P.A. Cox - Inorganic chemistry (793955), страница 11
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(l) is assumed to be a pure liquid or the major component (solvent) in a solution. Forsubstances dissolved in water the designation (aq) (for ‘aqueous’) is used. Thus solid sodium chloride dissolving in wateris expressed:NaCl(aq) means dissolved NaCl molecules and is incorrect for this substance.NamesThe systematic naming of chemical substances is not easy, and the authoritative guide to inorganic nomenclature fills abook of 299 pages.
Very many trivial names such as water (H2O) will always remain in use. Systematic nomenclatureis based on three systems.Binary namesSimple examples are sodium chloride (NaCl), phosphorus trichloride (PCl3) and dinitrogen tetroxide(N2O4). The oxidation state may be given as an alternative to the stoichiometry, as in manganese (IV) oxide, ormanganese dioxide, MnO2 (see Topic B4).
This is unnecessary when only one possibility is known, as inmagnesium bromide (MgBr2).Elements are named in the same order as they appear in the formula (see above). Although there is no implicationthat the compound is ionic, the names ending in -ide are the same as those used for anions (e.g. fluoride, F−). For theelements listed in Table 1. anion names are derived not from English but from the Latin name which gives the chemicalsymbol. For example, CsAu is cesium auride.Binary naming may be extended to include complex ions with recognized names as in ammonium chloride(NH4Cl), sodium cyanide (NaCN) and magnesium sulfate (Mg2SO4).
Some common oxoanions are listedelsewhere (Topic F7, Table 1). Many other complex ions can be named systematically as discussed below.B4—DESCRIBING INORGANIC COMPOUNDS43Table 1. Anion names derived from Latin roots; the -ate form is used for complex anionsSubstitutive namesThis is the system used in organic chemistry, as in dichloromethane, CH2Cl2, which can be regarded as derived frommethane CH4 by replacing two hydrogens with chlorine.
(There is no assumption that this is a chemically feasible routefor preparation.) It may be extended to inorganic molecules using the appropriate hydride names (see Topic F2,Table 1). Thus from silane (SiH4) we obtain names such as chlorosilane (SiH3Cl) and tetrachlorosilane (SiCl4), thelatter being also called silicon tetrachloride. For nitrogen compounds derived from ammonia (NH3) the root amine isused, as, for example, in hydroxylamine (NH2OH).Coordination namesThis system is used in compounds that can be regarded as complexes formed by the coordination of atoms or groups toa central atom.
Examples from transition metal chemistry are tetraamminecopper(2+) ion ortetraamminecopper(II) ion, [Cu(NH3)4]2+, and tetrachlorocuprate(2−) or tetrachlorocuprate(II), [CuCl4]2− (see Topic H6 for further examples, and nomenclature for isomers). Either the overall charge on the complex ion orthe oxidation state of the central atom is given (the latter always with Roman numerals). Anion names end in -ate anduse the Latin roots for elements shown in Table 1. Coordination names are also widely used for complex ions with maingroup elements, for example, tetrahydroborate, [BH4]−; in this case the charge or oxidation state may be omitted asonly one possibility is known.Examples of the use of coordination names in the binary system are the solids hexaamminenickel dibromide, [Ni(NH3)6]Br2, and potassium hexafluorophosphate(V), K[PF6].Structure and bondingThe complete description of a chemical structure involves specifying the relative coordinates of the atoms present,or alternatively giving all bond lengths and bond angles (see Topic B7).
A simple example is shown in 2. Less completeinformation is satisfactory for most descriptive purposes. The coordination number (CN) of an atom is the numberof bonded atoms, irrespective of the type (ionicity, multiplicity, etc.) of bond involved. For very simple molecularcompounds this is obvious from the formula (e.g.
O in H2O and C in CO2 (3) both have CN=2). However, polymericand ionic solids have greater CN values (e.g. 4, 6 and 8, respectively, for Si in SiO2, Ti in TiO2 and U in UO2), and itshould not generally be assumed that the CN is given directly by the stoichiometry.44SECTION B—INTRODUCTION TO INORGANIC SUBSTANCESThe geometrical arrangement around an atom is sometimes described as its coordination sphere. Differentgeometrical arrangements may be described by simple informal terms (e.g. H2O (2) is bent and CO2 (3) linear), orby the names of polyhedra, such as tetrahedra and octahedra (see Topics C2 and D3).
