Buffers, indicators and pH curves Flashcards
(18 cards)
Acid
Proton donor
What is a buffer solution
A weak acid and the salt of that weak acid (e.g. CH3COOH and CH3COONa) or a weak alkali and the salt of that weak alkali (NH4OH and NH4Cl)
Add acid to an acid buffer (CH3COOH and CH3COONa)
There is a large reservoir of the acid and its conjugate base, when H+ is added equilibrium position backwards (H+ reacts with CH3COO-) and ratio of H+ to CH3COO- hardly changes so pH stays roughly constant
Add alkali to acid buffer
There is a large reservoir of the acid and its conjugate base, so OH- reacts with H+ to produce water, so equilibrium shifts forwards and CH3COOH reacts with water (ionises) into the salt and the H+ ratio of H+ to CH3COO- hardly changes so pH stays roughly constant
Why is HCOOH stronger than CH3COOH
The methyl group donates electron density to the o- making it more likely to re attract the H+, however the H group cannot donate electron density
Why is trichloroethanoic acid stronger than chloroethanoic acid
The chlorine atoms are very electronegative meaning electrons are drawn out of the OH group and H+ ions are much less likely to be re attracted since charge density is spread out
Base
Proton acceptor
Equivalence point
Number of moles of H+ = number of moles of OH-
Half equivalence point
Number of moles of acid salt = number of moles of acid
Significance of half equivalence point
pH = pKa
Salt of a strong acid and strong base
Weak conjugates, pH = 7
Salt of a weak acid and strong base
Salt is a strong conjugate base, pH > 7
Salt of a strong acid and weak base
Conjugate salt is fairly acidic, pH > 6
What determines choosing an ideal indicator
pKin should be equal to or as close as possible to the equivalence point of the titration
Why are carboxylic acids acidic but alcohols which also contain an OH group are not?
Carboxylic acids are acidic because they can spread the delocalised charge (charge density is spread out) over the whole COO- group so they are less likely to be protonated again
