Reactions of the Alkali and Alkaline Earth Metals

Nowadays, the gold content of ores is much too low for these simple mechanical separation methods to be effective. Instead, the ore is treated with very dilute (0.01 M) sodium cyanide solution, through which air is blown. The following redox reaction takes place  

Write balanced equations for the reaction of (a) sodium with hydrogen. (b) barium with oxygen. 

Strategy The formulas of the products can be deduced from Table 20.1. Note that barium forms two products with oxygen, BaO and Ba02. 

Reaction of calcium hydride (CaHJ with water. 

Reaction of sodium with water. When a very small piece of sodium is added to water, it reacts violently with the water. The OH- ions formed in the reaction turn the acid-base indicator phenolphthalein from colorless to pink. 

TABLE 20.1 Reactions of the Alkali and Alkaline Earth Metals  

The compounds formed by the reaction of hydrogen with the alkali and alkaline earth metals contain H ions for example, sodium hydride consists of Na+ and H ions. These white crystalline solids are often referred to as saline hydrides because of their physical resemblance to NaCl. Chemically, they behave quite differently from sodium chloride for example, they react with water to produce hydrogen gas. Typical reactions are  

The heat evolved in the reaction frequently causes the hydrogen to ignite. 

Among the Group 2 metals, Ca, Sr, and Ba react with water in much the same way as the alkali metals. The reaction with calcium is 

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Section 20.1 deals with the processes by which these metals are obtained from their principal ores. Section 20.2 describes the reactions of the alkali and alkaline earth metals, particularly those with hydrogen, oxygen, and water. Section 20.3 considers the redox chemistry of the transition metals, their cations (e.g., Fe2+, Fe3+), and their oxoanions (e.g., Cr042-). ... 

As well as an acid-independent pathway, an acid-dependent pathway also occurs for the dissociation of a number of the alkali and alkaline earth metal cryptates (Table 7.1). Similar behaviour to that just discussed for methanol also occurs for reactions in a range of other non-aqueous solvents (Cox, Truong Schneider, 1984). 

Water, in its reaction with the alkali- and alkaline-earth metals, resembles ammonia, but the complexes with the halides of the platinum metals are different. The water molecule has two lone pairs of electrons, but these pairs seem to be less active in complex formation. There are many cases in which from the magnetic moment it can be concluded that the hydrates are still ionic, whereas in the corresponding NH3 complex there is covalency, the NH3 molecules sharing their lone electron-pairs with the metal atom. 

In the case of the alkali and alkaline earth metals, the sulphate is the sole product with copper, lead, bismuth and antimony, the product contains the metal, formed by reduction of the sulphate by unchanged sulphide. In cases where the sulphate is unstable at the temperature of reaction, as with zinc, cadmium, aluminium, tin, chromium, iron, cobalt and nickel, the oxide is the final product. The action may be catalytically accelerated by the addition of triferric tetroxide, Fe304.2... 

The alkylation reaction of various alkali and alkaline-earth metal carbonates with alkyl halides R(CH2)nX (X = Cl, Br, I) is a primary synthetic procedure in organic chemistry for obtaining various symmetrical and unsymmetrical dialkyl carbonates under phase-transfer conditions in polar aprotic solvents [45]. Excellent yields may be obtained by running the reaction at 383 K in ionic liquids such as... 

The relationship between the composition and structure of borates and their decomposition in aqueous solution has been reviewed (78, 226,414,417). Borates of the alkali and alkaline-earth metals give an alkaline reaction in solution, as the borates formed by hydrolysis possess a lower boron-to-metal ratio than in the initial material (414). 

The silicides of the alkali and alkaline-earth metals are prepared by melting the metal with Si. To obtain pure silicides, the synthesis must be carried out in an inert atmosphere. The reactions proceed at high T, e.g., ca. 600°C and above. Because silicides react slowly with carbon and oxides, syntheses are carried out in graphite or refractory oxide crucibles. 

The hydrides formed in reaction (a) may be classified as (1) saline or ionic hydrides, (2) metallic hydrides and (3) covalent hydrides. The saline hydrides include the hydrides of the alkali and alkaline-earth metals, except BeHj, which is covalent. Transition metals form binary compounds with hydrogen that are classified as metallic hydrides including rare-earth and actinide hydrides. Intermetallic compound hydrides, such as TiFeHj and LaNijH, may be thought of as pseudobinary metallic hydrides. 

Write chemical equations describing the reactions of O2 with each of the alkali and alkaline earth metals. Account for differences within each family. 

A number of hydroxides and carbonates of the alkali and alkaline-earth metals have been used as catalysts for these reactions. Lobry de Bruyn and Alberda van Ekenstein considered the hydroxide ion to be responsible for their effect. Michaelis and Rona examined the transformation of D-glucose in several alkaline buffers and found that the re-... 

Solubility only the cyanides of the alkali and alkaline earth metals are soluble in water the solutions have an alkaline reaction owing to hydrolysis. 

These processes are quite rapid with an apparent rate constant which exceeds lO" cm s" [5,6] The only example of electron transfer reaction which has been observed was between the hydrophobic ferro-cinium - ferrocene redox couple in nitrobenzene and the hydrophilic hexacyanoferrate redox couple in water [9]. A more complex mechanism is involved in the case of ion transfer facilitated by an iono-phore[10]. This is the case, for example in the transfer of the alkali and alkaline earth metal cations across a water/nitrobenzene interface facilitated by synthetic neutral cyclic or acyclic iono-phores derived from 3,6-dioxaoctanedicarboxylic acid [11]. 

Salt-inclusion solids described herein were synthesized at high temperature (>500°C) in the presence of reactive alkali and alkaline-earth metal halide salt media. For single crystal growth, an extra amount of molten salt is used, typically 3 5 times by weight of oxides. The reaction mixtures were placed in a carbon-coated silica ampoule, which was then sealed under vacuum. The reaction temperature was typically set at 100-150 °C above the melting point of employed salt. As shown in the schematic drawing in Fig. 16.2, the corresponding metal oxides were first dissolved conceivably via decomposition because of cor-... 

Most metals (other than the alkali and alkaline-earth metals) are corrosion resistant when cathodically polarized to the potentials of hydrogen evolution, so that this reaction can be realized at many of them. It has thus been the subject of innumerable studies, and became the fundamental model in the development of current kinetic concepts for electrochemical reactions. Many of the principles... 

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