Direct combination

For the formation of the hydrogen halides by the direct combination of the elements, the enthalpies of formation are ... 

The term oxidation was originally applied to the formation of a metal oxide by the direct combination of the metal and oxygen. For example,... 

Both boron and aluminium chlorides can be prepared by the direct combination of the elements. Boron trichloride can also be prepared by passing chlorine gas over a strongly heated mixture of boron trioxide and carbon. Like boron trifluoride, this is a covalent compound and a gas at ordinary temperature and pressure (boiling point 285 K). It reacts vigorously with water, the mechanism probably involving initial co-ordination of a water molecule (p, 152). and hydrochloric acid is obtained ... 

The tribromide and triodide of both boron and aluminium can be made by the direct combination of the elements although better methods are known for each halide. The properties of each halide closely resemble that of the chloride. 

Nitrogen does form a number of binary compounds with the halogens but none of these can be prepared by the direct combination of the elements and they are dealt with below (p. 249). The other Group V elements all form halides by direct combination. 

A complete set of trihalides for arsenic, antimony and bismuth can be prepared by the direct combination of the elements although other methods of preparation can sometimes be used. The vigour of the direct combination reaction for a given metal decreases from fluorine to iodine (except in the case of bismuth which does not react readily with fluorine) and for a given halogen, from arsenic to bismuth. 

Towards a simple Lewis base, for example the proton, phosphine is a poorer electron donor than ammonia, the larger phosphorus atom being less able to form a stable covalent bond with the acceptor atom or molecule. Phosphine is, therefore, a much weaker base than ammonia and there is no series of phosphonium salts corresponding to the ammonium salts but phosphonium halides. PH4X (X = Cl, Br, I) can be prepared by the direct combination of phosphine with the appropriate hydrogen halide. These compounds are much more easily dissociated than ammonium halides, the most stable being the iodide, but even this dissociates at 333 K PH4I = PH3 -t- HI... 

Nitrogen monoxide is the most stable of all the oxides of nitrogen. It can be prepared in small amounts by direct combination of the... 

Antimony forms both a + 3 and a + 5 oxide. The + 3 oxide can be prepared by the direct combination of the elements or by the action of moderately concentrated nitric acid on antimony. It is an amphoteric oxide dissolving in alkalis to give antimonates(III) (for example sodium antimonite , NaSb02), and in some acids to form salts, for example with concentrated hydrochloric acid the trichloride, SbCl3, is formed. 

Many of these sulphides occur naturally, for example iron(ll) sulphide, FeS (magnetic pyrites), and antimony(III) sulphide, Sb S, (stibnite). They can usually be prepared by the direct combination of the elements, effected by heating, but this rarely produces a pure stoichiometric compound and the product often contains a slight excess of the metal, or of sulphur. 

Halides of non-metals are usually prepared by the direct combination of the elements. If the element exhibits more than one oxidation state, excess of the halogen favours the formation of the higher halide whilst excess of the element favours the formation of the lower halide (e.g. PCI5 and PCI3). 

Ironilll) chloride is a black, essentially covalent solid, in which each iron atom is surrounded octahedrally by six chlorine atoms. It is prepared by direct combination of iron with chlorine or by dehydration of the hydrated chloride, by one of the methods given on p.343). 

Cobalt II) halides can be obtained by direct combination of the elements, or by dehydration of their hydrates. Anhydrous cobalt(II) chloride is blue, and the solid contains octahedrally-coordinated cobalt the hydrated salt C0CI2. bHjO is pink, with each cobalt surrounded by four water molecules and two chloride ions in a distorted octahedron. 

Nickel forms yellow anhydrous halides NiXjlX = F. Cl. Br) and a black iodide Nil2 all these halides are made by direct combination of the elements, and the chloride by reaction of sulphur dichloride oxide with the hydrated salt. All dissolve in water to give green solutions from which the hydrates can be crystallised the solutions contain the ion [NifHjOls], and the chloride crystallises as NiCl2.6H2O, nickel(II) chloride hexahydrate. 

