Acids and Bases
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Bronsted-Lowry acid?
proton donor
Bronsted-Lowry base?
proton acceptor
conjugate acid?
species formed when a proton is added to a base
conjugate base?
species formed when a proton is removed from an acid
Define buffer solution
solution that resists changes to pH when small amounts of an acid or a base is added
Bronsted-Lowry acid-base reaction
reaction involving the transfer of a proton
difference between monoprotic acid and diprotic acid
Monoprotic acid = acid that releases one H+ ion per molecule e.g. HCl (hydrochloric acid), HNO3 (nitric acid), CH3COOH (ethanoic acid) Diprotic acid = acid that releases two H+ ions per molecule e.g. H2SO4 (sulfuric acid), H2C2O4 (ethanedioic acid)
definition of pH? Rearrange it How many decimal places should pH be given to?
pH = – log [H+] [H+] = 10^-pH 2 d.p.
[H2SO4] when pH = 1.30
[H+] = 10^-1.30 = 0.0501 [H2SO4] = 0.0501/2 = 0.0251
ionic product of water expression? value of kW
[H+][OH-] 1.00 x 10^-14
Work out pH of strong acids Work out pH of weak acids Work out pH of acid buffers
Strong acids: -use pH equation Weak acids: Use Ka, assume that [H+] = [OH-] Acidic buffers: -the above does not apply -calculate [A-] -calculate [H+] using Ka -calculate pH
Why does Kw increase when temp increases?
ionisation of water is an endothermic process water only very slightly dissociates
Why does the expression for Kw not include concentration of water? (2 marks)
M1: [H2O] is very high OR very few H+ and OH- ions OR Only/very slightly dissociates M2: [H2O] is relatively constant
Which reaction is favoured when strong acids/bases dissociate?
Forward More H+/OH- ions produced
Which reaction is favoured when weak acids/bases dissociate?
Backwards Less H+/OH- ions produced
Acidic buffers are made from… Give an example of an acidic buffer
Weak acid and its salt E.g. ethanoic acid and sodium ethanoate
Purpose of an acidic buffer is to…
resist changes in pH keep the solution below pH 7
What happens when we add H+ to buffer with ethanoic acid and its salt
H+ reacts with CH3COO- ions in solution More CH3COOH produced Equilibrium shifts to the left
What happens when we add OH- to buffer with ethanoic acid and its salt
OH- ions react with H+ Lower concentration of H+ Equilibrium shifts to the right to replace the reacted H+ ions
Basic buffers are made from… Give an example of an basic buffer
Weak base and its salt E.g. NH3 and NH4+Cl-
Purpose of a basic buffer is to…
Resist changes in pH to keep the solution above pH 7
What happens when we add OH- to a buffer with ammonia and ammonium chloride
OH- ions react with NH4+ ions More NH3 and H2O produced Equilibrium shifts to the left
What happens when we add H+ to a buffer with ammonia and ammonium chloride
H+ reacts with OH- Lower conc of OH- Equilibrium shifts to the right
What happens when H⁺ ions are added to the acid buffer?
they combine with some of the large quantity of ethanoate ions to make more undissociated acid molecules Therefore: -amount of salt ions decreases: as the excess combines with the extra H⁺ to form undissociated acid -amount of undissociated acid increases: H+ combines with [A-], so more undissociated acid forms -pH stays more or less the same: extra H⁺ added immediately combine with [A-]