Chapter 11 Electrolyte Effects: Activity or Concentration?

9A Chapter 11 Electrolyte Effects: Activity or Concentration? Fig 9-1 why Ka, Kw, Ksp↑while adding [NaCl]↑ Effect of electrolyte concentration on concentration-based equilibrium constants

9A-1 [explain] dissociation ↑by↑ionic strength of soln.

9A-2 The effect of added electrolyte p.206 ionic strength = μ= ½ΣCιZι2 [I] ionic charges ex. 9-1, 9-2

Effect of charge on μ

[C]c[D]d [A]a[B]b • 9B Activity Coefficients • aA + bB → cC + dD • Without considering μ, k = • (2) The effect of μ; the [x] activities (ax) ax = γx[x] conc. of x activity of x activity coefficient of x ∴ k = =

ex. XmYn(s) mXn+ + nYm- Ksp = aXmaYn = γXmγYn[X]m[Y]n = γXmγYnK'sp thermodynamic equilibrium const. concentration solubility product const.

9B-1 properties of γx at p 208. • at μ→ 0, γx → 1, ax → [x] & Ksp → K'sp • at high μ(μ> 0.1 M), γ↑& could > 1 • (2) γxdepend on the of soln • not the nature of the X (electrolyte) • (3) at μ= const. For X with larger charge, the bigger △γx • ex. △γBaSO4 > △γAgCl at μ = k

(4) at any μ, for same charge ions: γX ≈ γY their difference (minor) could be from at any effective diameter of hydrated ion was formed (5) HCN + H2O  H3+ + CN- Ag+ + CN- AgCN  Ni2+ + 4CN-  Ni(CN)42- aCN= aCN = aCN

9B-2 The Debye – Hückel eqn. • Fine γX of the ions from Z (ionic charge) & αX (average size) -log γX = αX = effective diameter of the hydrated ion X (10-9m)

 for most single charged ions : αX ≈ 0.3 nm -log γX ≈  the larger charged ions ; the larger αX (table 9.1)  For μ< 0.01  3.3 αX << 1  1 + 3.3 αX ≈ 1 ∴ -log γX = 0.51 ZX2 see ex. 9-3

Ex: • What weight of Na2HPO4 and KH2PO4 would be required to prepare 200 mL of a buffer solution of pH 7.40 that has an ionic strength of 0.20?

Sol: • pH = pKa2 + log [HPO4-2]/[ H2PO4-] 7.40 = 7.20 + log X/Y (2) µ = 1/2ΣCiZi2 0.20 = 1/2 {[Na+](1)2 + [X](2)2 + [K+](1)2 + [Y](1)2} = 3X + Y • 解聯立方程式(1) and (2)

Chapter 11 Electrolyte Effects: Activity or Concentration?