acids and bases assumptions and random stuff

why is H2O not in the equation for Kw

• Dissociation of water is small so denominator will be huge and we can make assumption that it remains constant

what happens to Kw and pH of water as temp increases

• As temperature increases, the pH of pure water will be less than 7

  • Forwards reaction is endothermic - so increase in temp shifts equilibrium to right hand side to oppose the increase in temp

  • H₂O (l) <=> H⁺ (aq) + OH^- (aq)

what are the two assumptions of weak acids

the dissociation of the acid is greater than that of water so all H+ ions came from acid, so [A-] = [H+]

only a very small amount of HA dissociates so [HA] at start = [HA] at equilibrium

acidic buffer assumption

• IN ACIDIC BUFFER SOLUTIONS, THE A- FOR KA COMES FROM THE SALT AS WE ASSUME THAT THE DISSOCIATION OF THE WEAK ACID IS NEGLIGIBLE

  • Needs loads of weak acid and only a little base as we want both HA and A- in solution

for acidic buffer where strong base + excess weak acid

HA + OH- —> H2O + A-