Hydrides ionic

The reaction is carried out at about 400 °C for sodium, potassium, and rubidium and at about 700 °C with lithium. Metal compounds such as the nitrides may also be used  

The ionic hydrides are white solids with high melting points, and all of the alkali metal hydrides have the sodium chloride crystal structure. Because they resemble the salts of the alkali and alkaline earth metals, the ionic hydrides are often referred to as saline or salt-like hydrides. The properties of the alkali metal hydrides are shown in Table 6.3, and those of the alkaline earth hydrides are shown in Table 6.4. 

The apparent radius of the H- is determined from the M-H distance in the crystal by subtracting the known radius of M+. It is obvious from the data shown in Table 6.3 that the radius assigned to H in Li-H is smaller than in other hydrides of Group IA. The Li-H bond is considered to have a substantial amount of covalent character. Probably the greater covalency 

Compound A Hf° (k mol 1) Apparent Charge on H (e units) Density, Hydride (g cm 3) Density, Metal (g cm 3) 

Molten hydrides of Groups IA and IIA are good electrical conductors, and hydrogen is liberated at the anode as a result of the oxidation of IF  

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These solid ionic hydrides (having an ionic lattice and containing the hydride ion H ) react with water, for example... 

When the molten ionic hydrides are electrolysed, all yield hydrogen at the anode, the metal at the cathode. 

The most important characteristic of ionic hydrides is that they are strong Bronsted bases. The hydride ion will react with most molecules that contain a hydrogen atom bound to an atom of high... 

Although the formation of ionic hydrides is usually exothermic, the formation of interstitial hydrides may have positive enthalpy values. Physical characteristics of interstitial hydrides are determined by the fact that hydrogen atoms in interstitial positions cause some expansion of the lattice but contribute very little mass. Consequently, the interstitial hydrides always have lower densities than the metal itself, even though the crystal structure is normally the same. When interstitial positions contain hydrogen atoms, the flow of electrons in conduction bands within the metal is impeded, so the... 

Ionic hydrazides, 13 567 Ionic hydrides, 13 608-611 Ionic interactions... 

A first group of hydrides (ionic hydrides) is formed with the more electropositive elements of the 5-block of the Periodic Table. This group of hydrides includes the salt-like MeH (Me+H ) NaCl-type compounds of the alkali metals and the di-hydrides (Co2Si-type) formed by the divalent metals Ca, Sr, Ba and also by Eu and Yb. The thermal stability of these hydrides decreases from Li to Cs and from Ca to Ba the chemical reactivity on the contrary increases from Li to Cs and from Ca to Ba. While the reaction of NaH with water is very violent, the reaction of LiH or CaH2 can be used for a portable source of hydrogen. 

Figure 5.2. The phase diagram of the Sr-H system. An intermediate phase, stable in a range of temperature and composition values (solid solution [) is formed in the Sr-rich region. The other phase (practically a stoichiometric compound) corresponding to the formula SrH2 may be considered a representative of the ionic hydrides. The low-temperature form of this compound has the oP12-Co2Si-type structure.

In the third regime, at high [H2O], where Forster noted high water gas shift activity as well as carbonylation, the ionic species now included substantial amounts of the ionic hydride complex [IrH ( 0)213] . 

When heated under hydrogen it forms an ionic hydride, SrH2, a stable crystalline salt. Heating Sr metal in a stream of nitrogen above 380°C forms nitride, Sr3N2. 

Similarly, very few true ionic hydrides, borides, nitrides, and carbides exist (see Chapter 5). 

When discussing metal alloys (Section 4.3), we saw that atoms of non-metallic elements such as H, B, C, and N can be inserted into the interstices (tetrahedral and octahedral holes) of a lattice of metal atoms to form metal-like compounds that are usually nonstoichiometric and have considerable technological importance. These interstitial compounds are commonly referred to as metal hydrides, borides, carbides, or nitrides, but the implication that they contain the anions H, B3, C4, or N3- is misleading. To clarify this point, we consider first the properties of truly ionic hydrides, carbides, and nitrides. 

Ionic Hydrides. The ionic or saline hydrides contain metal cations and negatively charged hydrogen ions. They crystallize in the cubic lattice similar to the corresponding metal halide, and when pure, are white sulid.s. When dissolved in molten sales or hydroxides and electrolyzed, hydrogen gas is liberated al the anode. Their densities arc greater than those of the parent metal, and their rormalion is exothermic. All are strong bases. 

V I ionic hydrides are crystalline solids with high melting points. They are formed by direct reaction with only the most active metals. 

Ionic hydrides are saltlike, high-melting, white, crystalline compounds formed by the alkali metals and the heavier alkaline earth metals Ca, Sr, and Ba. They can be prepared by direct reaction of the elements at about 400°C ... 

The H anion is a good proton acceptor, and ionic hydrides therefore react with water to give H2 gas and OH ions ... 

This reaction of an ionic hydride with water is a redox reaction because the hydride reduces the water (4-1 oxidation state for H) to H2 (0 oxidation state). In turn, the hydride (—1 oxidation state for H) is oxidized to H2. In general, ionic hydrides are good reducing agents. Some, such as potassium hydride, catch fire in air because of a rapid redox reaction with oxygen ... 

Because lithium is a group 1A metal and barium is a group 2A metal, their hydrides have the formulas LiH and BaH2, respectively. Both are ionic hydrides and therefore react with water to give H2 gas and OH ions. Balance the equations either by inspection or by using the method of oxidation numbers (Section 4.9). 

Hydrogen forms three types of binary hydrides. Active metals give ionic hydrides, such as LiH and CaFF nonmetals give covalent hydrides, such as NH3, H2O, and HF and transition metals give metallic, or interstitial, hydrides, such as PdH,.. Interstitial hydrides are often nonstoichiometric compounds. 

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