2P32 Principles of Inorganic Chemistry Dr. M. Pilkington Lecture 22 The Acid-Base Character of Oxides and Hydroxides in Aqueous Solution Oxides; acidic, basic, amphoteric Classification of oxides - oxide acidity and basicity Hydrolysis of oxo anions Periodicity of oxide acid-base character Reactions of acid and basic oxides 1. Oxides Oxygen forms compounds with all elements except He, Ne, Ar, and Kr. It reacts directly with all elements except the halogens, a few noble metals e.g. Ag and Au and the noble gases. As encountered for the hydrides, there is variability in the types of bonding encountered. Metal oxides are ionic solids, nonmetal oxides are discrete molecular covalent gases and liquids. The oxides of heavier nonmetals and the semimetals tend to be covalent polymeric solids. Transition metal oxides are occasionally nonstiochiometric. 1
The variation of acid-base properties of the oxides in aqueous solution, is strongly correlated with the position of the metal-nonmetal line. Oxides in aqueous solution (Acidic and Basic Anhydrides) One of the most important aspects of the properties of oxides is their acid-base properties. Many oxides are basic or acidic anhydrides; that is they are compounds that are formed by the removal of water from a corresponding base or acid. Ionic oxides are usually basic anhydrides, whereas covalent oxides are usually acidic anhydrides. Oxides of the semimetals are amphoteric anhydrides, capable of acting as either an acid or base, depending on the circumstances. If we list the oxides of a given period, e.g. 3 rd, we find an orderly progression of their acid-base character. The acid strength increases with the acidity of the cation involved Na + Cl +7 Na 2 O, MgO, Al 2 O 3, SiO 2, P 4 O 10, SO 3, Cl 2 O 7 Na(OH) Mg(OH) 2 Al(OH) 3 Si(OH) 4 H 3 PO 4 H 2 SO 4 HClO 4 (sol) (insol) amphoteric weak acid very strong acid. BASIC OXIDE ACIDIC OXIDE AMPHOTERIC OXIDE strong base Amphoteric species Strong Acid 2
The ionic oxides are characterized by the presence of the oxide ion, O 2-, that like the H + ion, cannot exist alone in aqueous solution. The reaction between the oxide and a water molecule is shown below: O 2- + H O 2 O H - H The 2- charged oxide attacks and forms a bond with a partially positive hydrogen atom of the water molecule. The subsequent breaking of the O-H bond produced two hydroxide d ions. The equilibrium constant for this reaction is greater than 10 22, so this reaction lies far to the right. For example sodium oxide: Na 2 O (s) + H 2 O(l) 2NaOH (aq) 2Na + + 2OH - (aq) The process is a sequence from the metal oxide, to the metal hydroxide that dissociates into the aqueous hydroxide and metal ions. Sodium oxide is therefore a basic anhydride; it produces the base sodium hydroxide in aqueous solution. Note the greater degree of ionic character of the oxide, the more basic it is. Nonmetal oxides react with water to produce what are known as oxo-acids or oxyacids, i.e. acids containing NM-O-H unit, where NM = nonmetal. A non metal oxide is usually characterized by polar covalent bonds rather than the ionic bonds of a metal oxide. 3
For example, H + H O O 2H O 2 O - NMO 2-2 + 2H 3 O + NM H + NM O H oxoacid The partially negative oxygen atom of a water molecule will attack the partially positive nonmetal atom at the same time that the oxygen of the oxide is attracted to one of the hydrogen atoms of the water. The breaking of the O-H bond of the water produces an oxoacid that, splits to produce the corresponding aqueous anion and hydronium ions. For example, sulfur trioxide (non metal) produces sulfuric acid as the oxoacid that in turn dissaciates into sulfate and hydronium ions. Sulfur trioxide is the acid anhydride of sulfuric acid. Amphoteric oxides are often oxides of the semimetals. Although the oxides themselves are not too soluble in water, they can react with either acids or bases. ZnO is also amphoteric. + 6H + 2Al 3+ (aq) + 3H 2 O Al 2 O 3 (s) + OH - + 3H 2 O 2[Al(OH) 4 ] - (aq) 4
