Ch 15

• Common Ion Effect

  Shift in equilibrium position that occurs due to the addition of an ion already involved in the equilibrium reaction

  
  

  In solutions containing polyprotic acids:

  
  

  •Production of protons by the first dissociation significantly inhibits the succeeding dissociation
• Buffered Solutions

  •Solutions that resist changes in pH when either hydroxide ions or protons are added

  •Example –Human or animal blood

  •Absorbs acids and bases produced in biologic reactions without alterations to pH
• Solving Buffer Problems

  •Buffered solutions are solutions of weak acids or bases containing a common ion

  •When a strong acid or base is added to a buffered solution, it is best to deal first with the stoichiometry of the neutralization reaction
• Buffering Capacity

  •Amount of protons or hydroxide ions that a buffer can absorb without a significant change in pH

  •The greater the concentrations of buffering components in the buffer, the larger the buffering capacity. 

  •Buffer absorbs relatively large amounts of protons or OH–ions with minor changes in pH

  •Determined by the magnitudes of [HA] and [A–]
• Half e.q point = pKa
• Strong acids

  pH = 7.00 at eq point
• Methods Used to Determine the Equivalence Point

  •pH meter

  •Used to monitor the pH and then plot the titration curve

  •Acid–base indicator: Marks the end point of a titration by changing colour

  •Equivalence point is not necessarily the same as the end point

  •Careful selection of the indicator ensures that the margin of error is minimal
• Phenolphthalein

  •A commonly used indicator

  •Colorless in HIn form and pink in In–form

  •HIn -Weak acid form

  •In–-Basic form