Acids & bases

• Lowry-Brønsted theory


  Acid-base equilibria involve the transfer of protons between substances. Substances can be classified as acids or bases depending on their interaction with protons.
• Lowry-Brønsted acid


  Proton donor
• Lowry-Brønsted base


  Proton acceptor
• Strong acid


  Acid that (almost) completely dissociates into its ions
• Strong acids


  • HCl, H2SO4, HNO3
• Weak acid


  Acid that partially dissociates when in solution
• Weak acids


  • Organic carboxylic acids like ethanoic acid
• Strong base


  Base that (almost) completely dissociates into its ions
• Strong bases


  • NaOH, KOH, Ca(OH)2
• Weak base


  Base that partially dissociates when in solution
• Weak bases


  • NH3, CH3NH2
• Dissociation constant (Ka)


  Equilibrium constant for the dissociation of a weak acid or base

  
• Acid strength


  The stronger the acid, the higher the Ka value
• Ionic product of water (Kw)


  Equilibrium constant for the dissociation of water

  
• At 25°C, Kw has a constant value of 1 x 10-14
• Temperature increase


  Favours the forward reaction in the water dissociation equilibrium, decreasing the pH at which water is neutral
• Acid-base calculations


  Use relationships between Ka, pKa, Kw and [H+] to solve problems involving weak/strong acids and bases

  
• Titration curve


  Shows how pH of a solution changes during an acid-base reaction. Neutralisation point is the equivalence point.

  
• Neutralisation point occurs around pH 7 for strong acid-strong base reactions
• Buffer solution


  A solution able to resist changes in pH when small volumes of acid or base are added
• Buffer calculations


  Use Ka, pH, Kw to find pH of buffer solutions containing acid-base or acid-salt mixtures
• Uses of buffers


  • Maintain specific pH for enzymes/reactions in living organisms and industrial processes
• Chemical indicators


  Specific indicators can only indicate pH change within a certain range
• Common indicators


  • Methyl orange, phenolphthalein