Ammonium chloride, 523 reaction with barium hydroxide octahydrate

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. 

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. 

What s going on How can the spontaneous reaction of barium hydroxide octahydrate with ammonium chloride release energy yet absorb heat The answer is that, in the context of a chemical reaction, the words "energy" and "heat" don t refer to exactly the same thing. There is another factor in addition to heat that determines whether energy is released and thus determines whether a reaction takes place spontaneously. We ll take only a brief look at this additional factor right now and return for a more in-depth study in Chapter 17. 

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). 

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