Chapter 18 OXYGEN 18-1 Introduction Oxygen compounds of all the elements except He, Ne, and possibly Ar are known. Molecular oxygen (dioxygen, 02) reacts (at room temperature or on heating) with all other elements except the halogens, a few noble metals, and the noble gases. The chemistry of oxygen involves the completion of the octet (neon config­ uration) by one of the following means: 1. Electron gain to form the oxide 0 2-. 2. Formation of two single covalent bonds, usually in bent AB2E2 systems, such as water and ethers. 3. Formation of a double bond, as in ABE2 systems, such as ketones or CI 4Re=0. 4. Formation of a single bond, as well as electron gain, as in ABEs systems, such as OH- and RO-. 5. Formation of three covalent bonds, usually in pyramidal ABsE systems, such as HsO+ and RsO+. 6. Formation in rare cases of four covalent bonds, as, for example, in Be40 (CHSC02)6' The wide range of physical properties shown by the binary oxides of the el­ ements is due to the broad range of bond types from essentially ionic systems to essentially covalent ones. Thus we distinguish the highly ionic oxides (such as those of the alkali and alkaline earth metals) from the completely covalent, mo­ lecular oxides, such as CO2, There are, however, intermediate cases such as the oxides of boron, aluminum, or silicon. Ionic Oxides The formation of the oxide ion from molecular oxygen requires about 1000 kJ mol-I: ! 02(g) = O(g) !1H = 248 kJ mol- l (18-1.1) 2 1 O(g) + 2e- = 0 - !1H = 752 kJ mol- (18-1.2) In forming an ionic metal oxide, energy must also be expended to vaporize and to ionize the metal. Thus the stability of ionic metal oxides is a consequence only of the high lattice energies that are obtained with the small and highly charged oxide ion. 435 436 Chapter 18 I Oxygen Where the lattice energy is not sufficient to offset the energies for ionization, and so on, oxides with substantial covalent character are formed. Examples of oxides with some covalent character are BeO, Si02, and oxides of boron, such as B20 g . Covalent or Molecular Oxides Covalent or molecular oxides are compounds, such as CO2, S02, SOg, and N02, in which covalent bonding is dominant. Such compounds are well described by the ABxEy classification, as presented in Chapter 3, with some exceptions, as noted in the following subsection. Use of the p orbitals in 1t bonding with other atoms is an important aspect in the bonding of molecular oxides. This may be pn-pn bonding as in the ketones (R2C=0), or pn-d1t bonding as in phosphine oxides (RgP=O) or linear M=O=M systems. ABEg Systems Terminal oxygen atoms that bear three lone pairs of electrons are found in alkbxides (RO-), and hydroxide (OH-). Such oxygen atoms may be considered to be spg hybridized. AB2E2 Systems The compounds that fit into this class are usually angular due to the volume requirements of two lone pairs of electrons. Examples include water, alcohols, and ethers. The oxygen atoms are considered to be spS hybridized, but there are wide variations from the tetrahedral bond angles due to electronic repulsions be­ tween the two lone pairs of electrons: H 20 (104.5°) and (CHg)20 (Ill0). Where the atoms bound to oxygen have d orbitals available, some pn-d1t character is often present in the bond to oxygen, and the B-A-B angles may be even larger, for example, the angle Si-O-Si in quartz is 142° and in HgSi-O-SiHg it is greater than 150°. A linear B-A-B situation at oxygen occurs in some AB2E2 systems con­ taining transition metals (e.g., [CIsRu-O-RuCIs]4-). The () bonds to Ru are formed by sp hybrids on oxygen, thus leaving two pairs of1t electrons on oxygen in porbitals that are oriented perpendicular to the Ru-O-Ru axis. These filled p orbitals on oxygen interact with empty d orbitals on the Ru atoms, forming a 1t-bond system. AB3 E Systems The third example containing spg hybridized oxygen atoms is that of the ox­ onium ions :OH; and :OR;. The formation of oxonium ions is analogous to for­ mation of ammonium ions (NH:). Oxygen is less basic than nitrogen, and the oxonium ions are therefore less stable. Notice that ions of the type OH~+ are un­ likely (even though :OH; still has a lone electron pair), because of electrostatic repulsion of the :OH; ion towards another proton. As for :NRg, the pyramidal :OR; ions undergo rapid inversion. ABE2 Systems Oxygen atoms of this type include those of ketones, aldehydes, and other or­ ganic carbonyls. The oxygen atoms are Sp2 hybridized and have a roughly trigo­ nal arrangement around the oxygen of the lone pairs E and the carbonyl carbon. 