P.A. Cox - Inorganic chemistry (793955), страница 30
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Thus slight reduction of TiO2 introduces electrons and gives n-type behavior. Similarly, oxidation of NiOremoves some electrons and it becomes a p-type semiconductor.Instead of providing electrons, atoms in defect sites may themselves be mobile and thus provide ionic conductionin a solid. Ionic compounds such as NaCl have high conductivity in their molten form, and such conductivity isimportant for the manufacture of aluminum by electrolysis of molten cryolite (Na3AlF6). In most solids however, ionicconduction is much lower and arises largely from defects.
Interstitial ions and vacancies in ionic compounds must occurin combinations that provide overall electrical neutrality. Two important combinations are Schottky defects wherethere is an equal concentration of anion and cation vacancies, and Frenkel defects where vacancies of one ion arebalanced by interstitials of the same kind. For example NaCl has predominantly Schottky defects, and silver halides(AgCl and AgBr) mostly Ag+ Frenkel defects. Both interstitial ions and vacancies may be mobile and so contribute toionic conduction. Doping with ions of different charge may change the defect concentrations and thus the conductivity.For example if AgBr is doped with a small concentration of CdBr2, each Cd2+ replaces two Ag+ ions.
The concentrationof Ag+ vacancies is thereby increased and that of interstitials decreased. As the interstitials are more mobile than thevacancies in AgBr, the initial effect of doping is to decrease the ionic conductivity. However, as the concentration of Cd2+ is increased the vacancies become sufficiently numerous to dominate the conduction process, and so conductivity risesagain.Some solids, known as fast ion conductors show a degree of ionic conduction which is comparable to that of themolten form, and which cannot be attributed to low concentrations of defects.
For example above a transitiontemperature of 146°C, AgI adopts a structure with a body-centered cubic array of I−. The Ag+ ions move freelybetween a variety of sites where they have almost equal energy. One cannot think strictly of defects in a case like this,rather it is the absence of a unique ordered structure that gives rise to high ionic conductivity. Anions are mobile attemperatures well below the melting point in some compounds with the fluorite structure, such as PbF2 and ZrO2. Theoxide ion conductivity of ZrO2 can be increased by doping with CaO or Y2O3. Thus, in Ca0.1Zr0.9O1.9 (consistent withthe ionic charges Ca2+, Zr4+ and O2−) the ratio of anions to cations is less than the value 2:1 required for the normalZrO2 lattice, so that oxygen vacancies are present.
Doped ZrO2 is used as a ‘solid electrolyte’ in electrochemicalsensors and in fuel cells. One important application is in sensors that measure the O2 concentration of exhaust gasesfrom automobile engines, and is used in conjunction with ‘catalytic converters’ for removing pollutants (see Topic J5).Two platinum electrodes are placed on opposite faces of a sample. Oxygen gas reacts at one electrode according toOxide ions pass through the solid and the reverse reaction occurs at the other electrode. A potential difference isdeveloped between the two electrodes which depends on the ratio of O2 partial pressures on each side.Section E—Chemistry in solutionE1SOLVENT TYPES AND PROPERTIESKey NotesPolarity and solvationDonar and acceptorpropertiesIon-transfer solventsRelated topicsStrongly polar molecules form solvents with high dielectric constantsthat are good at solvating charged species.
At a molecular levelsolvation involves specific donor-acceptor interactions and othertypes of intermolecular force.Most good solvents have donor (Lewis base) and acceptor (Lewis acid)properties, responsible for solvation and other chemical reactions.The solvent-system acid-base concept depends on the possibility ofion transfer from one solvent molecule to another. Protic solvents actas H+ donors and can support Brønsted acid-base reactions. Oxideand halide ions may be transferred in other solvents.Inorganic reactions andLewis acids and bases (C9)synthesis (B6)Molecules in condensedphases (C10)Polarity and solvationA solvent is a liquid medium in which dissolved substances are known as solutes.
Solvents are useful for storingsubstances that would otherwise be in inconvenient states (e.g. gases) and for facilitating reactions that would otherwisebe hard to carry out (e.g. ones involving solids, see Topic B6). The physical and chemical characteristics of a solvent areimportant in controlling what substances dissolve easily, and what types of reactions can be performed.
The chemical aswell as the physical state of solutes may be altered by interaction with the solvent. A list of useful solvents is given inTable 1.The most important physical property of a solvent is its polarity. Molecules with large dipole moments such aswater and ammonia form polar solvents. The macroscopic manifestation is the dielectric constant (εr), the factorby which electrostatic forces are weakened in comparison with those in a vacuum (see Topic C10). For example, inwater εr=82 at 25°C, and so attractive forces between anions and cations will be weaker by this factor.At a microscopic level, solutes in polar solvents undergo strong solvation.
For example, the Born model predictsthat the Gibbs free energy of an ion with charge q (in Coulombs) and radius r will be changed in the solvent compared withthe gas phase by an amount(1)130SECTION E—CHEMISTRY IN SOLUTIONTable 1. Properties of some solvents, showing normal melting and boiling points (MP and BP, respectively), dielectric constant (εr, at 25°C or at theboiling point if that is lower), and donor and acceptor numbers (DN and AN, respectively)aDecomposes.This estimate of the solvation energy is highly approximate, as it assumes that the solvent can be treated as acontinuous dielectric medium on a microscopic scale.
