Ch08-TCF

Ch08-TCF - Quantitative Chemical Analysis Chapter 8...

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Quantitative Chemical Analysis Quantitative Chemical Analysis Activity and The Systematic Treatment of Equilibria Activity and The Systematic Treatment of Equilibria Chapter 8:
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Chemical Activity Consider the solubility of AgCN … AgCN ( s ) Ag + ( aq ) + CN ( aq ) H 2 O K sp = 2.0 × 10 16 [Ag + ] = 1.4 × 10 8 M ( Note : AgCN dissolves in excess CN) Now, let’s add KNO 3 (very soluble, strong electrolyte) to bring its concentration to 0.10 M. ( No common ion.) [Ag + ] increases to 1.9 × 10 8 M, a 36% change! The concentrations of other ions in solution affect the solubility of AgCN (or any other salt)!
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Ionic Strength We need a way to quantitate the ions in solution, so we define the ionic strength , µ , … µ = 1 2 Σ i C Z 2 i i where c i = concentration of ion i z i = charge on ion i So, if KNO 3 is 0.10 M, K + NO 3 µ = 1/2 [(0.10)(+1) 2 + (0.10)( 1) 2 ] = 0.10 M For a 1:1 electrolyte, µ = concentration Consider 0.10 M Ca(NO 3 ) 2 µ = 1/2 [(0.10)(+2) 2 + (0.20)( 1) 2 ] = 0.30 M Electrolyte Molarity Ionic Strength 1:1 M M 2:1 M 3M 3:2 M 6M 2:2 M 4M
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So, the concentrations of other ions in solution affect the solubility of any salt • How does this work? Harris, pp. 141, 142 Everything here is fine, up to the last sentence. The last sentence implies a kinetic argument, not thermodynamic.
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How does the ionic strength effect work? (cont.) • The presence of ions in water changes the dielectric properties of the solvent in such a way that all of the ions are more stabilized (more strongly “solvated”) (at least up to µ = 0.1 or 0.2 M) relative to the constant crystal lattice energy so they become more soluble. • Thermodynamic formalism accommodates this “non-ideal” solution behavior by defining the equilibrium constant in terms of activities – not just concentrations. • Thus, for A + B C + D K = [C] γ C [D] γ D a C a D [A] γ A [B] γ B a A a B [A] [B] [C] [D] γ C γ D γ A γ B == Where … a i = activity of species i γ i = activity coefficient of species i
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Activity coefficients can be measured , or they can be estimated by calculation from a number of theoretically derived equations. Harris uses the extended Debye Hückel equation log γ = 0.51 z 2 µ 1 + ( α µ 305) (at 25 o C) Eq 8-6 Where z = charge on the ion µ = ionic strength α = diameter of the hydrated ion 0.51 and 305 = products of several fundamental variables and constants evaluated at 25 ˚C.
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Ionic and Hydrated Sizes of Ions Solid blue = ions Dispersed blue = hydration sphere
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Solubility of KO 2 C-CHOH-CHOH-CO 2 H •Increases when MgSO 4 or NaCl is added.
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Ch08-TCF - Quantitative Chemical Analysis Chapter 8...

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