Acidity
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  1. Bronsted-Lowry, HA = H+ + A- (conjugate base)

  2. Ka = [H+] [A-] / [HA]

    1. For Ka < 1, compound mostly associated

    2. For Ka > 1, compound mostly disassociated into proton (hydronium in water) and conjugate base

  3. For H+ + A- = HA, K = [HA] / ([H+] [A-]) = 1/Ka

  4. pKa = -log (Ka), Ka = 10-pKa (anti log of pKa)

    1. lower pKa, more acidic compound, less basic conjugate base

    2. higher pKa, less acidic compound, more basic conjugate base

    3. pKa of water = -log([H+] [-OH] / 56) = -log((10-7) (10-7) / 56) = 7 + 7 + log(56) = 15.7

  5. pH = -log[H+]

  6. Ka = 10-pKa = (x) (x) / (I-x), compound of initial concentration I disassociates into [H+] = [A-] = x

    1. Ka (I-x) = x2

    2. x2 +Ka x -I Ka = 0

    3. [H+] = x = (-Ka + (Ka2+4 I Ka)) / 2, negative answer does not make sense

    4. pH = -log(x)

  7. For HA + B- = A- + HB, K = Kf Kb

    1. Where HA = H+ + A-, Kf = Kaf

    2. Where H+ + B- = HB, Kb = 1/Kab

    3. K = (10-pKaf) (10--pKab) = 10(-pKaf + pKab)

    4. If -pKaf + pKab > 0, pKaf < pKab, equilibrium favored towards the right, products

    5. If -pKaf + pKab < 0, pKaf > pKab, equilibrium favored towards the left, reactants

    6. A mixture of HA and B- with initial concentrations I1 and I2, respectively, has a final concentration of A- and HB of:

      1. K =  10(-pKaf + pKab) = (x) (x) / ((I1-x) ((I2-x))

      2. K (I1-x) ((I2-x) = x2

      3. K (I1I2 - (I1+I2) x + x2) = x2

      4. (K-1) x2 -K (I1+I2) x + K I1 I2 = 0

      5. x = (K (I1+I2) ((K (I1+I2))2-4 (K-1) K I 1 I2 )/(2 (K - 1)), check that the solution is positive.

      6. The percentage of HA that is disassociated is x / I1 100 %

      7. The final ratio of [A-]/[HA] = x / (I1-x)

  8. Lewis Acids and Bases

  9. Nucleophiles and Electrophiles

  10. Reasons why some compounds are more acidic than others

    1. Electronegativity, examples: CH4, NH3, H2O, HF

    2. Hybridization: sp3, sp2, sp

    3. Delocalization, Resonance effect, examples: carboxylic acids and phenols

    4. Inductive effect, examples: halo substituted carboxylic acids

    5. Sterics, examples: amines

    6. Hard (ionic) and Soft (covalent)

 

pKa values

CH4
49
NH3
36
H2O
15.7
HF
3.2
  PH3
26
H2S
7
HCl
-7
    H2Se HBr
-9
      HI
-10

 

 

 

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Thursday, April 06, 2017 13:18