P.A. Cox - Inorganic chemistry (793955), страница 38
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The great strength of thetriple bond limits its reactivity.Ammonia is basic in water and a good ligand. It is an importantindustrial and laboratory chemical. Related compounds includehydrazine and organic derivatives of ammonia (amines).The many known nitrogen oxides have unusual structures, all withsome degree of multiple bonding. Oxocations and oxoacids can beformed, of which nitric acid is the most important. All compoundswith oxygen are potentially strong oxidizing agents, but reactivity isoften limited by kinetic factors.Fluorides are the most stable halides. Many metals form nitrides butthese are not highly ionic.Introduction to nonmetalsIndustrial chemistry (J4)(F1)Phosphorus, arsenic andantimony (F6)The elementNitrogen is a moderately electronegative element but the great strength of the triple bond makes N2 kinetically andthermodynamically stable.
The atom can form three single bonds, generally with a pyramidal geometry (see Topics C1and C2), but also has a notable tendency to multiple bonding. Its unusually rich redox chemistry is illustrated in theFrost diagram in Fig. 1 (see below).Dinitrogen makes up 79 mol % of dry air. The element is essential for life and is one of the elements often in shortsupply, as fixation of atmospheric nitrogen to form chemically usable compounds is a difficult process (see Topics J3 andJ6).Nitrogen is obtained from the atmosphere by liquefaction and fractional distillation.
Its normal boiling point (77 K or−196°C) and its ready availability make it a useful coolant. It reacts directly with rather few elements and is often usedas an inert filling or ‘blanket’ for metallurgical processes. The majority of industrial nitrogen, however, is used to makeammonia and further compounds (see Topic J4).F5—NITROGEN165Fig. 1. Frost diagram showing the redox states of nitrogen in water at pH=0 (continuous line) and pH=14 (dashed line).Ammonia and derivativesAmmonia NH3 is manufactured industrially in larger molar quantities than any other substance. The Haber processinvolves direct synthesis from the elements at around 600 K at high pressure and in the presence of a potassiumpromoted iron catalyst. Ammonia is used to make nitric acid and other chemicals including many plastics andpharmaceuticals.Ammonia has a C3v pyramidal structure.
It is a good Lewis base and an important ligand in transition metal complexes(see Topics C9, E3 and H3). In water it acts as a Brønsted base through the equilibriumThe ammonium ion forms salts and has a similar radius to K+, although the structures are sometimes differentcan undergo hydrogen bonding. For example, NH4F has the tetrahedral wurtzite structure rather thanbecausethe rocksalt structure of KF; the tetrahedral coordination is ideal for formation of hydrogen bonds betweenand F− ions. Ammonium salts often dissociate reversibly on heating:Ammonia has a normal boiling point of −33°C. As with water, this value is much higher than expected from the normalgroup trend, a manifestation of strong hydrogen bonding. Liquid ammonia also undergoes autoprotolysis although to alesser extent than water (see Topics E1 and E2).
It is a good solvent for many ionic substances, and is much more basicthan water. Ammonium salts act as acids and amides as bases. Ammonia is kinetically inert under strongly reducingconditions, and will dissolve alkali metals to give solutions with free solvated electrons present (see Topic G2).Hydrazine N2H4 (1) can be made by the Rauschig synthesis:166SECTION F—CHEMISTRY OF NONMETALSIts combustion to give N2 and H2O is extremely exothermic (ΔH=−620 kJ mol−1) and it has been used as a rocket fuel.The explosive hydrogen azide HN3 is the conjugate acid of the azide ion(2). Another hydrogen compound ishydroxylamine NH2OH.Nitrogen forms an enormous variety of organic compounds.
Amines such as methylamine CH3NH2 andtrimethylamine (CH3)3N can be regarded as derived from ammonia by replacing one or more H atoms with alkyl oraryl groups. Like ammonia, amines are basic and form complexes with transition metals. Tetraalkyl ammoniumions such as [(C4H9)4N]+ are useful when large anions are required in inorganic synthesis (see Topic D6). Nitrogen alsoforms heterocyclic compounds such as pyridine C5H5N.Oxygen compoundsThe most commonly encountered oxides, oxocations and oxoanions, are shown in Fig. 2. All these species have somemultiple bonding, the single N—N and N—O bonds being comparatively weak. Nitrous oxide N2O can be made byheating ammonium nitrate.
