1120 acid base

• according to the arrhenius theory, an acid dissociates in water to give H+
• according to the arrhenius theory, a base dissociates in water to give OH-
• limitations of arrhenius theory

  • solvent can only be water
  • cannot explain acid-base behaviour in non aqueous solution
  • cannot why all salts are neutral
• according to the bronsted lowry theory, an acid is a proton donor
• according to the bronsted lowry theory, a base is a proton acceptor
• a bronsted base must have lone pair electrons
• according to the lewis theory, an acid is an electron acceptor
• according to the lewis theory, a base is an electron donor
• if water is pure and at 25 ºC ,

  [H3O+ ] = [OH− ] = 1 x 10^-7 M

  Kw = [H3O+ ][OH− ] = 1 x 10^-14 M
• pH = -log[H+]
• pOH = -log[OH− ]
• at 25ºC, pKw = pH + pOH = 14
• Ka is the acid ionisation constant
• Kb is the base ionisation constant
• strong acid/base has a large ionising constant (much larger than 1)
• weak acid/base has a small ionising constant (much smaller than 1)
• Ka= [H+][A-]/[HA]
• Kb= [BH+][OH-]/[B]
• the stronger the acid, the weaker its conjugate base
• the stronger the acid, the larger its Ka = the smaller its pKa
• the stronger the base, the larger its Kb = the smaller its pKb
• The percent ionization of a weak acid/base increases when the solution becomes more dilute.
• % ionisation = [H+] / Initial [HA] and then times 100%
• percent ionization depends on:

  1. Initial molarity
  2. ionization constant
• K <<< 1 = backward favoured
• K >>> 1 = forward favoured
• K = 1 = equilibrium
• Why does diluting the solution favour ionisation?

  Concentration is given by mole/volume.

  If volume increases, it means that concentration will decrease.

  Concerning Ka = [H+][A-]/[HA] , the number of moles of ions in this equation must increase to maintain the constant Ka. (constant = unchanged)

  Therefore this causes the equilibrium to shift to the right / product side, favouring ionisation
• contribution of self-ionization of water to pH can be ignored unless the solution is extremely dilute
• If the [H+] from the compound is greater than 1 x 10^-5 M, ignore self ionization of water
• Polyprotic acids are acids with more than one ionizable proton (can donate multiple H+).
• Each ionisation of H+ of a polyprotic acid has its own Ka value
• H3PO4 is a triprotic acid
• Ka3 < Ka2 < Ka1

  because it is more difficult to remove a proton from a more negatively charged species / weaker acid
• Phosphoric Acid H3PO4 is a weak acid
• Ka (acid) x Kb (its conjugate base) = Kw
• Kb (base) x Ka (its conjugate acid) = Kw (constant)
• In a conjugate acid base pair, larger Ka means a smaller Kb
• In a conjugate acid base pair, larger Kb means a smaller Ka
• When salt is dissolved in water, any pH change is caused by hydrolysis reaction