221-chapter-8

221-chapter-8 - Activity Introduction 1.) Hydration Ions do...

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Activity Introduction 1.) Hydration Ions do not act as independent particles in solvent (water) Surrounded by a shell of solvent molecules Oxygen has a partial negative charge and hydrogen partial positive charge Oxygen binds cations Hydrogen binds anions
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Activity Introduction
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Activity Introduction 3.) Size of Hydration Size and charge of ion determines number of bound waters Smaller, more highly charged ions bind more water molecules Activity – is related to the size of the hydrated species Small Ions bind more water and behave as larger species in solution
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Effect of Ionic Strength on Solubility 1.) Ionic Atmosphere Similar in concept to hydration sphere Cation surrounded by anions and anions are surrounded by cations - Effective charge is decreased - Shields the ions and decreases attraction Net charge of ionic atmosphere is less than ion - ions constantly moving in/out of ionic atmosphere Activity Each ion see less of the other ions charge and decreases the attraction Each ion-plus-atmosphere contains less net charge and there is less attraction between any particular cation and anion
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Activity Effect of Ionic Strength on Solubility 2.) Ionic Strength ( μ ) Addition of salt to solution increases ionic strength - Added salt is inert does not interact or react with other ions In general, increasing ionic strength increases salt solubility - Opposite of common ion effect The greater the ionic strength of a solution, the higher the charge in the ionic atmospheres More ions added, more ions can be present in ionic atmospheres
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Activity Effect of Ionic Strength on Solubility 2.) Ionic Strength ( μ ) Measure of the total concentration of ions in solution - More highly charged an ion is the more it is counted - Sum extends over all ions in solution ( 29 = + + = i 2 i i 2 2 2 2 1 1 z c 2 1 z c z c 2 1 μ
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Activity Effect of Ionic Strength on Solubility 2.) Ionic Strength ( μ ) Example: What is the ionic strength of a 0.0087 M KOH and 0.0002 M La(IO 3 ) 3 solution? Assume complete dissociation and no formation of LaOH 2+
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Activity Effect of Ionic Strength on Solubility 3.) Equilibria Involving Ionic Compounds are Affected by the Presence of All Ionic Compounds in the Solution Knowing the ionic strength is important in determining solubility Example: K sp = 1.3x10 -18 If Hg 2 (IO 3 ) 2 is placed in pure water, up to 6.9x10 -7 M will dissolve. If 0.050 M KNO 3 is added, up to 1.0x10 -6 M Hg 2 (IO 3 ) 2 will dissolve. Occurs Due to Changes in the Ionic Strength & Activity Coefficients
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Activity Equilibrium Constant and Activity 1.) Typical Form of Equilibrium Constant However, this is not strictly correct Ratio of concentrations is not constant under all conditions Does not account for ionic strength differences 2.) Activities, instead of concentrations should be used Yields an equation for K that is truly constant b a d c [B] [A] [D] [C] K = b B a A d D c C A A A A K = where: A A , A B , A C , A D is activities of A through D
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Activity Equilibrium Constant and Activity 3.)
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221-chapter-8 - Activity Introduction 1.) Hydration Ions do...

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