Exothermic acid-base reaction

During the formation of CBPCs, because of the exothermic acid-base reaction, the temperature of the reacting aqueous solution rises. This subsequently increases the dissolution... 

Precalcination reduces the rate of dissolution of MgO substantially, and as we shall see later in this chapter, a slurry of MgO and H3PO4 or an acid phosphate can be mixed for several minutes before the exothermic acid-base reaction kicks off. Often, large-scale production of ceramics, such as oil well cements, requires hours of pumping time (see Chapter 15) or long setting times. Even in a production line of refractories that produce castables of consistent quality, the reaction slurry must have a storage life of a few hours. In such cases, chemicals are added to retard the reactions in the slurry. 

Step 3 Take a proton away. Proton transfer between the carboiyl group and the alkoxide ion gives the carboxylate anion. This strongly exothermic acid-base reaction drives the whole reaction to completion. 

Although essentially inert m acid-base reactions alkanes do participate m oxidation-reduction reactions as the compound that undergoes oxidation Burning m air (combus tion) IS the best known and most important example Combustion of hydrocarbons is exothermic and gives carbon dioxide and water as the products... 

Cerium(III) oxide is quite a strong base compared with boron(IIl) oxide (see Table 9.1), and the acid/base reaction is strongly exothermic. No appreciable hydrolysis occurs for CeCl3, which dissolves in water to give Ce3+(aq). A detailed thermochemical analysis of the factors which lead to the dramatic difference in behaviour between B203 and Ce203 is not straightforward, but the considerations set out in Section 10.2 in the discussion of Table 10.1 are relevant. 

The various CBPC products discussed in the last chapter reveal that CBPC powder consists of one or more sparsely soluble oxides and an acid phosphate. When this mixture is stirred in water, the acid phosphate dissolves first and makes the solution acidic, in which the sparsely soluble alkaline oxides dissolve and an acid-base reaction is initiated. This reaction produces slurry that subsequently hardens and a ceramic hard product is formed. If the acid phosphate is phosphoric acid solution, the setting reaction is too rapid. Such a process becomes impractical for production of large ceramic objects because the rapid acid-base reaction is exothermic and that boils the reaction slurry. Therefore, less acidic acid phosphates (such as chhydrogen phosphates) are preferred for fabrication of practical ceramics. 

As shown in Table 4.2, the dissociation constants of the acid phosphates vary widely. (NH4)H2P04 and KH2PO4 have low pK o as well as low molar solubilities (see Table 4.1) and hence are suitable to form a ceramic. Salts with higher pAijo can form ceramics in small volumes but cannot be used for practical ceramics, because their solubility is high and the acid-base reactions are too rapid and also exothermic. 

As stated before, the overall acid-base reaction is exothermic and heats the reaction slurry. To avoid excessive heating of the slurry, the reaction rate of the dissolved species should be slow. Thus, following are the two requirements for forming a well-crystaUized CBPC. 

Note that dissolution of acid phosphates is an endothermic reaction that cools the slurry, while the subsequent reaction between dissolved ions is an exothermic reaction. The net acid-base reaction is, however, exothermic and generates heat. In the case of formation of MgKP04 6H20, for example, the initial cooling of the ceramic is significant and aids in... 

The Ceramicrete process is based on the acid-base exothermic reaction. As a result, the exothermic heat evolution and its rate depend on the size of the waste forms produced. Larger forms generate more heat, which does not dissipate rapidly. Thus, the setting slurry heats up and accelerates the acid-base reaction, and the mixture is able to set even in cold surroundings. 

As Table 15.3 indicates, the heat offormation of Ceramicrete-based permafrost cement is typically 50-60% of the heat of formation of conventional cement. Even though the acid-base reaction is highly exothermic, i.e., it releases a significant amount of heat during setting. In CBS compositions, the binder that produces heat is only a part of the entire CBS formulation, and the remaining components are extenders. Thus, the net amount of heat generated is about half that in the equivalent amount of conventional cement. 

Large excess binary fluorides are reacted with RTIL fluorohydrogenates when they are volatile. However, stoichiometric reaction is necessary in the case of involatUe fluorides. This Lewis acid-base reaction is usually highly exothermic and precautions are necessary. Table 18.3 shows some physical properties of... 

X 102 (qj. q 55 ] cal) of heat energy were released by this acid-base reaction to the surroundings, the solution the reaction is exothermic. 

Most chemical reactions that are exothermic will be either acid-base reactions or oxidation-reduction reactions. Thus, as you think about any single chemical as something that may have incompatible partners, you can ask ... 

Often the exothermicity of the acid-base reaction is enough to produce a violent reaction, but there are some instances where it is not the exothermicity per se of the reaction that is so dangerous as much as the products. We look at a few common examples. 

It may not be clear from the preceding section how the energy of the reaction correlates with the acid-base reaction that converts 1 to 2. The calculated value of AH° for the reaction of 1 with sodium amide assumes that AS° is small that term is ignored, so at room temperature assume that AG° AH° -—29.2 kcal moH. What does this value mean If the reaction is exothermic, the reaction of 1 with sodium amide is more favorable than the reaction of 2 with ammonia. 

These cements are based on the chemical reaction of solid basic magnesia powder and an aqueous acidic ammonium phosphate solution.The acid-base reactions that occur on mixing lead to the formation of insoluble magnesium ammonium phosphate hydrates, the primary binding material in the hardened product. Commercial products generally contain inert fillers and a set retarder to facilitate control of the exothermic setting process. 

For now, you can interpret "favorable as meaning downhill/exothermic. Such reactions are called thermodynandcalfy favorable. (pKa data tell you if an acid-base reaction is thermodynamically favorable.) Later we will discuss reactions that are downhill (thermodynamically favorable), but still do not produce products because they are too slow (kinetically unfavorable). 

Anesthesiologists appreciate that the simple acid-base reactions are highly exothermic. Consequently, a properly functioning system is warm to the touch. 

If a drybox is not available, the preparation can also be carried out by use of a dry, unreactive solvent (typically an alkane) as a blanket against hydrolysis. This has been suggested in the patent literature as a method for the large-scale industrial preparation of Eewis acid-based ionic liquids, as the solvent also acts as a heat-sink for the exothermic complexation reaction [28]. At the end of the reaction, the ionic liquid forms an immiscible layer beneath the protecting solvent. The ionic liquid may then either be removed by syringe, or else the solvent may be removed by distillation before use. In the former case it is likely that the ionic liquid will be contaminated with traces of the organic solvent, however. 

To evaluate the heat exchange/productivity performances of the device and its environment, an acid-base neutralization involving sulfuric acid and soda has been performed. It is an instantaneous and exothermic reaction with AH = —92.4 kj moP (NaOH). Each experiment is characterized by the initial concentration of the reactants (from 10 to 30% in mass of soda and from 5 to 12% in mass of sulfuric acid). These concentrations are varied in order to evaluate the behavior of the reactor with respect to different amounts of heat generated (from 0.4 to 1.3 kW). Each run is performed with a variable utility flow rate (from 1 to 3 m h ). 

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