Endothermic ammonium thiocyanate

FIGURE 6.9 The endothermic reaction between ammonium thiocyanate, NH4SCN, and barium hydroxide octahydrate, Ba(0H)2-8H20, absorbs a lot of heat and can cause water vapor in the air to freeze on the outside of the beaker. 

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. 

Certainly many favourable physical and chemical changes are exothermic. Some favourable changes, however, involve no release of energy. Others are actually endothermic. For example, a favourable endothermic chemical reaction occurs when barium hydroxide and ammonium thiocyanate are placed in an Erlenmeyer flask, as shown in Figure 7.4. 

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. 

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) ... 

FIGURE 8.10 A favorable but endothermic reaction. The reaction of barium hydroxide and ammonium thiocyanate is one of the uncommon examples of a favorable reaction that absorbs heat from its surroundings. 

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. 

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