The Diels-Alder Reaction consists in the direct combination of a compound containing a conjugated diene system u ith a reagent which possesses a double or triple bond activated bj suitable adjacent groups. Examples of such reagents are maleic anhydride, p-benzoquinone, acraldehyde and acetylene dicarboxylic esters. Combination always occurs at the 1,4 positions of the diene system ... 

Termination steps are m general less likely to occur than the propagation steps Each of the termination steps requires two free radicals to encounter each other m a medium that contains far greater quantities of other materials (methane and chlorine mol ecules) with which they can react Although some chloromethane undoubtedly arises via direct combination of methyl radicals with chlorine atoms most of it is formed by the propagation sequence shown m Figure 4 21... 

Other nylons are made this way from direct combinations of monomers to produce types 6/9, 6/10, and 6/12. 

Preparation. In the laboratory, sulfur tetrafluoride is made by combining SCI2 and NaF suspended in acetonitrile at ca 77°C (106). For commercial production, SF is made by direct combination of sulfur with elemental fluorine (107). Commercial appHcations of SF are limited. It is available from Air Products and Chemicals. 

Gallium Nitrogen Compounds. Direct combination of gaUium and nitrogen (40) involves a gas discharge at 750°C for the dissociation of... 

Interest has continued in on-site manufacture of hydrogen peroxide from the elements, particularly for remote sites located considerable distances from wodd-scale anthraquinone processes. However, no commercial-scale direct combination plants have been constmcted as of this writing. 

Indium also combines with nonmetaUic elements and with metalloids such as N, P, Sb, As, Te, and Se. Many of the latter compounds ate semiconducting as ate the oxide and sulfide. Indium antimonide [1312-41 -0], InSb indium arsenide [1303-11-3], In As and indium phosphide [22398-80-7], InP, ate the principal semiconducting compounds. These ate all prepared by direct combination of the highly purified elements at elevated temperature under controlled conditions. 

Ma.nufa.cture. Nickel carbonyl can be prepared by the direct combination of carbon monoxide and metallic nickel (77). The presence of sulfur, the surface area, and the surface activity of the nickel affect the formation of nickel carbonyl (78). The thermodynamics of formation and reaction are documented (79). Two commercial processes are used for large-scale production (80). An atmospheric method, whereby carbon monoxide is passed over nickel sulfide and freshly reduced nickel metal, is used in the United Kingdom to produce pure nickel carbonyl (81). The second method, used in Canada, involves high pressure CO in the formation of iron and nickel carbonyls the two are separated by distillation (81). Very high pressure CO is required for the formation of cobalt carbonyl and a method has been described where the mixed carbonyls are scmbbed with ammonia or an amine and the cobalt is extracted as the ammine carbonyl (82). A discontinued commercial process in the United States involved the reaction of carbon monoxide with nickel sulfate solution. 

The phosphides are usually made by direct combination of the elements at elevated temperature. The reactive phosphoms is typically red phosphoms, white phosphoms, or phosphoms vapor. Lithium phosphide [12057-29-3] sodium phosphide [12058-85-4] Na P and potassium phosphide [12260-14-9] iron(III) phosphide [26508-33-8] EeP, and diiron phosphide [1310-43-6] Fe2P, are made in this manner. 

Various sulfides of the halogens are formed by direct combination of sulfur with fluorine, bromine, and chlorine. No evident reaction occurs with iodine instead, the elements remain as components of a mixture. Mixtures of sulfur and potassium chlorate, or sulfur and powdered zinc, are highly explosive. 

Hydrogen sulfide has been produced in commercial quantities by the direct combination of the elements. The reaction of hydrogen and sulfur vapor proceeds at ca 500°C in the presence of a catalyst, eg, bauxite, an aluminosihcate, or cobalt molybdate. This process yields hydrogen sulfide that is of good purity and is suitable for preparation of sodium sulfide and sodium hydrosulfide (see Sodium compounds). Most hydrogen sulfide used commercially is either a by-product or is obtained from sour natural gas. 

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