Nevertheless, it gives a rough guide that is useful in interpretingsolubility trends (see Topic E4).In reality, solvation involves donor-acceptor interactions, which may not be purely electrostatic in nature (seebelow), so that neutral molecules may also be strongly solvated. Solvent molecules are ordered round the solute, notonly in the primary solvation sphere but (especially with ions) affecting more distant molecules.
Solvation thereforeproduces a decrease in entropy, which can be substantial with small highly charged ions, and contributes to acid-basestrength, complex formation and solubility trends (see Topics E2–E4).Nonpolar solvents such as hexane have molecules with little or no dipole moment and low dielectric constants.They are generally better at dissolving nonpolar molecules and for carrying out reactions where no ions are involved.The molecules interact primarily through van der Waals’ forces (see Topic C10). Nonpolar media are generally poorsolvents for polar molecules because the weak intermolecular forces cannot compete with the stronger ones in the puresolute.
Similarly, nonpolar solutes cannot compete with the strong intermolecular forces in a polar solvent and so maynot be very soluble. These generalizations have many limitations. Ionic substances can dissolve in solvents of lowerpolarity if the ions are efficiently solvated by appropriate donor and acceptor interactions (see Topic E4). As theelectrostatic forces between solvated ions remain relatively strong, however, they tend to form ion pairs.
Althoughliquid ammonia (εr=22) is a good solvent for some ionic compounds, ion pairing is much commoner than in water(εr=82).Donor and acceptor propertiesMost polar solvents have donor or Lewis base properties resulting from lone-pair electrons (see Topic C9). Gooddonor solvents include water, ammonia and pyridine, and are efficient at solvating cations and other Lewis acids.Acceptor or Lewis acid behavior is important for solvating anions, and results from empty orbitals or from hydrogenbonding.
Donor and acceptor numbers have been defined by measuring the strength of interaction between solventmolecules and the ‘standard’ acceptor (SbCl5) and donor (OPCl3) molecules, respectively. Values are shown in Table 1.and can provide a useful guide although they ignore many specific details of the interaction, and in particular make nodistinction between ‘hard’ and ‘soft’ character. As an example of this limitation, benzene is listed as having noE1—SOLVENT TYPES AND PROPERTIES131Table 2. Some ion-transfer solvents, with the characteristic solvent-system acid and base species, and other examples of acids and basesappreciable donor strength, yet will dissolve silver perchlorate AgClO4 because of a strong ‘soft’ donor-acceptorinteraction between Ag+ and a benzene molecule.In many cases a donor-acceptor interaction may be only the first step in a more substantial solvolysis reaction.These reactions are common with nonmetal halides and oxides in water and ammonia; for example,An example of the variety of products formed in different donor solvents is provided by the reactions of FeCl3, where Srepresents a coordinated solvent molecule:These differences are thought to result from the lower polarity of pyridine compared with the other two solvents, andthe better solvation of small ions such as Cl− in DMSO compared with MeCN.Ion-transfer solventsWater, ammonia and other protic solvents undergo a reaction known as autoprotolysis:Although the equilibrium constants may be small (around 10−30 for ammonia) the possibility of such reactions leads to adefinition of acids and bases based on a solvent system (see Table 2).
An acid is the positive species formed (inthe above example) or any solute that gives rise to it; similarly, a base is the negative speciesor anything producingit in solution. With protic solvents this corresponds to the Brønsted definition of acids and bases (see Topic E2). Theexamples in Table 2 show that something acting as an acid in one solvent can be a base in another.Aprotic solvents do not have transferable H+ but some other ion such as halide or oxide can be involved.
Table 2shows the example of BrF3, which undergoes some autoionization with F− transfer. Substances dissolving to produceF− ions act as bases, and Lewis acids that can react with F− act as acids:132SECTION E—CHEMISTRY IN SOLUTIONIn oxide melts the solvent system corresponds to the Lux-Flood acid/base definition: an oxide ion donor is a base,and an oxide acceptor an acid. In the reactionthe calcium oxide is basic, and the silica acidic.Lux-Flood acidities of oxides are important in reactions taking place in silicate melts, for example in glassmanufacture.
The values correlate well with other aspects of acid-base behaviour, for example that manifested inaqueous chemistry (see Topics B2 and F7). Acidity of EOn/2 generally increases with the oxidation state n, and is largerfor smaller ions En+ and for non-metallic elements. Strongly basic oxides that act as oxide donors include Na2O andCaO; acidic oxides acting as oxide acceptors include B2O3 and P2O5.Section E—Chemistry in solutionE2BRØNSTED ACIDS AND BASESKey NotesDefinitionspHStrong and weakbehaviorTrends in pK valuesRelated topicsA Brønsted acid is a proton donor and undergoes protolysis when abase is present. Acids and bases form conjugate pairs. Water andsome other substances are both acidic and basic.Water undergoes autoprotolysis (self-ionization) giving H3O+ andOH−.
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