Ammonium thiocyanate, reaction with barium hydroxide

Perhaps you have seen a demonstration of the endothermic but favorable reaction of barium hydroxide and ammonium thiocyanate. This reaction absorbs so much heat from its surroundings that it can freeze water in contact with the reaction flask (Figure 8.10) ... 

The acid-base chemistry reaction of barium hydroxide (Ba(OH)2) with ammonium thiocyanate (NH SCN) in water creates barium thiocyanate, ammonia, and water. The reaction is highly favorable, but is also so endothermic that the solutions cools to such an extent that a layer of frost forms on the reaction vessel. Explain how an endothermic reaction can be favorable. 

A favourable endothermic reaction occurs when barium hydroxide is mixed with ammonium thiocyanate. If you removed the stopper in photograph (B), you would detect the characteristic odour of ammonia. 

FIGURE 5.8 Endothermic and exothermic reactions, (a) When ammonium thiocyanate and barium hydroxide octahydrate are mixed at room temperature, the temperature drops, (b) The reaction of powdered aiummum with Fe203 (the thermite reaction) proceeds vigorousiy, reieasing heat and forming Ai203 and moiten iron. 

Barium thiocyanate was first prepared by Berzelius, who roasted barium hexacyanoferrate(II) with sulfur. It has also been obtained by reaction of barium carbonate with a solution of thiocyanic acid, by conversion of ammonium thiocyanate through the copper (I) thiocyanate by consecutive reactions with copper(I) chloride and barium hydroxide, by treatment of Prussian blue with barium sulfide, and by reaction of barium sulfide, sulfur, and cyanamide. The procedure described below makes possible the preparation of barium thiocyanate in any desired quantity from barium hydroxide and ammonium thiocyanate as starting materials. The 3-hydrate, " which is obtained first, is dehydrated readily to yield anhydrous barium thiocyanate. 

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