1.12 Acids and Bases

• • An acid is a proton donor.
  • A base is a proton acceptor.
  • Acid–base equilibria involve the transfer of protons
• The concentration of hydrogen ions in aqueous solution covers a very wide range. Therefore, a logarithmic scale, the pH scale, is used as a measure of hydrogen ion concentration. pH = –log10[H+ ]
• • Water is slightly dissociated.
  • Kw is derived from the equilibrium constant for this dissociation.
  • Kw = [H+ ][OH– ]
  • The value of Kw varies with temperature
• • Weak acids and weak bases dissociate only slightly in aqueous solution.
  • Ka is the dissociation constant for a weak acid.
  • pKa = –log10 Ka
• • A buffer solution maintains an approximately constant pH, despite dilution or addition of small amounts of acid or base.
  • Acidic buffer solutions contain a weak acid and the salt of that weak acid.
  • Basic buffer solutions contain a weak base and the salt of that weak base.
• 2022 AL P1 Q4.3

  The pH of a barium hydroxide solution is lower at 50 °C than at 10 °C

  At 50 °C a 25 cm3 sample of this barium hydroxide solution was neutralised by 22.45 cm3 of hydrochloric acid added from a burette.

  Deduce the volume of this hydrochloric acid that should be added from a burette to neutralise another 25 cm3 sample of this barium hydroxide solution at 10 °C

  22.45 cm^3 BECAUSE same number of [OH-] ions
• Explain why the expression for Kw does NOT include the concentration of water [2 marks]

  • [H2O] is very high (compared with [H+ ] and [OH− ])
  • OR Very few H+ and OH− ions
  • OR Only / very slightly dissociates
  • OR Equilibrium lies far to the left Not partially dissociates M1
  • [H2O] is effectively constant OR is incorporated into the constant K (Allow changes by only a very small amount) M2
• Explain why value of Kw increases as temperature increases [2 marks]

  • (Dissociation OR breaking bonds) is endothermic 1
  • ∴ Equilibrium moves to RHS (at higher T) to absorb heat or to lower T or oppose increase in T (Allow to oppose change only if increase T mentioned)
• A buffer solution maintains an approximately constant pH, despite dilution or addition of small amounts of acid or base
• Acidic buffer solutions contain a weak acid and the salt of that weak acid. Basic buffer solutions contain a weak base and the salt of that weak base.
• • At half equivalence point the pH = pKa
  • [HA] = [A-]
  • So cancel them in the equation
  • So [H+] = Ka
• Students should be able to Use pH curves to select an appropriate indicator

  • Must change colour within the vertical part of the titration curve to be effective at determining the endpoint
• Reacting weak acids and strong bases

  • HCN + KOH -> KCN + H2O
  • The solution is slightly alkaline
  • As KCN (salt) FURTHER reacts with Water forming
  • CN- + H2O -> OH- + HCN