Topic 12 - Acid-base Equilibria
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meaning Bronsted-Lowry base & Bronsted-Lowry acid
Bronsted-Lowry base = proton acceptor Bronsted-Lowry acid = proton donor
meaning pH
pH = - log [H+] (always 2 d.p.)
meaning: - monoprotic acid - diprotic acid
• monoprotic acid = acid that releases one H+ ion per molecule • diprotic acid = acid that releases two H+ ions per molecule
describe the difference between strong acids and weak acids
• strong acids: All molecules fully dissociate into their ions in water HX → H+ + X- • weak acids: Molecules that partially dissociate into their ions in water HX ⇌ H+ + X-
give examples of strong acids and weak acids
• strong acids: HCl, HNO3, H2SO4 • weak acids: carboxylic acids
give examples of strong bases and weak bases
• strong bases: NaOH, KOH, Ba(OH)2 • weak bases: NH3
which presumptions used when calculating pH of weak acid
• [H+] = [A-] - we neglect the ionisation of water • [HA] at equilibrium = [HA] initial - because weak acids dissociate very little
describe the equation linking Ka, H+, A-, and HA
Ka = ([H+] * [A-]) // [HA]
describe the equation linking pKa and Ka
pKa = - log Ka
How do the values of pKa and Ka relate to the strength of the acid
• the bigger the value of Ka, the stronger the acid • the bigger the value o pKa, the weaker the acid
which equation for Ka is used to calculate the pH of a weak acid in water
Ka = [H+]^2 // [HA]
meaning Kw (ionic product of water)
Kw = [H+][OH-] [H+] = 10^-14 // [OH-]
meaning indicator
weak acids where HA and A- are different colours HA ⇌ [H+] + [H-] - at low pH, HA is the main species present, whilst in high pH, A- is the main species present
describe the 2 common indicators used in acid-base reactions
methyl orange: - colour of HA = red - colour of A- = yellow - pH range of colour change = 3.2 - 4.4 Phenolphthalein - colour of HA = colourless - colour of A- = pink - pH range of colour change = 8.2 - 10.0
meaning equivalence point on a pH curve
when the moles of alkali added equals the moles of acid present
on a pH curve, when is phenolphthalein used, and when is methyl orange used
• pH at equivalence < 7: methyl orange • pH at equivalence > 7= phenolpthalein
meaning buffer solution
a solution that resists changes in pH when small amounts of acid or alkali are added (pH only changes a bit)
describe acidic buffers
• have a pH lower than 7 • made from a mixture of weak acid and one of its salts, or made from a mixture of excess weak acid and a strong alkali • [acid] and [salt] are much higher than [H+]
describe basic buffers
• have a pH higher than 7 • made from a mixture of weak alkali and one of its salts, or made from a mixture of excess weak alkali and strong base • [base] and [salt] are much higher than [OH-]
describe what happens when a small amount of H+ is added to an acidic buffer (CH3COOH & CH3COO-)
CH3COOH ⇌ H+ + CH3COO- the H+ is removed by the reaction from CH3COO- to CH3COOH The [CH3COO-] falls slightly and the [CH3COOH] rises slightly. But, as [CH3COO-] + [CH3COOH]»_space; [H+], the ratio of [CH3COOH] and [CH3COO-] remains roughly constant
describe what happens when a small amount of OH- is added to an acidic buffer (CH3COOH & CH3COO-)
CH3COOH ⇌ H+ + CH3COO- the OH- reacts with the H+, so some CH3COOH dissociates into CH3COO- to replace the lost H+ The [CH3COO-] rises slightly and the [CH3COOH] falls slightly. But, as [CH3COO-] + [CH3COOH]»_space; [H+], the ratio of [CH3COOH] and [CH3COO-] remains roughly constant
describe what happens when water is added to an acidic buffer (CH3COOH & CH3COO-)
the ratio of [CH2COOH] to [CH2COO-] remains constant and so the pH remains constant
describe what happens when a small amount of H+ is added to a basic buffer (NH3 & NH4+)
NH3 + H2O ⇌ NH4+ + OH- The added H+ is removed by reacting with OH-. So, some NH3 reacts to replace the OH- The [NH3] falls slightly and the [NH4+] rises slightly. But, as [NH3] + [NH4+]»_space; [OH-] the ratio of [NH3] and [NH4-] remains roughly constant
describe what happens when a small amount of OH- is added to a basic buffer (NH3 & NH4+)
NH3 + H2O ⇌ NH4+ + OH- The added OH- is removed by reacting with NH4+ to form NH3 The [NH3] rises slightly and the [NH4+] falls slightly. But, as [NH3] + [NH4-]»_space; [OH-], the ratio of [NH3] and [NH4-] remains roughly constant