P.A. Cox - Inorganic chemistry (793955), страница 12
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Many oxo-salts (carbonates, nitrates, etc.) and hydroxides of metallicelements decompose in a similar way to oxides on heating. The temperatures required to achieve these endothermicreactions can often be correlated with the size and charge of the metal ion as discussed in Topic D6.Exchange and metathesis reactionsThe simplest type of exchange reaction may be written AB+C→AC+B and is exemplified byIn this case the direct combination of iron and chlorine gives FeCl3, illustrating that exchange reactions may give adifferent product compared with direct combination. The reaction AB+C→AC+B can also be useful for preparing theelement B, many of the extraction methods for elements discussed in Topic B4 being of this type. One reason forpreferring exchange to direct combination is that one of the elements may not be easy to work with. Although fluorinecombines directly with nearly every element, it is a dangerous and unpleasant gas, and is often replaced by a fluorinatingagent such as ClF3 in industrial and laboratory processes:The more complex exchange process AB+CD→AD+CB is described as substitution or metathesis.
An examplealso involving fluorination iswhich is used to make PF3 as direct combination of P with fluorine gives the pentafluoride PF5.Metathesis reactions are common in the preparation of organic derivatives of elements, using organo-lithium or magnesium (Grignard) reagents produced by direct synthesis:B6—INORGANIC REACTIONS AND SYNTHESIS47They are especially valuable for making compounds that are not thermodynamically stable with respect to theirconstituents, and which therefore cannot be made by direct reaction.
For example, thermodynamically unstablehydrides may be made using LiAlH4 or NaBH4:The use of solventsAlthough some of the simpler types of reaction may be carried out with the reactants on their own, many reactions arecarried out in solution. The most obvious function of a solvent is to facilitate the mixing of solid substances, wherereaction would otherwise be very slow (see below). Suitable solvents depend on the nature of the compounds involved,the most important property being polarity (see Topic E1). Non-polar solvents such as hexane and toluene are oftenused for reactions involving organic and organoelement compounds, although more polar coordinating solvents such asethers (including tetrahydrofuran, THF, C4H8O) are sometimes required.
For example organomagnesium (Grignard)reagents are prepared and used in ether solution.More polar substances generally require more polar solvents. For example water (or sometimes ethanol) may be usedfor the preparation of many coordination compounds of transition metals (Topics E3, H3). Water is useful for reactionsinvolving ionic substances, especially when the desired product is insoluble and so may be formed directly byprecipitation. An example isThe principles discussed in Topic E4 can often be used to choose an appropriate counter-ion (for example one of similarsize) to achieve the desired precipitation. Solubility may also be manipulated by changing the solvent or thetemperature.
Hydrothermal methods use water under conditions of high temperature and pressure to increase thesolubility of reactants. Temperatures between 150 and 500°C may be used, with pressures between 100 and 2000 bar.Hydrothermal methods are common for the synthesis of some solids such as zeolites (see Topic D5) and also forgrowing single crystals of compounds such as quartz, SiO2.Solvents may however be themselves reactive. Sometimes this can be exploited, using the solvent as one of thereactants, as with the following reaction in liquid ammonia:More often however, such reactivity is undesirable and solvents may need to be chosen accordingly.
Both the acid/baseand the redox properties of the solvents can limit the range of conditions available and many reactions impossible inwater can be carried out in other solvents. Liquid ammonia is good for highly basic and for reducing conditions(especially using dissolved alkali metals as mentioned in Topic G2).
However, for strongly acid and/or oxidizingconditions solvents such as H2SO4 or liquid SO2 may be used (see Topic F8).Solid-state reactionsReactions between solid substances can be very slow, because the reactants meet directly only at the interface betweensolid particles, and the bulk reaction requires the diffusion of atoms through the solids.
Even when one reactant is48SECTION B—INTRODUCTION TO INORGANIC SUBSTANCESgaseous or liquid the barrier to diffusion may prevent bulk reaction. For example the formation of inert oxide films onsome reactive metals such as aluminum and titanium is important for their applications. Reactions confined to surfacelayers are exploited in the manufacture of electronic devices such as integrated circuits made from silicon.Conventional ceramic synthesis of mixed oxides uses finely divided starting materials, ground up together, andfired at high temperatures to speed up diffusion. An example is the preparation of the ‘high temperature’superconductor YBa2Cu3O6.8:Reaction takes place at 930°C followed by cooling in O2 to give the desired oxygen content of the product. BaCO3 isused rather than BaO as this oxide is very sensitive to water and CO2 and so is hard to obtain in pure form.
