2P32 Principles of Inorganic Chemistry Dr. M. Pilkington Lecture 21 Cations, Anions and ydrolysis in Water: 1. ydration.energy 2. ydrolysis of metal cations 3. Categories of acidity and observable behavior 4. Speciation of elements in water 1. ydration The formation of a solution involves the interaction of solute with solvent molecules. Many different liquids can be used as solvents for liquid solutions, and water is the most commonly used solvent. When water is used as the solvent, the dissolving process is called hydration. The interaction between water molecules and a sodium ion is illustrated in the diagram below. This is a typical ion-dipole interaction. At the molecular level, the ions interact with water molecules from all directions in a 3-dimensional space. 1
The Enthalpy of hydration, of an ion is the amount of energy released when a mole of the gaseous ion is dissolved in a large amount of water. M x+ (g) + n 2 O M x+ (aq) where M x+ (aq) represents ions surrounded by water molecules and dispersed in the solution. As the size of the cation decreases then the enthalpy of hydration increases. This is because the interaction between the water and the cation is stronger so more energy is released on hydration. Remember Coloumb s Law E is proportional to 1/r As the charge of the cation increases, the enthalpy of hydration increases as with increasing charge the atomic radii of the ions decreases. 2. ydrolysis of cations Metal ions in aqueous solution behave as Lewis acids. The positive charge on the metal ion draws electron density from the O- bond in the water. This increases the bond's polarity making it easier to break. When the O- bond breaks, an aqueous proton is released producing an acidic solution. The equilibrium constant for this reaction can be measured. [M( 2 O) n ] x+ + 2 O [M( 2 O) n-1 (O)] (x-1)+ + 3 O + 2
Note the similarity of the equation for the hydrolysis of a hydrated cation with the equation for the ionization of a weak acid in aqueous solution. The K eq for the hydrolysis of a hydrated cation is analogous to the K a for the ionization of a weak acid. K eq is an acid ionization constant. Generally, hydrolysis constants for cations are tabulated as -log K a. These tabulated hydrolysis constants are averages of different experimental measurements that sometimes differ by more than one pk a unit. For this reason, small differences are not considered significant when comparing the values of these constants. 3
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Factors Affecting pka lower values of pka and p correspond to greater extent of hydrolysis and higher acidity of the solution. Cation Radius, pm pka K + 152 14.5 Na + 116 14.2 Li + 90 13.6 (too small to measure directly) Ba 2+ 149 13.5 Ca 2+ 116 12.8 Mg 2+ 86 11.4 Rate of hydrolysis of metal or acidity of a hydrated cation INCREASES with INCREASING charge and DECREASING Radii. The smaller the metal the more tightly it holds the lone pairs on the oxygen waters. In turn the hydrogen s will become more positively charged and the O- bond weakened so it easily breaks to give the + ions. 5
Be 2+ has an atomic radii of 59 pm and an electronegativity of 1.57 From the table, Be 2+ is more electronegative than the Na +, K +, Li + cations. Be 2+ is more acidic than would otherwise be predicted. Why? 1. The partial covalent character in the BeO 2 bond weakens the O- bond. 2. Plus the small coordination number means the oxygen lone pairs are held very tightly by the cation weakening O- bonds more than would otherwise be expected. To summarize there are important consequences on the acidic properties and hydrolysis reactions of compounds of elements with high oxidation numbers or small radii and high electronegativities. 6
3. Categories of Acidity (see handout) 1. Non Acidic Cations: for example Na + (in NaCl) are usually not acidic in Group 1 with the exception of Li +. When we dissolve sodium in water it dissolves smoothly, the solution is definitely NOT detectably acidic. On addition of base we do not get the formation of an insoluble hydroxide (NaO is soluble in 2 O). 2. Feebly Acidic Cations: pka = 11-14 (e.g. Ba 2+ in BaCl 2 ) BaCl 3 is very soluble in water, it dissolves smoothly, the hydroxide is somewhat soluble in water but less soluble than the hydroxides of non acidic cations. 3. Weakly Acidic Cations: pka = 6-11.5 (e.g. Fe 2+ in FeCl 2 ). FeCl 2 is soluble in water with barely detectable change in the p towards a more acidic solution. Fe(O) 2 precipitates as the p is increased. 4. Moderately Acidic Cations: pka = 1-6 (e.g. Fe 3+ in FeCl 3 ) FeCl 3 is soluble in water, produced a distinctly acidic solution. Fe(O) 2 precipitates at p = 3 or higher. Insoluble in excess hydroxide. 5. Strongly Acidic Cations: pka +4 to +1 (e.g. TiCl 4 ). TiCl 4 reacts VIOLENTLY with water produces a lot of heat and insoluble TiO 2. TiCl 4 (aq) + 2 2 O TiO 2 + 4Cl 6. Very Strong Acidic Cations: pka = < -4 (e.g. P 5- and PCl 5 ) Extremely VIOLENT reaction with water produce solutions of the oxo acids i.e. 3 PO 4 when PCl 5 reacts with 2 O. The solution is highly acidic. When base is added no precipitate appears as the acid gets neutralized. 7
Distribution and predominance diagrams for a phosphate species (see handout) Indicates the fraction of all phosphorous present in the form of phosphoric acid 3 PO 4, the phosphate ion PO 4 3- and the two partially protonated phosphate ions PO 4 2- and 2 PO 4-. Natural waters usually have p values between 6 and 9, so we can see from the diagram what form of phosphorous might be present. 8
Past Exam Question 2006 Periodicity 1.(a) (4 marks) What is the inert-pair effect? ow is it related to the common oxidation states of Tl and Pb? (b) (3 marks) What is a diagonal relationship? To which elements does this term apply in the periodic table? (c) (4 marks) Explain the difference between hydrolysis and hydration of a metal ion. Illustrate your answer with an appropriate example. 1 (a) Answer In group 13-17, in 4th 5th and 6th periods ns 2 electroms are much harder to remove than expected (i.e. they are inert ). (these elements are less reactive than expected on the basis of ENC trend, atomic size and IE). Therefore the highest oxidation state in the group is less stable than the one that is 2 less than the highest. e.g. Tl 5d 10 6s 2 6p 1 +1 is more stable than +3 Pb 5d 10 6s 2 6p 2 +2 is more stable than +4 The 6s 2 are the inert pair, not easily removed. Al is +3 is only known state so no inert pair. 9
1(b) Answer An element in the second row (period) has behavior more like the element in the next group and next period than to its own congeners. e.g. BeO and Al 2 O 3 are amphoteric when other group II oxides are basic. appens because the second period element ions are small and their charge density (charge/size of ion) is similar to the diagonal element in the period below. The three diagonal pairs are similar in atomic radii, electronegativity and charge density: Li Be B Mg Al Si 1 (c) Answer ydration involves interaction of chemical species with water. For ions the result is: O O +Z O δ+ δ- O δ- O δ+ δ+ δ+ O δ- O -Y δ+ O ydrolysis involves further chemical reaction of hydrated ions with the solvent (water). Metal ions in aqueous solution behave as Lewis acids. The positive charge on the metal ion draws electron density from the O- bond in the water. This increases the bond's polarity making it easier to break. When the O- bond breaks, an aqueous proton is released producing an acidic solution. e.g. [M(O 2 ) 6 ] n+ + 2 O [M(O 2 )(O)] (n-1)+ + 3 O + 10