Calcium hydroxide reactivity

Suitable catalysts include the hydroxides of sodium (119), potassium (76,120), calcium (121—125), and barium (126—130). Many of these catalysts are susceptible to alkali dissolution by both acetone and DAA and yield a cmde product that contains acetone, DAA, and traces of catalyst. To stabilize DAA the solution is first neutralized with phosphoric acid (131) or dibasic acid (132). Recycled acetone can then be stripped overhead under vacuum conditions, and DAA further purified by vacuum topping and tailing. Commercial catalysts generally have a life of about one year and can be reactivated by washing with hot water and acetone (133). It is reported (134) that the addition of 0.2—2 wt % methanol, ethanol, or 2-propanol to a calcium hydroxide catalyst helps prevent catalyst aging. Research has reported the use of more mechanically stable anion-exchange resins as catalysts (135—137). The addition of trace methanol to the acetone feed is beneficial for the reaction over anion-exchange resins (138). 

Po22olans contain reactive siUca which reacts with cement and water by combining with the calcium hydroxide released by the hydration of the calcium siUcates to produce additional calcium sihcate hydrate. If sufficient siUca is added, about 30% of the weight of cement, the calcium hydroxide can... 

The low permittivity of these liquids compared with water inhibits dissociation of the acids so that cement formation demands much more reactive basic oxides. Oxides and hydroxides that are capable of cement formation are ZnO, CuO, MgO, CaO, Ca(OH)2, BaO, CdO, HgO, PbO and BiaOj (Brauer, White Moshonas, 1958 Nielsen, 1963). In practice these are confined to two calcium hydroxide and special reactive forms of zinc oxide. 

As for solvents, liquid ammonia or dimethylsulfoxide are most often used. There are some cases when tert-butanol is used as a solvent. In principle, ion-radical reactions need aprotic solvents of expressed polarity. This facilitates the formation of such polar forms as ion-radicals are. Meanwhile, the polarity of the solvent assists ion-pair dissociation. This enhances reactivity of organic ions and sometimes enhances it to an unnecessary degree. Certainly, a decrease in the permissible limit of the solvent s polarity widens the possibilities for ion-radical synthesis. Interphase catalysis is a useful method to circumvent the solvent restriction. Thus, 18-crown-6-ether assists anion-radical formation in the reaction between benzoquinone and potassium triethylgermyl in benzene (Bravo-Zhivotovskii et al. 1980). In the presence of tri(dodecyl)methylammonium chloride, fluorenylpi-nacoline forms the anion-radical on the action of calcium hydroxide octahydrate in benzene. The cation of the onium salts stabilizes the anion-radical (Cazianis and Screttas 1983). Surprisingly, the fluorenylpinacoline anion-radicals are stable even in the presence of water. 

Crystallization of magnesium hydroxide by a continuous mixed suspension mixed product removal crystallizer was conducted to make clear the characteristics of reactive crystallization kinetics of magnesium hydroxide, which was produced by the precipitation from magnesium chloride with calcium hydroxide. The following operating factors were investigated affecting the crystallization kinetics the initial concentration of feeds, residence time of reactants, feed ratio of reactants, and concentrations of hydroxide and chloride ions. 

The present work deals with the reactive crystallization of magnesium hydroxide, a well-known sparing soluble material, from magnesium chloride with calcium hydroxide. Magnesium hydroxide is produced industrially by the precipitation from brine with... 

The lower reactivity of benzaldehyde with respect to acetaldehyde was found also in the vapour phase aldolisation over lithium phosphate [390]. Over the same catalyst, the reactivity order in the self-condensations of aldehydes could be estimated as CH3CHO > CH3CH2CHO (CH3)2-CHCHO. The reactivity of isobutyraldehyde in the self-condensation was almost undetectable, probably due to steric hindrance on the a-carbon, but this substance was able to react as a hydrogen acceptor with cyclohexanone. With propionaldehyde over a calcium hydroxide catalyst, a Cannizzaro-type reaction occurred to some extent simultaneously with the aldolisation [390]. This unexpected result was also recorded by other authors [391], who established that the tendency to aldolisation decreased, and the tendency to the Cannizzaro reaction increased, with... 

Calcium Sulfide. (CAS 20548-54-3. CaS, grayish-white solid, reactive with HjO. formed by reaction of calcium sulfate and carbon at high temperatures. Calcium hydrogen sulfide. CadlSh. formed in solution by saturating calcium hydroxide suspension with H S pK,p of CaS = 7.24. 

Hydroxides of reactive metals show no decomposition when they are heated. The hydroxides of moderately reactive metals do decompose to produce the metal oxide and water. This process is used to convert calcium hydroxide (slaked lime) into calcium oxide (lime). 

Because calcium oxide is a fairly reactive powder, it forms calcium hydroxide when in contact with water. This reaction is exothermic and hence heats water during formation of the hydroxide. Because of this excess heat, it cannot directly be used to form phosphate ceramics by reacting it with an acid phosphate solution and must be used in a less soluble form as sparsely soluble silicate or hydrophosphate. In spite of this difficulty, because human bones contain calcium phosphate, there have been sufficient efforts in developing methods of forming biocompatible CBPCs of calcium phosphate by using partially soluble phosphates of calcium rather than using oxide itself. A similar approach may also be taken if one uses partially soluble silicate or aluminate of calcium. These routes are discussed in Chapter 13. 

