Endothermic barium hydroxide

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. 

We saw in Section 8.5 that an enthalpy change is often called a heat of reaction because it is a measure of the heat flow into or out of a system at constant pressure. If the products have more enthalpy than the reactants, then heat has flowed into the system from the surroundings and AH has a positive sign. Such reactions are said to be endothermic (endo means "within," so heat flows in). The reaction of 1 mol of barium hydroxide octahydrate with ammonium chloride, for example, absorbs 80.3 kj from the surroundings (AH° = +80.3 kj). The surroundings, having lost heat, become so cold that water freezes around the outside of the container (Figure 8.6). 

FIGURE 8.6 The reaction of barium hydroxide octahydrate with ammonium chloride is so strongly endothermic and draws so much heat from the surroundings that the temperature falls below 0°C and moisture freezes around the container. 

Bassam Z. Shakhashiri, "Endothermic Reactions of Hydrated Barium Hydroxide and Ammonium Salts," Chemical Demonstrations, A Handbook for Teachers of Chemistry, Vol. 1 (The University of Wisconsin Press, Madison, 1983) pp. 10-12. 

We ve said on several occasions that chemical and physical processes occur spontaneously only if they go "downhill" energetically so that the final state is more stable and has less energy than the initial state. In other words, energy must be released for a process to occur spontaneously. At the same time, though, we ve said that some processes occur perfectly well even though they are endothermic and absorb heat. The reaction of barium hydroxide octahydrate with ammonium chloride shown in Figure 8.6, for example, absorbs 80.3 kj of heat (AH° = +80.3 kj) and leaves the surroundings so cold that moisture freezes around the outside of the container. 

Some chemical changes are also endothermic processes. Figure 3 shows a chemical reaction that occurs when barium hydroxide and ammonium nitrate are mixed. Notice in Figure 3 that these two solids form a liquid, slushlike product. Also, notice the ice crystals that form on the surface of the beaker. As barium hydroxide and ammonium nitrate react, energy is absorbed from the beaker s surroundings. As a result, the beaker feels colder because the reaction absorbs energy as heat from your hand. Water vapor in the air freezes on the surface of the beaker, providing evidence that the reaction is endothermic. 

Figure 15-2 An endothermic process, (a) When solid hydrated barium hydroxide,...

The reaction of ammonium chloride and barium hydroxide octahydrate is endothermic. 

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. 

Owhen solid hydrated barium hydroxide, Ba(0H)2 8H2O, and excess solid ammonium nitrate, NH4NO3, are mixed, an endothermic reaction occurs. 

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. 

The polybutadienes prepared with these barium t-butoxide-hydroxide/BuLi catalysts are sufficiently stereoregular to undergo crystallization, as measured by DTA ( 8). Since these polymers have a low vinyl content (7%), they also have a low gl ass transition temperature. At a trans-1,4 content of 79%, the Tg is -91°C and multiple endothermic transitions occur at 4°, 20°, and 35°C. However, in copolymers of butadiene (equivalent trans content) and styrene (9 wt.7. styrene), the endothermic transitions are decreased to -4° and 25°C. Relative to the polybutadiene, the glass transition temperature for the copolymer is increased to -82°C. The strain induced crystallization behavior for a SBR of similar structure will be discussed after the introduction of the following new and advanced synthetic rubber. 

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