Hydroxides and oxoacids

The ricji oxoacid chemistry of sulfur (pp. 705-21) is not paralleled by the heavier elements of the group. The redox relationships have already been summarized (p. 755). Apart from the dark-brown hydrated monoxide Po(OH)2 , which precipitates when alkali is added to a freshly prepared solution of Po(ll), only compounds in the +4 and +6 oxidation states are known. 

Selenous acid, 0=Se(0H)2, i.e. H2Se03, and tellurous acid, H2Te03, are white solids which can readily be dehydrated to the dioxide (e.g. in a stream of dry air). H2Se03 is best prepared by slow crystallization of an aqueous solution of Se02 or by oxidation of powdered Se with dilute nitric acid  

Hydrated polonium dioxide, PoO(OH)2, is obtained as a pale-yellow flocculent precipitate by addition of dilute aqueous alkali to a solution 

In the -1-6 oxidation state the oxoacids of 8e and Te show little resemblance to each other. H28e04 resembles H28O4 (p. 710) whereas orthotelluric acid Te(OH)g and polymetatelluric acid (H2Te04) are quite different. 

The acid dissociation constants of H28e04 are close to those of H28O4, e.g. (H28e04) 

---

A number of compounds— hydrates, hydroxides, and oxoacids—that contain water or its components lose water when heated. Hydrates, compounds that contain water molecules, lose water to form anhydrous compounds, free of molecular water. 

In Chapter 9 we established the first five components of our interconnected network of ideas for understanding the periodic table. These included the periodic law, the uniqueness principle, the diagonal effect, the inert-pair effect, and the metal-nonmetal line. These components are summarized individually and collectively in colored figures located on the front inside cover of the book The icons for each component are shown there as well as on the bookmark pullout in the back of the text. In Chapter 10 we discussed hydrogen and the hydrides (as well as basic nuclear processes). In Chapter 11 we discussed the chemistry of oxygen, reviewed and extended our knowledge of the nature of water and aqueous solutions, and added a sixth component to our network the acid-base character of oxides and their corresponding hydroxides and oxoacids. The network with this additional component is shown in color on the top left side of the back inside cover of the book. 

Oxides, oxoacids and hydroxides Oxides and oxoacids of carbon... 

Our network of ideas can be applied to oxides, which divide into metal ionic and nonmetal covalent types. Ionic oxides are basic anhydrides that produce metal hydroxides and hydroxide ions in aqueous solution. Nonmetal oxides are acidic anhydrides that produce oxoacids and hydronium ions in solution. These correlations have become the sixth component of our network of ideas. The relative strengths of oxoacids and hydroacids can be rationalized by using other parts of the network. A systematic approach to the nomenclature of the oxoacids is based on the five representative -ic acids. 

For each of the following oxides, write a two-part equation showing its reaction when placed in water. The equation should show (i) the corresponding hydroxide or oxoacid in molecular form and (ii) the ionized products in aqueous solution ... 

Metal oxides produce metal hydroxides and hydroxide ions in aqueous solution. Non metal oxides produce oxoacids and hydronium ions in aqueous solution. 

The eight chapters on the representative groups each include sections on (1) the history and discovery of the elements, (2) their fundamental properties as they relate to the growing network of ideas (including an overview of the hydrides, oxides, hydroxides and/or oxoacids, and halides of the group), (3) reactions and compounds... 

Apart from the binary hydrides of Groups 16 and 17, Lowry/ Brpnsted acids in aqueous solution are nearly all oxoacids, i.e. substances containing O-H bonds which ionise in aqueous solution to give oxo-anions and H+(aq) (or H30+). Most oxoacids are molecular hydroxides E(OH) , such as B(OH)3, Ge(OH)4 and Te(OH)6, or oxohydroxides EOm(OH) . In addition, we have more complex species containing E-E bonds or E-O-E bridges. In EOm(OH) - for example, N02(0H), PO(OH)3, S02(0H)2,103(0H) - the m O atoms are held to E by bonds having at least some double bond character, via p -p or d -p overlap. Oxohydroxides may be seen as being derived from hydroxides by elimination of H20, and are favoured by elements E whose atoms form double bonds to O atoms. 

Hydroxides MOH are important compounds for all the alkali metals. They can easily formed by reaction of oxides with water (or atmosphere moisture). They are soluble in water and give strong base. Compounds of oxoacids are commonly encountered, such as carbonate, nitrate, sulphate, etc. as these anions are fairly large, lithium compounds tend to be the most soluble in the series. Many of these compounds crystallise in a variety of hydrated forms (e.g. Na2C03. H2 O with n = 1, 7 or 10). 

Although oxoacids and hydroxides are Arrhenius acids and bases (they release or OYi (aq) into aqueous solution), acid and base anhydrides do not fall into this classification because they contain neither nor OH. Acid anhydrides are acids in the Lewis sense, (they accept electron pairs), and base anhydrides are bases in the Lewis sense, (their ions donate electron pairs). The reaction between an acid anhydride and a base anhydride is then a Lewis acid-base reaction. An example of such a reaction is... 

The acid strength of nonmetal hydrides increases towards the right and to the bottom of the periodic table. Acid strengths of oxoacids can be predicted approximately from their formulae by Pauling s rules. Metal cations with polarizing character are acidic in water, and some form amphoteric oxides or hydroxides. 

The matter consists in the fact that although alkali metal hydroxides are classified among strong bases, they are nevertheless distilled without decomposition at temperatures near 1300 °C. Sodium and potassium carbonates melt without decomposition at 854 and 890 °C, respectively, and the partial pressure of C02 over the said melts is low enough even at 1000 °C. Thermal dissociation of Na202 starts at temperatures close to 460 °C, whereas K202 melts without appreciable decomposition at 490 °C [114, 121]. Hence, these bases should be sufficiently stable under the experimental conditions provided in most experimental oxoacidity studies (200-800 °C). 

As to the problem of completeness of hydroxide ion dissociation in the atmosphere of inert gas, which is not purified from water traces, the situation is as follows. By taking into account the value of the equilibrium constant of reaction (2.5.68), which is equal to 0.006 at 700 °C in the molten KCl-NaCl equimolar mixture, one can estimate the degree of dissociation of hydroxide ion as 0.995 ( 1) at the partial pressure of water vapour near 10-5 atm. (this is the usual concentration for gaseous extra-purity A), 0.857 at 0.001 atm. (for pure Ar) and 0.376 at 0.01 atm. (which is appropriate for nitrogen of technical quality). This means that only extra-pure argon can be used without preliminary purification for the creation of an inert atmosphere over the melts for oxoacidity studies, where the degree of hydroxide-ion dissociation approaches 1 (unity). 

---