Chemical leach tests on the bulk settled dust samples showed that the dusts are quite chemically reactive. Leach solutions have high alkali-nities, due to the rapid partial dissolution of calcium hydroxide from concrete particles. Indoor dust samples produced higher pH levels (11.8-12.4) and alkalinities (—600 mg CaCOa) than outdoor dusts (pH 8.2-10.4 alkalinity —30mgL CaCOa), indicating that outdoor dust samples had reacted with rainfall or other water prior to collection. Thurston et al (2002) found that the leachate pH of the dusts decreased with decreasing particle size. Some metals or metalloids in the dusts (aluminum, chromium, antimony, molybdenum, barium, copper, zinc, cobalt, nickel) are readily leached by deionized water many of these form oxyanion species or carbonate complexes that are most mobile at the alkaline pH s generated by the leachates. 

Although, for obvious reasons, fractional precipitation from salt solutions is by far the most economical of the methods discus.sed for the industrial fractionation of starch, such processes as that of Cantor and WimmeH might possibly become of interest for the production of reactive intermediates of each starch fraction. Several types of chemical derivatives of both starch components can be synthesized by way of the calcium hydroxide complexes. 

An important factor that affects the rate of a chemical reaction is the reactive nature of the reactants. As you know, some substances react more readily than others. For example, calcium and sodium are both reactive metals however, what happens when each metal is added to water is distinctly different. When a small piece of calcium is placed in cold water, as shown in Figure 17-7a, the calcium and water react slowly to form hydrogen gas and aqueous calcium hydroxide. 

In dry slaking calcium hydroxide is obtained as a dry powder. Use is made of the fact that so much heat is produced by the reaction of calcium oxide with water that the amount of water bound is approximately equal to that which can be evaporated. The so-called reactivity of the quicklime influences the product quality. Highly reactive lime leads, upon slaking, to a granular material, which has to be ground again. 

Portland cement is typically composed of about 25% P-dicalcium silicate (lamite), and 50% tricalcium silicate with the balance made up of various calcium aluminates and calcium iron aluminate (brownmillerite). Setting occurs when the cement is hydrated all the components show varying degrees of reactivity with water, but the most significant hydraulic activity is associated with the tricalcium silicate, which forms a cohesive mixture of calcium hydroxide and calcium silicate hydrate (C-S-H)... 

The most common absorbents are lime (calcium hydroxide) and limestone (calcium carbonate) slurries. Limestone is the preferred absorbent in many modern scrubbers, because of its low cost compared with lime and other absorbents.However, lime is also used because of its higher reactivity, which allows it to absorb sulfur more rapidly. This makes it possible to use smaller scrubbers to treat a given quantity of gas when lime is the absorbent. When lime or limestone... 

The trend to increasing reactivity with increasing size of atom for the alkaline earth metals is illustrated by the reaction of the elements with water, as shown in Figure 8.7. Beryllium does not react with water. Magnesium reacts with hot water. But calcium reacts with water to form calcium hydroxide [Ca(OH)2], as shown by this equation. 

Calcium is fairly reactive—too reactive to be found as a pure metal in nature. When it reacts with water, it forms calcium hydroxide [Ca(OH)2], and when it reacts with oxygen, it forms calcium oxide (CaO), or lime, one of calciums most common compounds. (Limestone is a rock primarily made out of calcium carbonate.) Calcium compounds are used to make building materials such as concrete, marble, and gypsum. 

EXPLOSION and FIRE CONCERNS flammable NFPA rating Health 4, Flammability 4, Reactivity 2 bums in air with a blue flame very dangerous fire hazard may polymerize explosively at 122-140°F or in the presence of traces of alkali severe explosion hazard when subjected to chemical reaction with oxidizers gas forms explosive mixtures with air reacts violently with acetaldehyde flashback along vapor trail may occur vapor may explode if ignited in confined area closed containers may rupture violently when heated reaction with water, steam, acid, or acid fumes emits toxic fumes of CN incompatible with amines, strong oxidizers, acids, sodium hydroxide, calcium hydroxide, sodium carbonate, water, caustics, and ammonia use water spray, dry chemical, alcohol foam, or carbon dioxide for firefighting purposes. 

EXPLOSION and FIRE CONCERNS flammable liquid NFPA rating Health 1, Flammability 3, Reactivity 1 very dangerous fire hazard containers may explode in fire vapor may explode if ignited in enclosed area flashback along vapor trail may occur violent reaction with calcium hydroxide, hydrocarbons, hydroxides, and inorganic bases may ignite on contact... 

Calcium metal is moderately reactive. If pieces of calcium are added to water, the metal begins to bubble as hydrogen gas is formed. The water begins to turn cloudy as solid calcium hydroxide begins to form. Write the unbalanced chemical equation... 

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