Activity and Activity Coefficients: Chemical Equilibrium and Electrolyte Effects

Activity and Activity Coefficients: Chemical Equilibrium and Electrolyte Effects
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Electrolytes are substances that produce ions in solutions and can have an impact on chemical equilibria. One such effect is the Common Ion Effect,

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About Activity and Activity Coefficients: Chemical Equilibrium and Electrolyte Effects

PowerPoint presentation about 'Activity and Activity Coefficients: Chemical Equilibrium and Electrolyte Effects'. This presentation describes the topic on Electrolytes are substances that produce ions in solutions and can have an impact on chemical equilibria. One such effect is the Common Ion Effect,. The key topics included in this slideshow are . Download this presentation absolutely free.

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Slide1Activity and ActivityCoefficients

Slide2Chemical Equilibrium Electrolyte EffectsElectrolytes: Substances producing ions in solutions Can electrolytes affect chemical equilibria? (A) “ Common Ion Effect ”    Yes Decreases  solubility of BaF 2  with NaF F -  is the “common ion”

Slide3fig. 10.3.  predicted effect of excess barium ion on solubility of BaSO4 . The common ion effect is used to decrease the solubility. Sulfate concentration is the amount in equilibrium and is equal to the BaSO 4  solubility. In absence of excess barium ion, solubility is 10 -5  M. The common ion effect is used to decrease the solubility. Sulfate concentration is the amount in equilibrium and is equal to the BaSO 4  solubility. In absence of excess barium ion, solubility is 10 -5  M. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

Slide4(B) No common ion: “ inert electrolyte effect ”or “ diverse ion effect ” Add Na 2 SO 4  to saturated solution of AgCl Increases  solubility of AgCl   Why??? shielding of dissociated ion species

Slide5predicted effect of increased ionic strength on solubility ofBaSO 4 .  Solubility at zero ionic strength is 1.0 x 10 -5  M. K sp  = K sp 0 /f Ag + f SO42- Solubility increases with increasing ionic strength as activity coefficients decrease. K sp  = K sp 0 /f Ag + f SO42- Solubility increases with increasing ionic strength as activity coefficients decrease. ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

Slide6Activity and ActivityCoefficients Activity of an ion, a i  = C i ƒ i C i  = concentration of the ion ƒ i   =  activity coefficient    ( @ C i  < 10 -4 M ) =  1 Ionic Strength,    =  ½  C i Z i 2 Z i  = charge on each individual ion.

Slide7Activity and ActivityCoefficients Calculation of Activity Coefficients Debye-Huckel Equation: -log  ƒ i  = 0.51 Z i 2  ½     i   ½   i  = ion size parameter in angstrom ( Å ) 1  Å  = 100 picometers (pm, 10 -10  meters) Limitations:  singly charged ions = 3  Å -log  ƒ i  = 0.51 Z i 2  ½        ½ 

Slide8Chemical Equilibria ElectrolyteEffects Diverse ion (Inert) electrolyte effect For   < 0.1 M, electrolyte effect depends on   only, NOT on the type of electrolyte Solute activities: a x  = activity of solute X a x  = [X]  x    x   = activity coefficient for X As        x    1 ,  a x     [X]

Slide9Chemical Equilibria ElectrolyteEffects • Diverse Ion (Inert Electrolyte) Effect: Add Na 2 SO 4  to saturated solution of AgCl K sp  =  a Ag +  .  a Cl -  =  1.75 x 10 -10 At high concentration of diverse (inert) electrolyte: higher ionic strength,   a Ag +   < [Ag + ] ;  a Cl -   < [Cl - ] a Ag +  .  a Cl -  < [Ag + ] [Cl - ] K sp   <  [Ag + ] [Cl - ] ;    K sp   <  [Ag + ] = solubility Solubility = [Ag + ] >   K sp

Slide10Diverse Ion Effect onSolubility: Presence of diverse ions will increase the solubility of precipitates due to shielding of dissociated ion species. K SP  and Activity Coefficients AgCl (s)  (AgCl) (aq)   Ag +  + Cl - Thermodynamic solubility product K SP K SP   = a Ag+  .  a Cl-  = [Ag + ] ƒ Ag+ .  [Cl - ]ƒ Cl- Ḱ  SP  = [Ag + ] .   [Cl - ] K SP   = Ḱ  SP    ƒ Ag+ .  ƒ Cl- Ḱ  SP  = K SP /( ƒ Ag+ .  ƒ Cl )

Slide11Chemical Equilibria Electrolyte Effects“Diverse ion (Inert) electrolyte effect” Is dependent on parameter called “ionic strength (  ”     = (1/2) {[A]Z A 2  + [B]Z B 2  +  …  +  [Y]Z y 2 }  0.1 M Na 2 SO 4  ; [Na + ] = 0.2M [SO 4 ] = 0.1M     = (1/2) {[A]Z A 2  + [B]Z B 2 }     = (1/2) {[0.2](1+) 2  + [0.1](2-) 2 }   =  0.3M

Slide12Chemical Equilibria ElectrolyteEffects Solute activities: When   is not   zero ,   a x   = [X]  x Equilibrium effects : mM + xX    zZ K  =( a z ) z /(a m ) m (a x ) x K  =( [Z]  Z   ) z /( [M]  M   ) m ( [X]  x   ) x K  ={( [Z] ) z /( [M] ) m ( [X] ) x  }{  Z   z /    M   m   x   x } K   = K  {  Z   z /    M   m   x   x } Ḱ = K    {  M   m   x   x  /   Z   z }

Slide13The Diverse Ion EffectThe Thermodynamic Equilibrium Constant and Activity Coefficients thermodynamic equilibrium constant, K case extrapolated to infinite dilution At infinite dilution, activity coefficient,  ƒ   =  1 Dissociation  AB    A +  + B - K = a A  a B /a AB  = [A + ]  ƒ A   .  [ B - ]  ƒ B  /  [AB]  ƒ AB K   = K ( ƒ A   .  ƒ B  /  ƒ AB ) Ḱ = K ( ƒ AB   /  ƒ A   .  ƒ B  )

Slide15Chemical Equilibria ElectrolyteEffects • Calculation of Activity Coefficients • -log  ƒ x   = 0.51 Z i 2  ½     i   ½  • Where   x  = effective diameter of hydrated ion, X (in angstrom units, 10 -8 cm),  Å •   Ion H 3 O + Li + F - Ca 2+ Al 3+ Sn 4+  x , ,  Å 9 6 3.5 6 9 11 ƒ x  @ 0.05 M 0.86 0.84 0.81 0.48 0.24 0.10

Slide16Chemical Equilibrium ElectrolyteEffects • Equilibrium calculations using activities : Solubility of PbI 2  in 0.1M KNO 3            2   (ignore Pb 2+ ,I - ) ƒ Pb  = 0.35   ƒ I  = 0.76 K sp   =  (a Pb ) 1 (a I ) 2  = ( [Pb 2+ ]  Pb  ) 1 ( [I - ]  I  ) 2 K sp  =  ( [Pb 2+ ] [I - ] 2 )(  Pb   I 2   ) =  Ḱ  sp  (  Pb   I 2   ) Ḱ  sp   = K sp  / (  Pb   I  ) K sp  = 7.1 x 10 -9  /(( 0.35)(0.76) 2 ) = 3.5 x 10 -8 (s)(2s) 2  =  K sp    s = (Ksp/4) 1/3 s =2.1 x 10 -3 M Note: If s = (K sp o /4) 1/3   then s =1.2 x 10 -3 M Solubility calculation difference approx. –43%