A classification according tosymmetry is also useful, as described in Topic C3.Describing bonding in a consistent way is much harder. The term valency, meaning the number of bonds formed byan atom, is useful in simple molecular substances. Stoichiometries such as CH4, CO2 and H2O can be rationalized byassuming the valencies C(4), H(1) and O(2). One can extend the idea by recognizing the possibility of variable valency;for example, three for phosphorus in PCl3 and P2O3, and five in PCl5 and P2O5.
Unfortunately, the simple valence ideahas serious limitations and can be misleading outside a narrow area. For example:• Given the ‘normal’ valencies of C and O, how can one account for the stability of CO, and the fact that it apparentlyhas a triple bond (see Topic C1)?• PCl5 in its solid form contains [PCl4]+ and [PCl6]− ions. What is the valency of P here?Much more serious problems arise with transition metal compounds.
For these and other reasons the word valency hasbeen largely abandoned by inorganic chemists. When it is necessary to distinguish different stoichiometries such as PCl3and PCl5 the oxidation state is more frequently used. This is defined according to clearer rules than valency, but asthey depend on the electronegativity difference of atoms, the oxidation state can be very uninformative about bonding.For example, every sulfur atom forms two covalent bonds in the compounds H2S, H2S2 (4), S2Cl2 and SCl2, and yet theoxidation state of sulfur is respectively −2, −1, +1 and +2.As a final example, consider phosphorous acid H3PO3 (5). The oxidation state of phosphorus is +3, its coordinationnumber 4, and its valency 5. All these numbers give useful information, but they must not be confused.Section B—Introduction to inorganic substancesB6INORGANIC REACTIONS AND SYNTHESISKey NotesDirect combination anddecompositionreactionsExchange andmetathesis reactionsThe use of solventsSolid-state reactionsRelated topicsDirect combination of elements (A+B→AB) may often be used toprepare binary compounds.
Combination with polyatomic reactantssuch as organic compounds is also possible. Decomposition reactionsinclude the formation of oxides by heating the oxo-salts orhydroxides of metals.The exchange of an atom or group (AB+C→AC+B) or themetathesis (substitution) reaction (AB+CD→AD+CB) are usefulalternatives to direct combination when either one of the elements isdifficult to work with, or when the desired product is notthermodynamically stable.Solvents may be used to facilitate reaction between solids, andsometimes to form the desired product by precipitation.
Reactivity ofa solvent can be exploited, but may also limit the types of productpossible.Reactions between solids are slow because of the high barrier to thediffusion of atoms. Conventional solid-state reactions involve hightemperatures, but methods including vapor transport can be used toaccelerate the reaction.Stability and reactivity (B3)Solvent types and propertiesOxidation and reduction(E1)(B4)Industrial chemistry (J4 andMore complex solids (D5)J5)Direct combination and decomposition reactionsThe reactions of inorganic substances, and the methods used in the synthesis of compounds, are exceedingly diverse.This Topic summarizes some of the major reaction types and their applications.The simplest type of reaction is the direct combination of two elements to form a binary compound, A+B→AB.For exampleMany binary compounds, especially halides and oxides of elements, may be made in this direct way, although there arelimitations.
It is clear that the formation of the desired compound AB must be thermodynamically favourable. Theremay also be kinetic problems, and the above synthesis of LiH requires a temperature of 600°C in order to overcome the46SECTION B—INTRODUCTION TO INORGANIC SUBSTANCESactivation energy associated with breaking the H-H bond. In some cases the reaction may be facilitated by using acatalyst, as in the synthesis of ammonia from H2 and N2 (see Topics H5 and J5).The scope of direct combination reactions is greatly extended if one or more of the species A and B is a polyatomicgroup rather than an element. An example also involving lithium isIn this case thermodynamic stability of LiCl aids the formation of the desired product butyl lithium (C4H9Li), which is auseful reagent for making alkyl derivatives of other elements.The reverse of combination is decomposition, AB→A+B.
The thermal decomposition of mercury oxide,was historically important in the discovery of oxygen, but simple reactions of this type are rarely useful in practice,although the decomposition of compounds by electrolysis is important in the production of some elements (seeTopic B4). More complex thermal decomposition reactions such asmay however be useful in the preparation of compounds (NiO in this example) which are hard to make in pure andstoichiometric form by direct combination.
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