Barium hydroxide reaction with ammonium

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. 

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. 

OC-Hydroxycarboxylic Acid Complexes. Water-soluble titanium lactate complexes can be prepared by reactions of an aqueous solution of a titanium salt, such as TiCl, titanyl sulfate, or titanyl nitrate, with calcium, strontium, or barium lactate. The insoluble metal sulfate is filtered off and the filtrate neutralized using an alkaline metal hydroxide or carbonate, ammonium hydroxide, amine, or alkanolamine (78,79). Similar solutions of titanium lactate, malate, tartrate, and citrate can be produced by hydrolyzation of titanium salts, such as TiCl, in strongly (>pH 10) alkaline water isolation of the... 

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. 

Tetrammino-palladous Hydroxide, [Pd(NII3)4](OH)2, may be obtained by decomposing the sulphate with barium hydroxide. It separates as a colourless crystalline substance which is soluble in water and has a strong alkaline reaction. The aqueous solution is capable of precipitating copper, iron, cobalt, and nickel from solutions of their salts, and it also decomposes ammonium salts. 

An intimate mixture ot 274 grms. of rubidium iron alum, or 260 grms. of rubidium aluminium alum with 100 grms. of calcium carbonate, and 27 grms. of ammonium chloride, is heated in a nickel crucible to a dull red heat until ammonia vapours are no longer evolved, and then the temp, is raised to redness. The product is ground with a litre of cold water for 15 minutes filtered by suction and washed with 400 c.c. of water, added in small portions at a time. The combined sulphuric acid is precipitated by the addition of barium hydroxide, and the filtered liquid boiled while a stream of carbon dioxide is passed through the soln. If the soln. loses its alkaline reaction, and yet retains some calcium, a little rubidium carbonate must be added to precipitate calcium carbonate. The soln. is then treated with hydrochloric acid and evaporated. 

Ammonia and primary aliphatic and aromatic amines convert 4-pyrones into 4-pyridones ° this must involve attack at an a-position, then ring opening and reclosure in some cases ring-opened products of reaction with two mole equivalents of the amine have been isolated, though such structures are not necessarily intermediates on the route to pyridones. The transformation can also be achieved by, first, hydrolytic ring opening using barium hydroxide (see above), and then reaction of the barium salt with ammonium chloride. ... 

To overcome the restrictions of the Barium method, ammonium carbonate was, in some cases, replaced by anionic exchange resins that are less aggressive and enable the treatment of tempera even in the presence of copper pigments. It must be added that the reaction of anion exchange resins is restricted to the surface in contact with the resin. Furthermore, the first-stage transformation producing barium sulfate is left out and barium hydroxide is applied directly. This technique is convenient with the many restrictions imposed by preconsolidation by synthetic resins. 

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