18- 1 Introduction 437 The Sp2 hybridization of the carbon atom leaves one p orbital available for for­ mation of a TC bond perpendicular to the trigonal plane. Acid-Base Properties of Oxides Generally, the oxides of the metals are basic, whereas those of the nonmetals are acidic. There are also a number of important amphoteric oxides. Basic Oxides 2 Although X-ray studies show the existence of discrete oxide ions (0 -) [as well as peroxide (O~-) and superoxide (02") to be discussed later], these ions cannot exist in aqueous solution owing to the hydrolysis reactions shown in Reactions 18-1.3 through 18-1.5. 2 0 - + H 20 -----c> 2 OH- (18-1.3) O~- + H 20 -----c> H02" + OH- (18-1.4) 2 02" + H 2 0 -----c> O 2 + H02" + OH- (18-1.5) Consequently, only those ionic oxides that are insoluble in water are inert to it. Ionic oxides function as basic anhydrides. When insoluble in water, they usually dissolve in dilute acids, as in Reaction 18-1.6. (18-1.6) However, some ionic oxides (e.g., MgO) become very slow to dissolve in acids after high-temperature ignition. Acidic Oxides The covalent oxides of the nonmetals are usually acidic, dissolving in water to produce solutions of acids. They are termed acid anhydrides. An example is given in Reaction 18-1.7, in which N 20 5 is seen to be the acid anhydride of nitric acid. (18-1.7) Even when these oxides are insoluble in water (e.g., as in the case of Sb20 5 ) , they will generally dissolve in bases (as in Reaction 18-1.8). (18-1.8) Acidic oxides will often combine directly, by fusion, with basic oxides to form salts, as in Reaction 18-1.9. fu~n Na + SI·o 2 ) N a S·OI 3 (18-1.9) 2° 2 Amphoteric Oxides These oxides behave acidicly towards strong bases and as bases towards strong acids. The example ofZnO is illustrated in Reactions 18-1.10 and 18-1.11. 438 Chapter 18 I Oxygen 2 ZnO(s) + 2 H+(aq) -------+ Zn +(aq) + H 20 (18-1.10) ZnO + 2 OH- + H 20 -------+ Zn (OH)~- (18-1.11) Other Oxides There are other oxides, some of which are relatively inert, which dissolve in neither acids nor bases (e.g., N20, CO, Pb02, and Mn02)' When Mn02 and Pb02 do react with acids (e.g., conc HCl) they do so by a redox rather than an acid-base reaction, as in Reaction 18-1.12. (18-1.12) 18·2 Occurrence, Properties, and Allotropy Oxygen has three isotopes, 160 (99.759%), 170 (0.0374%), and 180 (0.2039%). Fractional distillation of water allows concentrates containing up to 97 atom % 180 or up to 4 atom % 170 to be prepared. Oxygen-18 is used as a tracer in study­ ing reaction mechanisms of oxygen compounds. Although 170 has a nuclear spin (~), its low abundance means that even when enriched samples are used spectrum accumulation and/or the Fourier transform method are required. An example of 170 resonance studies is the distinction between H 20 in a complex, for example, [Co(NH3)sH20]3+, and solvent water. Oxygen has two allotropes; dioxygen (02) and trioxygen or ozone (03), Dioxygen is paramagnetic in all phases and has the rather high dissociation en­ ergy of 496 kJ mol-1. Simple valence bond theory predicts the electronic struc­ ture :0=0: which, though accounting for the strong bond, fails to account for the paramagnetism. However, simple MO theory (Section 3-5) readily accounts for the triplet ground state having a double bond. There are several low-lying sin­ glet states that are important in photochemical oxidations. Like NO, which has one unpaired electron in an antibonding (11:*) MO, oxygen molecules associate only weakly, and true electron pairing to form a symmetrical 0 4 species does not occur even in the solid. Both liquid and solid O 2 are pale blue. Ozone The action of a silent electric discharge on O 2 produces 0 3 in concentrations up to 10%. Ozone gas is perceptibly blue and is diamagnetic. Pure ozone obtained by fractional liquefaction of02-0 3 mixtures gives a deep blue, explosive liquid. The action of UV light on O 2 produces traces of 0 3 in the upper atmosphere. The maximum concentration is at an altitude of about 25 km. It is of vital im­ portance in protecting the earth's surface from excessive exposure to UV light. Ozone decomposes exothermically, as in Reaction 18-2.1: I1H = -142 kJ mol-1 (18-2.1) but it decomposes only slowly at 250°C in the absence of catalysts and UV light.