EQUILIBRIA
1.1 Acids and bases
A Brønsted acid is a species that can donate a proton
A Brønsted base is a species that can accept a proton
A conjugate acid-base pair is two species that differ from each other by an H+ ion
In the forward reaction:
o NH4+ is the conjugate acid of
the base NH3
o OH- is the conjugate base of
the acid H2O
In the reverse reaction
o NH3 is the conjugate base of
the acid NH4+
o H2O is the conjugate acid of
the base OH-
1.2 Calculating pH, Ka, pKa & Kw
pH – power of hydrogen
The pH can be calculated using: pH = -log10 [H+] where [H+] = concentration of H+ ions (mol dm-3)
The concentration of H+ ions in solution: [H+] = 10-pH
o pH of ethanoic acid when the hydrogen concentration is 1.32 × 10-3 mol dm-3
o pH = -log [H+] = -log 1.32 x 10-3 = 2.9
Ka & pKa – ionisation of weak acids
The Ka is the stability constant of the dissociation of a weak acid at 298 K
HA (aq) ⇌ H+ (aq) + A- (aq)
, A high value of Ka means that:
o The equilibrium position lies to the right
o The acid is almost completely ionised
o The acid is strongly acidic – lower pH
A low value of Ka means that:
o The equilibrium position lies to the left
o The acid is only slightly ionised (there are mainly HA and only a few H+ and A- ions)
o The acid is weakly acidic – higher pH
Since Ka values of weak acids are very low, pKa values are used instead to compare the strengths of
weak acids with each other
o pKa = -logKa
o The less positive the pKa value is, stronger the acid is
Kw – ionisation of water
The Kw is the stability constant for the ionisation of water at 298 K
o Its value is 1.00 x 10-14 mol2 dm-6
o H2O (l) ⇌ H+ (aq) + OH- (aq)
Kw = [H+] [OH-] concentration is same for both, so simplified to Kw = [H+]2
Concentration of water is regarded as constant and removed
1.1 Acids and bases
A Brønsted acid is a species that can donate a proton
A Brønsted base is a species that can accept a proton
A conjugate acid-base pair is two species that differ from each other by an H+ ion
In the forward reaction:
o NH4+ is the conjugate acid of
the base NH3
o OH- is the conjugate base of
the acid H2O
In the reverse reaction
o NH3 is the conjugate base of
the acid NH4+
o H2O is the conjugate acid of
the base OH-
1.2 Calculating pH, Ka, pKa & Kw
pH – power of hydrogen
The pH can be calculated using: pH = -log10 [H+] where [H+] = concentration of H+ ions (mol dm-3)
The concentration of H+ ions in solution: [H+] = 10-pH
o pH of ethanoic acid when the hydrogen concentration is 1.32 × 10-3 mol dm-3
o pH = -log [H+] = -log 1.32 x 10-3 = 2.9
Ka & pKa – ionisation of weak acids
The Ka is the stability constant of the dissociation of a weak acid at 298 K
HA (aq) ⇌ H+ (aq) + A- (aq)
, A high value of Ka means that:
o The equilibrium position lies to the right
o The acid is almost completely ionised
o The acid is strongly acidic – lower pH
A low value of Ka means that:
o The equilibrium position lies to the left
o The acid is only slightly ionised (there are mainly HA and only a few H+ and A- ions)
o The acid is weakly acidic – higher pH
Since Ka values of weak acids are very low, pKa values are used instead to compare the strengths of
weak acids with each other
o pKa = -logKa
o The less positive the pKa value is, stronger the acid is
Kw – ionisation of water
The Kw is the stability constant for the ionisation of water at 298 K
o Its value is 1.00 x 10-14 mol2 dm-6
o H2O (l) ⇌ H+ (aq) + OH- (aq)
Kw = [H+] [OH-] concentration is same for both, so simplified to Kw = [H+]2
Concentration of water is regarded as constant and removed
