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Constructing curves. Shapes of curves will depend on the strength of the acid and baseConstruct curve by calculating [H3O ] as the base (acid) is added.The strong-strong case is easy: all species are always completely ionized. At the equivalence point, pH = 7Weak acid - strong base will use pKa

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ph titration curves
pH titration curves
  • pH measured as an acid is neutralized by a base follows a characteristic curve that enables the equivalence-point to be determined with precision
constructing curves
Constructing curves
  • Shapes of curves will depend on the strength of the acid and base
  • Construct curve by calculating [H3O+] as the base (acid) is added.
  • The strong-strong case is easy: all species are always completely ionized. At the equivalence point, pH = 7
  • Weak acid - strong base will use pKa for the acid
titration of weak acid with strong base in four zones
Titration of weak acid with strong base in four zones
  • Zone 1: Initial pH of acid with no base added
  • Zone 2: Addition of OH- up to equivalence point
  • Zone 3: At equivalence point: all the HAc is converted into Ac-
  • Zone 4: After equivalence point
constructing a ph curve for the weak acid strong base case
Constructing a pH curve for the weak acid - strong base case
  • Zone 1: Initial pH of acid with no base added
  • Zone 2: Addition of OH- up to equivalence point
    • Treat like a buffer solution
    • NOTE: Concentrations change as volume increases with addition of base!
explicit contains adult themes calculation for zone 2
Explicit (contains adult themes) calculation for zone 2
  • Determination of [H+] up to equivalence point
  • [HAc]o is initial concentration of HAc and VH is the initial volume of HAc used
  • [OH-] is initial concentration of the base and VOH is the volume added at that point in the titration
weak acid strong base continued
Weak acid – strong base continued
  • Zone 3: At equivalence point: all the HAc is converted into Ac-
    • Treat like solution of basic salt
    • Where [Ac-] is the initial acetate concentration at the neutralization point prior to dissociation
weak acid strong base continued7
Weak acid – strong base continued
  • Zone 4: After equivalence point
    • All OH- results from the excess base
ph curves assist in identifying suitable indicator
pH curves assist in identifying suitable indicator
  • The equivalence point with the weak acid is at pH>7
  • Above pH 7, both curves coincide (strong base controls pH)
  • The initial rise in pH is greater with the weak acid (but at much lower [H3O+]
end point detection gets harder with weaker acids
End-point detection gets harder with weaker acids
  • Initial pH is higher
  • Initial change in pH is greater
  • Change in pH at equivalence point is lower
  • Harder to detect equivalence point in weak acid
weak base strong acid
Weak base - strong acid
  • Analogous to the weak acid – strong base case
  • pH at equivalence point < 7
  • Dictates use of different indicator
  • pH after equivalence point controlled by strong acid
polyprotic acids lots to note
Polyprotic acids – lots to note
  • An amino acid has two dissociations:

H2A+ + H2O = HA + H3O+

HA + H2O = A- + H3O+

example for alanine
Example for alanine
  • Initially, pH determined by pKa1
  • Halfway to 1st equivalence pt pH = pKa1
  • 1st equivalence pt,
  • Halfway to 2nd equivalence pt, pH = pKa2
  • 2nd equivalence pt, pH determined by pKb for the base A- (Kb obtained from Ka2)
  • Beyond 2nd equivalence pt, pH determined from OH- from NaOH
solubility products equilibrium between solute and solid
Solubility products – equilibrium between solute and solid
  • An electrolyte completely dissociated in equilibrium with undissolved solid
  • The solid phase is ignored
  • Writing Ksp expressions for salts
calculations
Calculations
  • Determine Ksp from solution concentrations (concentrations of individual ions may not be equal to those you would get by simple dissociation of compound)
  • Determine solubility from Ksp

MgF2 = Mg2+ + 2F-

Ksp = x.(2x)2 x =[Mg2+]

factors affecting solubility
Factors affecting solubility
  • Common ion effect – the addition of an ion from another source
  • Solution pH – where there is a weak acid or base
  • Complex ion formation
  • Amphotericity
common ion effect
Common ion effect
  • In a solution of a salt AB, addition of a additional B ions from another source will cause [A] to decrease because of Ksp
calculation of solubility under these conditions
Calculation of solubility under these conditions
  • Calculate solubility of MgF2 in a solution of 0.1 M NaF(aq)?
    • At equilibrium, [Mg2+] = x, [F-] = 0.1 + 2x
solution ph
Solution pH
  • Basic anions are protonated in acid
    • CaCO3 is insoluble in pure water
    • In acid solution, H+ converts CO32- to HCO3-
    • More Ca2+ is drawn into solution (Le Chatelier)
  • Salts of basic anions increase solubility in acid conditions
  • pH does not affect anions that are not basic
complex ion formation affects solubility
Complex ion formation affects solubility
  • AgCl is ordinarily highly insoluble (test for chloride ions)
  • Addition of aqueous ammonia redissolves the precipitate by formation of the complex ion Ag(NH3)2+
amphoteric substances
Amphoteric substances
  • Dissolve in both acid and basic solutions
  • Examples of complex ion formation
  • Oxides and hydroxide of Al are examples
prediction of precipitation and ion products
Prediction of precipitation and ion products
  • Predict the formation of a precipitate when solutions are mixed
  • Ion product is not an equilibrium quantity

IP = [Ca2+][F-]2

  • If IP is found to be larger than Ksp then precipitation occurs
selective precipitation
Selective precipitation
  • Mixtures of ions can be separated by combining with an anion that has wide range of solubiity.
    • For example, sulphides of Zn, Pb, Cu and Hg will precipitate leaving the sulphides of Mn, Fe, Co and Ni in solution
    • Ksp for former group are much larger than for latter group
qualitative analysis
Qualitative analysis
  • Apply a sequence of precipitation steps to divide a group of many metal ions into smaller groups. These smaller groups will be further analyzed to identify the members therein
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