It is isoelectronic with CO2 and somewhat unreactive, and is used as an anaesthetic(‘laughing gas’) and as a propellant for aerosols. Nitric oxide NO and nitrogen dioxide NO2 are the normalproducts of reaction of oxygen and nitrogen at high temperatures, or of the oxidation of ammonia. They are both oddelectron molecules. NO2 dimerizes reversibly at low temperatures to make N2O4, but NO has very little tendency todimerize in the gas phase, probably because the odd electron is delocalized in a π antibonding orbital (see Topic C5; themolecular orbital diagram is like that for CO but with one more electron).
NO reacts with oxygen to give NO2. It canact as a ligand in transition metal complexes. The other oxides of nitrogen are less stable: N2O3 is shown in Fig. 2. N2O5is normally found as [NO2]+[NO3]−; and NO3 is an unstable radical that (like NO and NO2) plays a role in atmosphericchemistry.(isoelectronic with CO and CO2, respectively) can be formed by the action of strong oxidizing agentsNO andon NO or NO2 in acid solvents such as H2SO4, and are known as solid salts (e.g. NO+[AsF6]−).
The nitrite andnitrate ionsand NO3− are formed respectively from nitrous acid HNO2 and nitric acid HNO3. As expectedfrom Pauling’s rules, HNO2 is a weak acid in water and HNO3 a strong acid (see Topic E2). Metal nitrates and nitritesare strong oxidizing agents, generally very soluble in water. Other less stable oxoacids are known, mostly containing N—N bonds. Although the free acid corresponding to phosphoric acid H3PO4 is unknown, it is possible to makeorthonitrates containing the tetrahedralion (see Topic F1, Structure 2).
Nitric acid is a major industrialchemical made from ammonia by catalytic oxidation to NO2, followed by reaction with water and more oxygen:It is used to make NH4NO3 fertilizer, and in many industrial processes (see Topic J4).F5—NITROGEN167Fig. 2. Structures of some oxides, oxocations and oxoanions of nitrogen.The redox chemistry of nitrogen compounds in aqueous solution is illustrated in the Frost diagram in Fig. 1 (seeTopic E5 for construction and use).
All oxides and oxoacids are strong oxidizing agents, and all oxidation states except−3, 0 and +5 are susceptible to disproportionation. The detailed reactions are, however, mostly controlled by kineticrather than thermodynamic considerations. In conjunction with oxidizable groups, as in ammonium nitrate NH4NO3 orin organic nitro compounds, N—O compounds can be powerful explosives.Other compoundsCompounds with sulfur are described in Topic F8. Apart from its fluorides, nitrogen halides are thermodynamicallyunstable and very explosive.
The trifluoride NF3 can be prepared by direct reaction of NH3 and F2. It is kinetically inertand nontoxic. Further fluorination gives the NV speciesThe oxofluoride ONF3 is also known. Likeit is isoelectronic withand must be described by a similarvalence structure (3). N2F4 is interesting in that like N2O4 it readily dissociates into NF2 radicals. Double-bonded N2F2exists in cis (4) and trans (5) forms, the former being thermodynamically more stable. The point groups are C2v (4) andC2h (5).Nitrogen reacts directly with some electropositive metals to form nitrides such as Li3N and Ca3N2. Although these canbe formulated with nitride ion N3− the bonding may be partially covalent. Other compounds with metals are amidesand imides (containingand NH2−, respectively) and azides containing.
Metal azides are thermodynamicallyunstable and often explosive.Section F—Chemistry of nonmetalsF6PHOSPHORUS, ARSENIC AND ANTIMONYKey NotesThe elementsHydrides and organicderivativesHalidesOxides and oxoacidsOther compoundsRelated topicsElemental structures are based on E4 molecules or three-coordinatepolymeric structures. Phosphates are widespread minerals, As and Sbbeing found as sulfides.Hydrides are less stable than ammonia and less basic. Many organicderivatives can be made.Compounds in the +3 and +5 oxidation state are known, althoughAsV is strongly oxidizing. Some halides are good Lewis acids, andhalide transfer reactions are common.Oxides in the +3 and +5 oxidation state are increasingly polymericwith heavier elements.
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