Hydrolysis of Oxo anions Non metals exist in aqueous solution as oxo acids that ionize to form oxoanions in solution. Oxo anions (combinations of oxygen and a NM) are hydrated in aqueous solution. In this case the electrostatic attraction is between the electron pairs on the oxo anion oxygen atoms and the partially positive H-atoms of the water molecules. The hydration is an exothermic process. Hydration energies increase with increasing charge and decreasing size. If the interaction between the anion and the H-atom of the water is sufficiently strong, the H can be removed from water generating a hydroxide ion resulting in a basic solution. MO y- x + H 2 O [MO (x-1) OH] (y-1) + OH - When determining the basicity of an oxo anion we have to take into consideration, charge, number of oxo groups and electronegativity. Oxoanions can be placed into categories that describe the extent to which they hydrolyze. Effect of charge on basicity Increasing charge on an anion increases its tendency to hydrolyze and form basic solutions. The pk b values of an oxoanion decrease by 4-5 units for each additional negative charge on the anion. Effect of number of oxo groups Since most nonmetals exhibit more than one oxidation state, they can form oxo anions that differ in the number of oxo groups (oxygen s bound directly to the metal). For example chlorine forms four different oxo anions: ClO -, ClO 2-, ClO 3-, ClO 4-. Adding additional oxygen s decreases the bascity of the oxoanion. Effect of electronegativity As the electronegativity of the nonmetal atom decreases the basicity of the oxo anion increases. 5
The classification of oxo anions is summarized below Classification Resulting Formula Nonbasic contains oxo groups & no charge Feebly basic no charge & no oxo groups Moderately basic no oogro oxo groups & a charge of -1/2 or -1 Very strongly basic no oxo groups & a charge more negative an -1 Example 1. Consider an oxo anion of formula MO 4-. For this formula, the effect of one unit of negative charge will cancel out the effect of two oxo groups. If we remove the negative charge and two oxo groups the resulting formula is MO 2. This anion falls into the category of nonbasic. 2. Consider an oxo anion of formula MO 4-3. The three oxo groups cancel 1.5 unit of negative charge. The resulting formula becomes M-2.5. This oxo anion will be very strongly basic. A table on the next slide lists calculated pkb s for the important simple oxo anions of the elements. 6
Summary of Trends in Numbers of Oxo Groups in Oxo Anions TheoxoanionshavingthesmallestcentralatomsarethoseofthePeriod2pblock. These can have a maximum coordination of 3 and will accommodate either 3 oxo groups or two oxo groups plus one unshared p electrons. 7
Central atoms of the p-block of Periods 3 and 4 and those of the 3 block 4 and those of the d-block of Periods 4 and 5 have larger radii and can have a maximum coordination number of 4. If the valence orbitals are 3p, 4p or 4d, the central atom can accommodate 4 oxo groups and unshared p electrons. Central atoms of the p-block and d-block of Period 6, have a maximum coordination number of 6. If the valence orbitals on the central atom are 5p, 5d, 5f, 6p or 6d, the central atom can accommodate 4-6 oxo groups and unshared p- orbital pairs. 8
2. To Summarize - classification of oxides Acidic Oxide produces an acid when it reacts with H 2 O e.g. SO 3 + H 2 O H 2 SO 4 SO 2 + H 2 O H 2 SO 3 CO 2 + H 2 O H 2 CO 3 Basic Oxide produces a base when reacts with H 2 O e.g. Na 2 O + H2O 2NaOH CaO + H 2 O Ca(OH) 2 Amphoteric Oxides react with H 2 O to produce a hydroxide that reacts further with either acids or bases. For Example ZnO, BeO -, Al 2 O 3-, Ga 2 O 3-, Sn 2 O 2-, PbO - 2 The E-OH Unit in Aqueous solution; (where E = an element) We know both metal and monmetal oxides react with water to produce compounds with an E-O-H unit. If E is a metal, the unit acts as a base releasing hydroxide ions in solution. If E is a nonmetal, hydronium ions are released. Why the difference? How does the nature of E determine whether the unit will be an acid or a base or amphoteric? To answer this we look closely at the relative electronegativities of the atoms within the unit. 9
