Lecture 21 – Cations, Anions and Hydrolysis in Water

1 12P32 Principles of Inorganic ChemistryDr. M. of metal of acidity and observable of elements in waterLecture 21 Cations, Anions and Hydrolysis in Water :1. Hydration The formation of a solutioninvolves 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.

2 At the molecular level, the ions interact with Water molecules from all directions in a 3-dimensional space. 2 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 +(g) + nH2O Mx+(aq) where Mx+(aq) represents ions surrounded by Water molecules and dispersed inthe 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.

3 Metal ions in aqueous solution behave as Lewis acids. The positive charge on the metal ion draws electron density from the O-H bond in the Water . This increases the bond's polarity making it easier to break. When the O-H bond breaks, an aqueous proton is released producing an acidic solution. The equilibrium constant for this reaction can be measured. [M(H2O)n]x++ H2O [M(H2O)n-1(OH)](x-1)++ H3O+ 2. Hydrolysis of cations3 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.

4 The Keqfor the Hydrolysis of a hydrated cation is analogous to the Ka for the ionization of a weak acid. Keqis an acid ionization constant. Generally, Hydrolysis constants for cations are tabulated as -log Ka. These tabulated Hydrolysis constants are averages of different experimental measurements that sometimes differ by more than one pKaunit. For this reason, small differences are not considered significant when comparing the values of these constants. 45 Factors Affecting pKa lower values of pKa and pH correspond to greater extent of Hydrolysis and higher acidity of the , pmpKaK+ + + (too small to measure directly)Ba2+ + + Rate of Hydrolysis of metal or acidity of a hydrated cationINCREASES with INCREASING charge and DECREASINGR adii.

5 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-H bond weakened so it easily breaks to give the H+ Be2+has an atomic radii of 59 pm and an electronegativity of From the table, Be2+is more electronegative than the Na+, K+, Li+cations. Be2+is more acidic than would otherwise be predicted. Why? partial covalent character in the BeOH2bond weakens the O-H the small coordination number means the oxygen lone pairs are held very tightly by the cation weakening O-H bonds more than would otherwise be expected.

6 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 Categories of Acidity(see handout) 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 addition of base we do not get the formation of an insoluble hydroxide (NaOH is soluble in H2O). Acidic Cations:pKa = 11-14 ( Ba2+in BaCl2)BaCl3is very soluble in Water , it dissolves smoothly, the hydroxide is somewhat soluble in Water but less soluble than the hydroxides of non acidic Acidic Cations:pKa = ( Fe2+in FeCl2).

7 FeCl2is soluble in Water with barely detectable change in the pH towards a more acidic solution. Fe(OH)2precipitates as the pH is Acidic Cations:pKa = 1-6 ( Fe3+in FeCl3)FeCl3is soluble in Water , produced a distinctly acidic (OH)2precipitates at pH = 3 or higher. Insoluble in excess Acidic Cations:pKa +4 to +1 ( TiCl4).TiCl4reacts VIOLENTLY with Water produces a lot of heat and insoluble (aq) + 2H2O TiO2 + Strong Acidic Cations:pKa = < -4 ( P5-and PCl5)Extremely VIOLENT reaction with Water produce solutions of the oxo acids H3PO4when PCl5reacts with H2O. The solution is highly acidic.

8 When base is added no precipitate appears as the acid gets and predominance diagrams for a phosphate species (see handout) Indicates the fraction of all phosphorous present in the form ofphosphoric acid H3PO4, the phosphate ion PO43-and the two partially protonated phosphate ions HPO42-and H2PO4-. Natural waters usually have pH values between 6 and 9, so we can see from the diagram what form of phosphorous might be Exam Question 2006 Periodicity1.(a) (4 marks) What is the inert-pair effect? How is it related to the common oxidation states of Tl and Pb?(b) (3 marks) What is a diagonal relationship?

9 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) AnswerIn group 13-17, in 4th 5th and 6th periods ns2electroms are much harder to remove than expected ( 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. Tl 5d106s26p1+1 is more stable than +3Pb 5d106s26p2+2 is more stable than +4 The 6s2are the inert pair, not easily removed.

10 Al is +3 is only known state so no inert (b) AnswerAn element in the second row (period) has behavior more like the element in the next group and next period than to its own BeO and Al2O3are amphoteric when other group II oxides are 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 three diagonal pairs are similar in atomic radii, electronegativity and charge density:Li Be BMg Al Si1 (c) AnswerHydration involves interaction of chemical species with Water . For ions the result is: Hydrolysis involves further chemical reaction of hydrated ions with the solvent ( Water ).