Acids, bases, pH and buffers

• Define Bronsted-Lowry acid


  A proton donor

  
• Define Bronsted-Lowry base


  A proton acceptor

  
• What are conjugate acid-base pairs?


  Contains two species that can be interconverted by transfer of a proton

  
• Strong acids...?


  Fully dissociate. High H+ concentration. There is one arrow pointing to the product direction
• Weak acids...?


  Partially dissociate. Low H+ concentration. It is a reversible reaction
• Strong bases...?


  Fully dissociates. High OH- concentration

  
• Weak bases...?


  Partially dissociates. Low OH- concentration

  
• Monobasic acid


  1x H+ ions can be replaced

  
• Dibasic acid


  2x H+ ions can be replaced

  
• Tribasic acid


  3x H+ ions can be replaced

  
• acid + metal -->


  salt + hydrogen

  
• acid + carbonate -->


  salt + water + carbon dioxide

  
• acid + base -->


  salt + water (neutralization)

  
• acid + alkali -->


  salt + water

  
• What is the mathematical relationship between pH and [H+(aq)]?

  
• What is the reverse of the relationship?


  [H+(aq)] = 10^-pH
• [H+(aq)] is ___ to the concentration of the acid [HA]


  Equal

  [H+(aq)] = [HA]
• What is the acid dissociation constant?


  Ka

  
• At what temperature are Ka values usually recorded at?


  25 °C
• The larger the numerical value of Ka...


  the further the equilibrium is to the right
• The larger the value of Ka...


  the greater the dissociation and the stronger the acid

  
• pKa =


  pKa values are much more manageable than Ka values. It is easier to compare relative acidic strengths using pKa than Ka values.

  
• Ka =

  
• What does [H+(aq)] depend on?


  The concentration of acid [HA]

  The acid dissociation constant
• Approximation for calculating the pH of weak acids - 1


  [H+(aq)] = [A-(aq)]

  HA dissociates to produce equilibrium constants of H+ and A- that are equal. There is usually a small concentration of H+ from the dissociation of water, but this will be very small and so can be neglected compared to concentration of H+ from the acid.
• Approximation for calculating the pH of weak acids - 2


  [HA(aq] (at equilibrium) = [HA(aq)] (at the start)

  The second approximation assumes that the concentration of the undissociated acid is much greater than the H+ concentration at equilibrium.

  We assume that the concentration of HA at equilibrium = concentration of undissociated HA
• Simplifying Ka expression

  
• Limitations of approximations in calculating pH of weak acids - 1


  At 25 °C, H+ from the dissociation of water = 10^-7. If the pH>6, then H+ from the dissociation of water will be significant compared with the dissociation of the weak acid
• Limitations of approximations in calculation pH of weak acids - 2


  This approximation is not justified/may not work for stronger weak acids and very dilute solutions
• What is Kw?


  The ionic product of water

  Kw = [H+][OH-]

  [H+] = [OH-] at 25 °C (neutral)

  
• What is the value of Kw at 25°C?

  
• What is an alkali?


  A soluble base that releases OH- ions in an aqueous solution
• Kw =


  [H+][H+]

  
• As temperature increases...


  Kw increases (endothermic)
• What is a buffer solution?


  A system that minimises pH changes when small amounts of an acid or base are added

  
• What 2 components does a buffer solution contain to remove an added acid or base?


  Weak acid (HA) --> Removes added alkali

  Conjugate base (A-) --> Removes added acid
• What ways can a buffer solution be prepared?


  Preparation from a weak acid and its salt

  Preparation by partial neutralisation

  Two reservoirs to remove added base and alkali

  
• Preparation from a weak acid and its salt

  
• Preparation by partial neutralisation

  By adding an aqueous solution of an alkali (e.g. NaOH) to an excess weak acid, the weak acid is partially neutralised by the alkali, forming the conjugate base.

  Some of the weak acid is left over unreacted - the resulting solution would consist of a mixture of the salt of the weak acid and any unreacted weak acid
• Two reservoirs to remove added acid and alkali