acids and bases

• Acid


  Proton donor
• Base

  Proton acceptor
• Strong acids

  Fully dissociate in solution -> form H+

  (HCl, HNO3, H2SO4)
• Strong bases

  Fully dissociate in solution -> form OH-

  (NaOH, KOH, Ca(OH)2)
• Weak acids

  Do not fully dissociate in solution

  (H3PO4, HF, CH3COOH)
• Weak bases

  Do not fully dissociate/partially react with H2O -> OH-

  (NH3, CH3NH2)
• pH


  -log[H+]
• pH of a strong acid

  Fully dissociate so [acid] : [H+]

  -log[H+] = pH
• pH of a weak acid

  Ka = [H+][X-]/[HX]

  Ka = [H+]²/[HX]

  Rearrange to find [H+] then -log[H+]
• pH of a strong base


  Kw = [H+][OH-]

  Rearrange to find [H+] then -log[H+]
• Amphoteric

  Acts as an acid and a base
• What is Kw

  Ionic product of water
• How does Kw vary with temperature?

  H2O ⇌ OH- + H+

  Forward reaction is endothermic

  If temperature increases

  Equilibrium shifts to the right

  Favouring forward reaction i.e increasing value of Kw
• Why are pKa and pKw used?


  Nicer way of writing Ka and Kw (how pH is used for H+ concentration)
• pKa


  -log[Ka]
• What happens if Ka increases?

  pKa decreases and smaller pKa = stronger acid
• pH curves

  
• Strong acid - Strong base curve

  
• Strong acid - Weak base curve

  
• Weak acid - Strong base curve

  
• Weak acid - Weak base curve

  
• How to find Ka of weak acid from pH curve

  Weak acid - Strong base curve

  1) find volume needed to reach equivalence

  2) halve the volume

  3) find pH from the half volume

  4) pH = pKa so find Ka
• How to find [OH-] from pH curve

  Find the pH at the very end of the curve

  Find [H+]

  Rearrange Kw formula to find [OH-]
• Buffers


  Maintain a constant pH in a solution despite the addition of water, acid or an alkali
• How are acid buffers made?


  Weak acid + salt of the weak acid

  CH3COOH ⇌ CH3COO- + H+
• Adding water to an acidic solution containing an acid buffer

  Decreases [H+] by dilution

  Equilibrium shifts to oppose the change - towards the right therefore increasing [H+]
• Adding an alkali to an acidic solution containing an acid buffer

  Decreases [H+] by neutralisation

  Equilibrium shifts to oppose the change - towards the right therefore increasing [H+]
• Adding an acid to an acidic solution containing an acid buffer

  Increases [H+]

  Equilibrium shifts to the left to oppose the change and decreases [H+]

  Salt of the weak acid associates with the H+ to reform the weak acid
• Basic buffers

  Weak base + salt of the weak base

  NH3 + H2O ⇌ NH4+ + OH-
• Adding water to a basic solution containing a basic buffer

  Decreases [OH-]

  Equilibrium shifts to the right to oppose the change i.e increasing [OH-]
• Adding acid to a basic solution containing a basic buffer

  Decreases [OH-]

  Equilibrium shifts to the right to oppose the change i.e increasing [OH-]
• Adding alkali to a basic solution containing a basic buffer

  Increases [OH-]

  Equilibrium shifts to the left to oppose the change

  Salt associates with OH- to reform the equilibrium