Tài liệu Chapter 8A: Activities doc

Chapter 8A: Activities I. The Effect of Electrolytes on Chemical Equilibrium ¾ The concentrations and charge strength of "spectator“ or non-reacting electrolytes ¾ The tendency to form precipitates will be decreased with increasing ionic strength 1) interactions (but not necessarily reactions) of the reactant species with other charged species 2) interactions of the charged product species with the spectator ions. Examples: ¾ AgCl (s) <==> Ag + + Cl - K sp increases with increasing ionic strength, so AgCl becomes more soluble. ¾ H 2 O + H 2 CO 3 <===> H 3 O + + HCO 3 - K a1 increases with increasing ionic strength, so the solution becomes more acidic. Figure 1 Figure 2 Effect of Ionic Strength ¾ Ionic strength is a measure of the concentration and charge of the ions in solution. It is defined by the equation: ¾ where A, B, C, … are charged species in the solution ¾ [A], [B], [C], … represent the species molar concentrations of ions A, B. C ¾ Z A, Z B, Z c, … are their respective charges. () )C]Z[B]Z[A]Z([ 2 1 μStrength Ionic 2 c 2 B 2 A +++= KNO 3 K + + NO 3 - 1 mole 1 mole 1 mole 0.1 M 0.1 M 0.1 M 2 H 2 O <==> H 3 O + + OH - 10 -7 M 10 -7 M () )C]Z[B]Z[A]Z([ 2 1 μStrength Ionic 2 c 2 B 2 A +++= M Na 2 SO 4 2 Na + + SO 4 2- 1 mole 2 mole 1 mole 0.1 M 0.2 M 0.1 M () )C]Z[B]Z[A]Z([ 2 1 μStrength Ionic 2 c 2 B 2 A +++= M M KNO 3 K + + NO 3 - 1 mole 1 mole 1 mole 0.05 M 0.05 M 0.05 M () 35.0)21.012.0105.0105.0( 2 1 ]ZSO[]ZNa[]ZNO[]ZK([ 2 1 )C]Z[B]Z[A]Z([ 2 1 μStrength Ionic 2222 2 SO -2 4 2 Na 2 NO - 3 2 K 2 c 2 B 2 A -2 4 - 3 =+++= +++= +++= ++ ++ xxxx Na 2 SO 4 2 Na + + SO 4 2- 1 mole 2 mole 1 mole 0.1 M 0.2 M 0.1 M M Effect of Ionic Strength ¾Dependent on 1) concentrations 2) charges ¾Independent on The chemical properties of the ions Therefore, the effect of ionic strength on a reaction (e.g., solubility or dissociation constant) is not dependent on the types of spectator ions, but on the concentration and charges of spectator ions. [...]... by neglecting activities in calculating the solubility of Ba(IO3)2 in a 0.033 M solution of Mg(IO3)2 The thermodynamic solubility product for Ba(IO3)2 is 1.57x10-9 Ba(IO3)2 Ba2+ + 2IO32+ solubility = [Ba ] [IO3 ] = 0.033 x 2M 1) Consider activities K sp = a Ba 2+ a 2 IO3 2+ - 2 3 = [Ba ]γ Ba 2+ [IO ] γ ' K sp = [Ba 2+ ][IO3 ]2 2+ solubility = [Ba ] = ' K sp - 2 3 [IO ] 2) Neglecting activities K sp... effective diameter of the hydrated ion X in picometers (10-12 m) Table 8-1 pm 900 600 400-450 350 300 250 800 600 500 450 400 900 400 1100 500 Hydronium ion concentrations in solutions of weak acids (See Chapter 7) HA + H2O H3O+ + A2 H2O H3O+ + OH- Ka Kw since Ka usually >> Kw, we normally can neglect the water hydronium ion contribution (but not always, particularly if the cHA is very low)... a+b pm 900 600 400-450 350 300 250 800 600 500 450 400 900 400 1100 500 γ+ = γMg2+= 0.755 γ- = γCl- = 0.925 Properties of Activity Coefficients A Rule I: (Figure 8-1) • As ionic strength increases, its activities coefficient (γ) decreases • At moderate ionic strengths, γ < 1 • As the solution approaches infinite dilution, γ ~ 1 and hence a ~ [X] and K ~ K’ • At high ionic strengths (μ > 0.1 M), the activity... Neglecting activities K sp = [Ba 2+ ][IO3 ]2 2 IO3 K sp ' sp = K γ Ba 2+ γ = 2 IO3 K sp [IO3 ]2 γ Ba 2+ γ 2 IO 3 1.57 x10 −9 solubility = [Ba ] = = = 3.60 x10 −7 M [IO3 ]2 (0.033 x 2) 2 2+ Ba(IO3)2 1) Consider activities 2+ solubility = [Ba ] = Ba2+ + 2IO3' K sp - 2 3 [IO ] = K sp [IO3 ]2 γ Ba 2+ γ 2 IO 3 1 2 Ionic Strength (μ ) = ([ A]Z 2 + [ B]Z 2 + [ C]Z c + ) A B 2 1 1 2+ 2 2 = ([ Mg ]Z Mg 2 + + [ IO 3 ]Z . Chapter 8A: Activities I. The Effect of Electrolytes on Chemical Equilibrium ¾ The. Activity Coefficients A. Rule I: (Figure 8-1) • As ionic strength increases, its activities coefficient (γ) decreases. • At moderate ionic strengths, γ <