- 

 tems in a large excess of water. This w 7 ould give the hypothetical heat 

 of reaction at 20°, to which a correction would have to be added in order 

 to compute its value at 50°. Now, Thomsen has found that the heat of 

 solution of a gram-molecule of crystallized sodium bromide in water is 

 — 4.71 Calories, or — 19.7 kilojoules, and that the heat of solution of 

 anhydrous sodium bromide is only — 0.19 Calorie, or — 0.8 kilojoules. 

 Neglecting the heat of dilution of the part dissolved in transition, we 

 may thus compute the heat of transition to be — 19.7 + -j 6 ^ X .8 = 19.2 

 kilojoules. It may be of interest to compare this with the latent heats 