Ceramicsynthesis is facilitated by the intimate mixing of the starting materials, and this can sometimes be achieved by thecoprecipitation from solution of a suitable mixture of precursors. For example a mixed oxide such as CaMnO3 canbe made by starting with stoichiometric quantities of calcium and manganese nitrates in aqueous solution and addingNaOH to coprecipitate the metals as hydroxides, followed by firing at 1000°C.An important way of overcome the diffusion barrier in solid state synthesis is the technique of vapor transport, wherean agent is added to the reactants to produce a volatile intermediate in a sealed tube.
For example the formation ofAl2S3 is slow even at 800°C where Al is liquid and S gaseous, because of the formation of an impermeable skin of sulfideon the surface of the metal. Adding a trace of I2, and using a temperature gradient in the tube, accelerates the reactionbecause of the reversible formation of volatile AlI3. The reactants are placed at the hot end of the tube, and the volatileiodide passes to the cooler end where the equilibriumshifts back to the left and the product is formed.Specialized low-temperature techniques known collectively as chimie douce (‘gentle chemistry’) methods can beused for certain types of compound. The ready formation of intercalation compounds, by insertion of species betweenlayers of a host lattice, is described in Topic D5.Section B—Introduction to inorganic substancesB7METHODS OF CHARACTERIZATIONKey NotesIntroductionElemental analysisMass spectrometrySpectroscopic methodsDiffraction methodsRelated topicsCharacterization may involve simple fingerprinting of compoundsalready known, or more extensive investigation designed to establishthe formula and structure of a new compound.The proportions of each element allow a stoichiometric formula to beobtained.
Chemical methods can be used, but instrumental methodsare more routine and include combustion analysis (for C, H, N andsometimes S) and methods based on atomic spectroscopy of samplesatomized at high temperature.Mass spectrometry is the most important way of determining themolecular formula. Characteristic patterns arising from differentisotopes aid the interpretation. Fragmentation patterns can giveinformation about the structural arrangement and are also useful forfingerprinting.Infrared (IR) and nuclear magnetic resonance (NMR) are valuablefingerprinting techniques for molecular compounds.
They can also giveinformation on new compounds about functional groups present andmolecular symmetry. Visible/UV absorption spectroscopy and othertechniques are useful for investigating electronic structure.X-ray diffraction on powder samples is used for fingerprintingcrystalline substances. Single crystal X-ray diffraction is the mostimportant method for complete structure determination to give bondlengths and angles.DescribinginorganicComplexes:electroniccompounds (B5)spectra and magnetism (H8)Introduction to solids (D1)IntroductionMethods of characterization aim to determine the products of a reaction. The level of detail expected depends on thecircumstances, and determines the range of methods required.
If the aim has been to make a known compound, oneneeds to check its identity and purity. Fingerprinting techniques measure a spectrum or some other property andcompare it with results published for known compounds and available in literature databases. Such techniques may alsoshow whether impurities are present, but it is often desirable to check the purity of the compound independently, forexample by elemental analysis. However, if the compound prepared is a new one, more thorough investigation is50SECTION B—INTRODUCTION TO INORGANIC SUBSTANCESappropriate.
The stoichiometric formula may be found by elemental analysis, and the full molecular formula in principleby mass spectrometry (MS). MS combined with other spectroscopic techniques, especially infrared (IR) andnuclear magnetic resonance (NMR), may give valuable information about the functional groups present (e.g.which atoms are bonded to which other ones) but do not provide a complete structure determination. Detailedinformation on the positions of atoms, bond lengths and angles, etc. is most often determined by X-ray diffraction.The aim of this account is to summarize only the type of information that can be obtained from the most importantmethods of characterization. Accounts of the principles behind them can be found elsewhere.Elemental analysisElemental analysis is important in establishing the purity and identity of a known compound, or the empirical(stoichiometric) formulae of a new one.
Elemental composition is usually quoted as percent by mass, from which thestoichiometry can be determined from atomic mass (RAM) values. Consider a compound (X) with the followingcomposition by mass:Dividing each mass percent by the corresponding RAM (Appendix 1) gives the following relative molar quantitiesThese are very nearly in the proportions 1:9:6:3, suggesting a stoichiometric formula CrC9H6O3.Traditional methods of elemental analysis depend on specific chemical reactions for given elements, either in solutionusing titrations (known as volumetric analysis) or precipitation of solids that can be weighed (gravimetricanalysis). Although such methods are still used for specific and very accurate purposes, they have been replaced inroutine work by automated instrumental methods. Combustion analysis is used to determine C, H, N, andsometimes S, by complete oxidation of the compound forming CO2, H2O, N2 and SO2.
The gases are separated anddetermined automatically by gas chromatography. The technique is most valuable for organic compounds, but is alsoused for organometallic compounds of inorganic elements.Techniques for determining the majority of elements rely on measuring the line spectra of atoms (see Topics A2,A3), from a sample that has been heated sufficiently to give complete atomization.
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