acids, bases, buffers

• kw


  Increasing/decreasing Oh- or H+ will not affect kw as long as it returns to equilibrium and the temperature is constant
• acid strength


  acid is stronger if it dissociates more
• Ka formula


  Ka = (H+)(OH-) divided by / (HA)
• KA


  Ka meaures the proportion of molecules that dissociate into ions
• using ka to find ph approximations

  1.equilibrium HA = initial HA - works well for weak acids as they dissociate slightly

  2.(H+)=(A-) - works fine for both weak and strong acids, but doesnt work for weak or strong acids in solution alongside other substances
• pka formula


  pka = -logka
• ka and pka correlation

  the lower the ka the higher the pka value
• pka


  allows us to easily compare strengths of different WEAK acids
• dibasic acids


  eg sulfuric acid H2SO4 the second H only dissociates slightly AFTER the first H fully dissociates
• neutralisation reactions involving weak acids


  if we react equal amounts of weak acid and strong base, the resulting salt with lightly react with water to form an alkaline solution
• why do we us titrtions

  when we want to work out concentration of something,
• methyl orange


  red if added to acidic solution, yellow if added to alkaline solution
• phenopthalein


  colourless in acidic solution, turns pink
• indicator is suitable if its ph range is within the vertical region of ph curve
• equivalence point


  the point at which acid is completely neutralised by base
• end point


  point at which an indicator changes colour
• at half equivalence point


  -(HA)=(A-) because half of acid has reacted to form A- ions and

  exactly half of acid is unreacted

  -because Ka= (H)(A)/(HA) , (A) and (HA) can be cancelled out as they arr equal to each other at half equivalence point

  so formula is Ka=(H+)

  -pka= ph , as -log(ka)=-log(h+)
• buffer definition


  resists change in ph when small amounts of acid or base are added
• what makes it a buffer

  1.must contain high conc of undissociated(HA) and that acids conjugate base(A-)
• buffer


  must contain some undissociated acid and its conjugate base and the concentrations (of undissociated HA and dissociated A-) must be high
• how to make a buffer solutionpart 1

  weak acid + salt(of its conjugate base)

  this works as weak acids would provide with high conc of undissociated acid and the conjugate base salt would give a high conc of acids conjugate base
• how to make a buffer part 2

  PARTIALLY neutralising weak acid with a strong base in order for the acid to make conjugate base pair
• exam technique- how does a buffer resist changes in ph upon addition of a small amount of acid

  -write down reversible reaction for specific buffer

  -state that adding the acid increases the (H+)

  -state that the equilibrium shifts to left to lower conc of (H+)

  -state that,overall, (H+) remains roughly constant

  E.g.

  • CH3COOH ⇌ H+ + CH3COO-
  • When acid is added, [H+] increases
  • As a result of this increase, the equilibrium shifts to the left - reducing the concentration of H+ ions to roughly what it was originally
  • This means, overall, [H+] remains almost constant.
• how to make buffer part 3

  can use weak buffer and its conjugate acid
• acidic and alkaline buffer solutions

  acidic buffer solutions contain high concentrations of an uncharged weak acid and its conjugate base

  alkaline buffer solutions contain high concentrations of an uncharged weak base and its conjugate acid
• how does a buffer resist changes in ph upon addition of a small amount of alkali

  concentration of OH- ions increase, the OH- react with H+, which reduces concentration of H+ so equilibrium shifts to right thus replenshing the lost H+