ANCH WEEK 8: Effect of Electrolytes on Chemical Equilibria

• Equilibrium Constants


  for chemical reactions should be written in terms of the activities of the participating species
• Activity Coefficient


  activity of species is related to its concentrations by a factor
• Ionic Equilibria 


  activities and concentrations can be substantially different. Also affected by the concentrations of electrolytes in solution that may not participate directly in the reaction
• Usually an activity of a reactant = concentration, if not equal called IONIC EQUILIBRIA
• Most solution equilibria depends on the electrolyte concentration of the medium, even if added electrolyte contains no ion
• Electrolyte, such as barium nitrate, potassium sulfate , sodium perchlorate added to the solution, the color of triiodide ion becomes less intense

  Decrease in color intensity indicates decreased concentration and the equilibrium has been shifted to the left by the added electrolyte

  Pink - less intense color

  Red or dark red to brownish - more intense color
• The effect of electrolyte charges on equilibrium


  Extensive studies reveal that the magnitude of the electrolyte effect is highly dependent on the charges of the participants in an equilibrium
• Only neutral species


  involved, position of equilibrium is essentially independent of electrolyte concentration
• Ionic Participants


  magnitude of the electrolyte effect increases with charge
• The effect of ionic strength


  systematic studies have shown that the effect of added electrolyte on equilibria is independent of the chemical nature of electrolyte but depends on a property of the solution called IONIC STRENGTH
• Ionic Strength


  A, B, C = Molar Concentration

  Z2A, Z2B, Z2C = Charges

  Unit used: M

  
• Salt Effect


  Also known as Electrolyte Effect

  results from electrostatic attractive and repulsive forces between the ions of an electrolyte and the ions involved in an equilibrium

  these forces cause each ion from the dissociated reactant from surrounded by a sheath of solution that contains a slight excess of electrolytes ions of opposite charge
• Salt Effect


  "opposites attract"

  Ba - Barium, positively charged

  • attracts chloride (negatively charged) and repels sodium (positively charged)
  • creates slightly negative ionic atmosphere

  Na - Sodium, negatively charged

  • attracts barium (positively charged) and repels chloride (negatively charged)
  • creates slightly positive ionic atmosphere
• Salt Effect


  • Result of this shielding effect is a decrease in overall attraction between barium and sulfate ions
  • Solubility becomes greater as the number of electrolyte ions in the solutions becomes larger
  • effective concentration becomes less as the ionic strength of the medium becomes greater
• Activity Coefficient


  Chemists use the term called, activity , to account for the effects of electrolytes on chemical equilibria. Depends on the ionic strength of the medium

  ax - activity of species, x

  [X] - Molar Concentration, x

  Yx - Dimensionless quantity, called Activity Coefficient, x

  
• Very concentrated solution


  high ionic strength but decreased activity coefficient and loses effectiveness
• Very diluted solutions


  minimal ionic strength with constant activity coefficient and high effectiveness
• Debye-Huckel Equation


  1923, P. Debye and E. Huckel, used the ionic atmosphere model, described Section 10A-3 to derive an equation that permits the calculation of the activity coefficients of ions from their charge and their average size

  Constant, 5.1 and 3.3 are applicable to aqueous solutions at 25 degree Celcius

  Yx - activity coefficient

  Zx - charge

  u - ionic strength

  ax - effective diameter, nanometer (10^-9 m)

  
• Properties of activity coefficient



  1. The activity coefficient of a species measure of the effectiveness
  2. If not too concentrated, activity coefficient is independent of the nature of the electrolyte and dependent only on the ionic strength
  3. Activity coefficient of an ion decreases more dramatically from unity as the charge on the species increases
• Properties of activity coefficient



  4. Activity coefficient of an uncharged molecule is approximately unity, no matter what the level of ionic strength

  5. Activity coefficient of ions of the same charge are approximately equal

  6. The activity of a coefficient of a given ion describes effective behavior in all equilibria in which it participates