hydroxide = OH - = base X O H hydronium = H 3 O + = acid where does the bond break? For a metal (ENs 0.7-1.5) X O H X + + OH - Producing a base in solution. The greater difference in electronegativity is between the metal and the oxygen, making the M-O bond the more polar. The more polar M-O bond is susceptible to attack by polar water molecules. This results in breaking the M-O bond to afford the aqueous metal cation and the hydroxide anion in solution. For a Non Metal (ENs 2.3-3.5) In this case the O-H bond is the more polar and susceptible to attack by water molecules, and this results in the oxoanion and hydronium ions in solution. X O H XO - + H + Producing an acidic solution. For a semimetal The two bonds of the E-O-H unit are approximately of the same polarity, and either can be broken, depending on the circumstances. In this case the unit is amphoteric. 10
3. Periodicity of oxide acid-base character We have just seen that for a non metal, how the X-O-H unit is split between the oxygen and the hydrogen atoms by the attack of polar water. Two additional factors are also important in determining the relative acid strengths: 1. The electronegativity ti it of the central atom 2. The number of nonhydroxyl oxygens bound to the central atom. As the electronegativity of the central atom increases, so does its ability to withdraw electron density from its neighboring atoms. The surrounding oxygen atoms have high electronegativity, so the central atom cannot withdraw electron density from these. The electron density comes from the O-H bonds. The H atom has low electronegativity. Electron density is withdrawn from the O-H bond, the H becomes more partially positive and therefore the O-H bond is more polar and susceptible to attack by water molecules. The acid strength of an oxoacid increases as the electronegativity of the central atom increases. For example, sulfuric acid is a stronger acid than selenic acid, phosphoric acid is stronger than arsenic acid and perchloric acid is stronger than perbromic acid. As the number of nonhydroxyl oxygens increases, they withdraw more electron density from the central atom, making it partially positive. In turn the central atom now withdraws more electron density from the only source available, i.e the O-H bond. Again the H atom becomes partially positive, and the O-H bond becomes more polar and therefore more susceptible to attack by water. The result is that as the number of nonhydroxyl oxygens increases so does the strength of the oxoacid. For example, acid strength increases from nitrous acid to nitric acid and from sulfurous to sulfuric acid. 11
What about Transition Metals? Cr II O is basic and dissolves in acids. Cr III 2O 3 is amphoteric it dissolves in acids (Cr 3+ ) and bases [Cr(OH) 3 ] - Cr VI O 3 is an acidic oxide it dissolves in water to produce H 3 CrO 4 (chromic acid). Acidity of the oxide increases s as the charge on the cation increases s (so with increasing oxidation number of the central atom). Acidity of the oxide also increases as the size decreases ( as the ionic radius decreases the X-OH bond strength increases and the H + ion dissociates). CrO 3 is analogous to SO 3 H 2 CrO 4 H 2 SO 4 Chromic acid Sulfuric acid 4. Reactions of acidic and basic oxides 1. With H 2 O to produce acid or bases CaO + H 2 O Ca(OH) 2 basic oxide N 2 O 5 + H 2 O 2HNO 3 acidic oxide 2. React with each other to afford salts. Na 2 O(s) + SO 3 (g) Na 2 SO 4 (s) basic acidic salt oxide oxide 12
CaO + Cl 2 O Ca(OCl) 2 basic acid salt oxide oxide 3. Acid oxide + base salt or salt + H 2 O CO 2 + NaOH NaHCO 3 CO 2 + 2NaOH Na 2 CO 3 + H 2 O (CO 2 is the anhydride of H 2 CO 3 ) 3. Basic oxide + acid salt + H 2 O MgO + 2HCl MgCl 2 + H 2 O 13