Acid/ Base equilibria

• Bronsted Lowry definition


  Transfer of protons (proton is a H+)
• Bronsted Lowry (BL) Acid


  Proton donor
• Bronsted Lowry (BL) Base


  Proton acceptor
• Bronsted Lowry definition example


  • Water and HCl
  • Water is BL base, HCl is BL Acid
• Lewis definition


  Transfer of electron pairs
• Lewis acid


  Electron pair acceptor
• Lewis base


  Electron pair donor
• Lewis definition example


  • Water and BF3
  • Water is a Lewis Base, BF3 is a Lewis acid
• BF3 as a Lewis acid

  • Boron does not have octet (only 6 electrons), it is sp2 hybridized with an empty orbital that can accept an electron pair
• The H2O HCl reaction can only be explained by the Lewis definition, not the Bronsted Lowry definition

  
• H2O as a Lewis base

  Electron pair donor
• H+ on HCl as a Lewis acid

  Electron pair acceptor

  
• Acid-base equilibrium


  H2O + HA ⇌ H30+ + A-

  
• Water


  Bronsted Lowry base
• HA


  Bronsted Lowry acid
• Hydronium


  Conjugate acid
• 1. 
     

  Conjugate base
• Ka


  Acid dissociation/ionization constant

  
• Pure solids and liquids are not included in the equilibrium constant expression
• Strong acids


  Donate protons very easily, making the equilibrium shift to the right towards products
• Conjugate base of a strong acid

  Rather poor at accepting protons
• Large Ka is a good way to recognize a strong acid
• Weak acids


  More likely to 'stay protonated', are less likely to donate their proton, equilibrium is to the left (towards the reactants)
• Small Ka is a good way to recognize a weak acid
• Water


  Amphotetic, can act as both an acid or as a base
• Autoionization of water


  One water molecule acts as a base, the other as an acid, producing H3O+ and OH-

  
• At 25°C, Kw = 1.0 x 10-14
• Equilibrium in autoionization of water lies far to the left
• Neutral solution


  [H3O+] = [OH-]
• Acidic solution


  [H3O+] > [OH-]

  
• Basic solution


  [H3O+] < [OH-]
• pH of water at 25°C is 7.00
• pH scale


  pH = 7 is neutral, pH < 7 is acidic, pH > 7 is basic
• Calculating hydronium ion concentration from pH

  10-pH = [H3O+]

  
• pOH of water at 25°C is 7.00
• pH + pOH = 14
• Calculating pH of aqueous ammonia solution

  [H3O+][OH-] = 1.0 x 10-14

  pH = 14 - pOH

  
• If first number is not 1.0 in [H3O+], subtract 0.5 from the exponent to estimate pH
• Strong acids


  Ionize 100% in solution
• Calculating pH of 0.0030M HNO3 solution

  pH = -log[H3O+] = -log(0.030M) = 1.52