Acid-base equilibria

• Bronsted-Lowry acids


  Proton donors
• Bronsted-Lowry bases


  Proton acceptors
• What happens when acids are mixed with water

  1. Acid dissociates

  2. Hydrogen ions (H+) or hydronium ions (H3O+) are released

  3. This makes the solution acidic
• What happens when bases are mixed with water

  1. Bases react with H+ ions

  2. Hydroxide ions (OH-) are formed

  3. This makes the solution basic
• Types of acids and bases

  • Strong acids
  • Weak acids
  • Strong bases
  • Weak bases
• Strong acids


  Dissociate or ionize almost completely
• Weak acids


  Dissociate poorly
• Strong bases


  Dissociate or ionize almost completely
• Weak bases


  Dissociate poorly
• Examples of strong acids

  • Hydrochloric acid
  • Nitric acid
  • Sulfuric acid
• Examples of strong bases

  • Sodium hydroxide
  • Potassium hydroxide
• Example of a weak base


  • Ammonia (NH3)
• Conjugate pair


  • Linked by the transfer of a proton
  • Any species that gains a proton is the conjugate acid
  • Any species that loses a proton is the conjugate base
• Acid-base reactions


  1. Protons are exchanged

  2. Acids donate protons

  3. Bases accept protons
• If more acid or base is added


  Equilibrium shifts to use up the added reactant (Le Chatelier's principle)
• Water behaves as a base when an acid is added to it</b>
• Reaction of acids and bases

  1. Acids and bases react to form salts and water

  2. The pH is neutral
• Salts


  Formed from the metal from the base and the non-metal from the acid
• Ammonia (NH3) reacts with water to form ammonium ions (NH4+) and hydroxide ions (OH-)
• You'll be given examples where you are expected to write the formula of a salt just from using the acid and the base that they've given you
• Nitric acid reacts with sodium hydroxide

  1. Nitrate (non-metal part of acid)

  2. Metal part of base

  3. Form potassium nitrate

  4. Produce water
• Ammonia doesn't produce OH- ions directly, it reacts with water first to accept a proton and produce ammonium ions and OH- ions
• Ammonia is the only reaction that doesn't produce water as part of a neutralization reaction
• Forming ammonium salts


  1. Ammonia reacts with acids

  2. Forms ammonium salts
• Enthalpy change of neutralization

  Energy change per mole when acid and base solutions react to produce water
• Neutralization reactions are always exothermic, so enthalpy changes of neutralization are always negative
• Weak acids and bases

  • They dissociate weakly
  • OH- and H+ ions are used up quickly in neutralization reactions as there is only a small number of them in solution
• Enthalpy of dissociation


  Energy required to break apart the ionic substances
• Enthalpy of neutralization


  Energy released when OH- and H+ ions react
• For weak acids and bases, the enthalpy of neutralization varies depending on the acid or base
• Strong acids and bases

  • They dissociate fully
  • Only one type of enthalpy change involved - when OH- and H+ ions react
• The standard enthalpy of neutralization is similar for all reactions of strong acids and bases
• pH


  Logarithmic scale that measures the concentration of H+ ions in solution
• Calculating pH


  pH = -log[H+]
• For a strong acid like HCl, the concentration of HCl equals the concentration of H+ ions
• Polyprotic/polybasic acids can donate more than one proton
• Monoprotic/monobasic


  • 1 mole of acid produces 1 mole of H+
• Diprotic/dibasic


  • 1 mole of acid produces 2 moles of H+
• Triprotic/tribasic


  • 1 mole of acid produces 3 moles of H+
• Calculating pH of strong acids

  Assume full dissociation

  For monoprotic: [H+] = [acid]

  For diprotic: [H+] = 2[acid]

  For triprotic: [H+] = 3[acid]