Hydrides, metal, water-reactive

Chemicals that are water or air reactive pose a significant fire hazard because they may generate large amounts of heat. These materials may be pyrophoric, that is, they ignite spontaneously on exposure to air. They may also react violently with water and certain other chemicals. Water-reactive chemicals include anhydrides, carbides, hydrides, and alkali metals (e.g., lithium, sodium, potassium). 

The most useful reagents for reducing aldehydes and ketones are the metal hydride reagents. Complex hydrides are the source of hydride ions, and the two most commonly used reagents are NaBlTj and LiAlH4. Lithium aluminium hydride is extremely reactive with water and must be used in an anhydrous solvent, e.g. dry ether. 

As mentioned earlier, direct thermal dissociation of water requires temperatures above approximately 2500 K. Since there are not yet technical solutions to the materials problems, the possibility of splitting water instead, by various reaction sequences, has been probed. Historically, the reaction of reactive metals and reactive metal hydrides with water or acid was the standard way of producing pure hydrogen in small quantities. These reactions involved sodium metal with water to form hydrogen or zinc metal with hydrochloric acid or calcium hydride with water. All these... 

Table 2.10 c, x and reactivity of 3d metal ions towards CO, hydride and water... 

Contamination of metal hydrides - oxygen and water reactivity... 

EXPLOSION and FIRE CONCERNS not combustible NFPA rating Heath 3, Flammability 0, Reactivity 0 nonflammable gas reacts violently with acrolein, aluminum, cesium oxide, chlorates, chromium, manganese, potassium chlorate, sodium, sodium carbide, stannous oxide, fluorine, cesium acetylene carbide, and potassium acetylene earbide reacts with water or steam to produce toxic and corrosive fumes incompatible with halogens, lithium nitrate, metal acetylides, metals, metal oxides, polymeric tubing, potassium chlorate, and sodium hydride use water spray or suitable agent for firefighting purposes. 

Binary (meaning two) salts are made up of two elements a metal and a nonmetal, except oxygen. They end in ide, such as potassium chloride. Binary salts, as a family, have varying hazards. They may be water-reactive, toxic, and, in contact with water, may form a corrosive liquid and release heat. Chemical reactions often release heat, which is referred to as an exothermic reaction. The hazard of an individual binary salt cannot be determined by the family. To determine the hazards of the binary salts, they have to be researched in reference materials. This varying hazard applies to all binary salts, except for nitrides, carbides, hydrides, and phosphides. 

Water-reactive chemicals Store in dry, cool, location protect from water from fire sprinkler. Sodium metal, potassium metal, lithium metal, lithium aluminum hydride Separate from all aqueous solutions and oxidizers... 

Water reactive materials are those that react violently with water. Alkali metals (e.g., hthium, sodium, and potassium), many organometallic compounds, and some hydrides react with water to produce heat and flammable hydrogen gas, which can ignite or combine explosively with atmospheric oxygen. Some anhydrous metal halides (e.g., aluminum bromide), oxides (e.g., calcium oxide), and nonmetal oxides (e.g., sulfur trioxide) and halides (e.g., phosphorus pentachloride) react exothermically with water, and the reaction can be violent if there is insufficient coolant water to dissipate the heat produced. 

Fire hazards posed by water-reactive substances such as alkali metals and metal hydrides, pyrophoric substances such as metal alkyls, strong oxidizers such as perchloric acid, and flammable gases such as acetylene require procedures beyond the standard prudent practices for handling chemicals described here (see sections 5.C and 5.D) and should be researched in LCSSs or other references before work begins. In addition, emergency response to incidents involving these substances must take their special hazards into account. 

Water is a moderately reactive nucleophile involved in several well-known catalytic cycles, such as hydroxycarbonylation and Wacker oxidation of olefins. Besides these, palladium, as many other late transition metals, is reactive in the water gas shift reaction (WGS reaction) (Scheme 3), which is a source of metal hydride complexes. Further transformations triggered by the WGS reaction are versatile. 

Mercury Acetate 1629 53 Metal Hydrides, water-reactive, 1409 40... 

Water-reactive chemicals. Certain chemicals, including sodium, potassium, and many metal hydrides, react with water, producing heat and flammable gases. Other chemicals may burn upon contact with water. Storage areas used to hold water-reactive chemicals should be designed to prevent accidental contact with water. For example, these areas should not be equipped with automatic water sprinkler systems. Segregate water-re-active chemicals from other combustible materials, and be certain that the area is of fire-resistant construction. 

Alkali metal hydrides contain the hydride ion, H , and have the sodium chloride structure. All the alkali metal hydrides are very reactive. For example, they react readily with water to give a hydroxide salt and hydrogen gas ... 

Storage of uranium foil in closed containers in presence of air and water may produce a pyrophoric surface [1], Uranium must be machined in a fume hood because, apart from the radioactivity hazard, the swarf is easily ignited. The massive metal ignites at 600-700°C in air [2]. The finely divided reactive form of uranium produced by pyrolysis of the hydride is pyrophoric [3], while that produced as a slurry by reduction of uranium tetrachloride in dimethoxyethane by potassium-sodium alloy is not [4],... 

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. 

The actinides. The actinides metals are electropositive and very reactive they are pyrophoric in finely divided form. They tarnish rapidly in air forming an oxide protective coating in the case of Th, but more slowly for the other actinides. The metals react with most non-metals. With steam or boiling water, oxide is formed on the surface of the metal and H2 evolves in this way hydrides are produced that react rapidly with water and facilitate further attack on the metals. The oxidation states observed in the chemistry of lanthanides and actinides are shown in Fig. 5.9. Notice the predominant oxidation state III for the lanthanides... 

The collected papers of a symposium at Dallas, April 1956, cover all aspects of the handling, use and hazards of lithium, sodium, potassium, their alloys, oxides and hydrides, in 19 chapters [1], Interaction of all 5 alkali metals with water under various circumstances has been discussed comparatively [2], In a monograph covering properties, preparation, handling and applications of the enhanced reactivity of metals dispersed finely in hydrocarbon diluents, the hazardous nature of potassium dispersions, and especially of rubidium and caesium dispersions is stressed [3], Alkaline-earth metal dispersions are of relatively low hazard. Safety practices for small-scale storage, handling, heating and reactions of lithium potassium and sodium with water are reviewed [4],... 

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