05ionsinsolution-2

05ionsinsolution-2 - Ions in Solution Chemical Principles...

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Unformatted text preview: Ions in Solution Chemical Principles of Marine Systems SIO141/CHEM174 Wednesday, April 8, 2009 1 Understanding the solubilities of salts 2 Definition of solubility product: ! Ks = a(M+)a(X) G(sol) = RT ln Ks and hence ! G(sol)/(kJ mol1) 5.7 log Ks A factor of 10 change in solubility product thus corresponds to quite a small change in the free energy 3 Thermodynamic cycle for dissolution of a salt in water MX (c) Salt dissolving in w ater (A) (B) Separating ions from lattice state M+ (g) + X (g) (C) Hydrating the ions M+ (aq) + X (aq) 4 free energy of the lattice (> 400 kJ mol1) Gs = GL + Gh free energy of hydration free energy of dissolution (80 < Gs < + 80 kJ mol1) What does this imply about the magnitude of Gh ? 5 Thermodynamic cycle for dissolution of a salt in water MX (c) Salt dissolving in w ater (A) (B) Separating ions from lattice state M+ (g) + X (g) (C) Hydrating the ions + M (aq) + X (aq) 6 Q1 What is required for a salt to be "soluble" ? 7 Q2 G = H - TS What do the various terms mean? 8 Entropy of solids What do we notice about these? 9 Changes of size Group 1 2 4 3 4 5 6 7 8 9 10 11 12 13 5 14 6 15 7 1s 16 8 1 2 H 17 9 He 18 10 1.008 4.003 s 3 Li Na K Rb Cs Fr Be Mg Ca Sr Ba Ra p B Al C Si Ge Sn Pb N P As Sb Bi O S Se Te Po F Cl Br I At Ne Ar Kr Xe Rn 6.941 9.012 12 11 22.99 24.30 20 19 39.10 40.08 38 37 85.47 87.62 56 55 132.9 137.3 88 87 (223) (226) 10.81 12.01 14.01 16.00 19.00 20.18 15 18 14 17 16 13 26.98 28.09 30.97 32.06 35.45 39.95 33 36 32 35 34 31 d 21 22 23 24 25 26 27 28 29 30 Sc Y Lu Lr Ti Zr Hf Rf V Nb Ta Db Cr Mo W Sg Mn Tc (98) 75 Fe Ru Os Hs Co Rh Ir Mt Ni Pd Pt Ds Cu Ag Au Rg Zn Cd Hg Ga In Tl 44.96 47.87 50.94 52.00 54.94 55.84 58.93 58.69 63.55 65.38 69.72 72.64 74.92 78.96 79.90 83.80 46 53 43 52 41 45 49 42 51 47 54 44 50 40 48 39 88.91 91.22 92.91 95.96 72 73 74 71 101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.3 78 85 84 77 81 83 79 86 76 82 80 Re Bh 175.0 178.5 181.0 183.8 186.2 190.2 192.2 195.1 197.0 200.6 204.4 207.2 209.0 (209) (210) (222) 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 (262) (267) (268) (271) (272) (270) (276) (281) (280) (285) (284) (289) (288) (293) UUb UUt UUq UUp Uuh UUs UUo (?) (294) f 57 58 59 60 61 62 63 64 65 66 67 68 69 70 La Ac Ce Th Pr Pa Nd U Pm Np Sm Pu Eu Am Gd Cm Tb Bk Dy Cf Ho Es Er Fm Tm Md Yb No 2007 138.9 140.1 140.9 144.2 (145) 150.4 152.0 157.2 158.9 162.5 164.9 167.3 168.9 173.0 95 92 90 94 98 91 100 101 102 96 93 89 99 97 (227) 232.0 231.0 238.0 (237) (244) (243) (247) (247) (251) (252) (257) (258) (259) 10 Cases where entropic considerations dominate: KNO3 ! (soluble, despite endothermic dissolution) LaPO4 ! (very insoluble) 11 However it is usually important to look at the free energy as a whole: Gs = GL + Gh We will look for semi-empirical expressions for GL & Gh that depend on ionic radius 12 13 14 Q3 What is the source of the contrasting behavior? 15 16 (A) = (B) + (C) 17 Q5 What patterns can you see in this table? Can you explain them? 18 ? ? 19 Other things to note 1. Hh(AgX) - Hh(NaX) 62 kJ mol1 Thus Hh(Ag+) > Hh(Na+) 2. HL(AgX) - HL(NaX) increases from F to I Thus crystal lattice is increasingly stabilized 3. SL consequence of new translational degrees of freedom compared to the lattice Sh consequence of restrictions on translational movement due to solvent-ion interactions 4. Sh(MF) entropy loss attributed to H-bonding between F and H2O 20 Clearly there are substantial difficulties in providing simple generalizations to rationalize solubilities! 21 HOMEWORK: Calculate the enthalpy of hydration of bromide ions given that the hydration enthalpy of barium ions is -1360 kJmol-1, the lattice enthalpy of formation for BaBr2 is -1937 kJmol-1 and the enthalpy of solution of BaBr2 = -38 kJmol-1. 22 Ionic hydration energies 23 Relation between ionic charge and hydration enthalpy (Born Model*) This model assumes that the ion is transferred from a vacuum into a continuous dielectric, and calculates G for the process Mz+(g) -- Mz+ (aq) (*A simple electrostatic model) 24 Relation between ionic charge and hydration enthalpy (Born Model*) NAz e G = 8 0r 2 2 1 1- (reflects the difference in charging energies) In water at 298 K (r in pm) 68,500z 2 G = - kJ mol1 r 4,090z 2 S = - kJ mol1 K 1 r 69,700z 2 H = - kJ mol1 r 25 7500 z=4 5000 Hydration energies of ions of different charge / radius -H 1 kJ mol 2500 z=2 z=1 0 2 z=3 0 10 z (r + 85 pm) 2 2 4 6 8 10 26 Q1 What is the significance of the 85 pm? 27 7500 z=4 5000 Hydration energies of ions of different charge / radius -H 1 kJ mol 2500 z=2 z=1 0 2 z=3 Allows for limitations of Born approach 4 6 8 10 0 10 z (r + 85 pm) 2 2 Empirical factor 28 ...
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This note was uploaded on 09/11/2009 for the course CHEM 174 taught by Professor Staff during the Spring '09 term at UCSD.

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