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Arrhenius definition• An acid is a substance that when added
to water increases the concentration of H+(aq) ions.
HA + (aq) H+(aq) + A-(aq)
• A base is a substance that when added to water increases the concentration of OH-(aq) ions.
BOH + (aq) B+(aq) + OH-(aq)
Bronsted-Lowry definitions• An acid is a proton donor.
HA H+ + A-
HA + H2O H3O+ + A-
• A base is a proton acceptor. B + H+ BH+
B + H3O+ BH+ + H2O
Ionisation of water
• Water is amphoteric.
• Can you write an expression for the equilibrium constant?
Ionic product of water
• Kw has a value of 1 x 10-14 at 25ºC.
• Known as the ionic product of water.
• Value varies with temperature.
Dissociation of acids
This is a measure of the strength of an acid
- the larger the value of Ka, the stronger the acid.
Dissociation of bases
This is a measure of the strength of a base
- the larger the value of Ka, the weaker the base.
• HIn and In- have different colours
• Their ratio is dependant on [H3O+]
• The colour of an indicator in any given solution therefore depends on the ratio, which in turn is determined by pH
• The theoretical point at which the colour change occurs is when [HIn] = [In-]
• Therefore the colour change occurs when KIn = [H3O+]
• pKIn = pH
• In practice, the colour change is not visible when [HIn] = [In-]
• Instead, they must differ by a factor of 10
– i.e. when [H+] = KIn ± 10
– OR when pH = pKIn ± 1
Choice of indicator• Colour change must occur as close to
the equivalence point as possible.
• Equivalence point – the point at which all of the acid has been exactly “neutralised” by all of the alkali.
• Does this always occur at pH 7?
• The colour chance must occur in the region of rapid pH change.
• This means that the addition of half a drop of acid/base will cause a colour change.
• The choice of indicator must therefore be made with reference to titration curves.
Buffer solutions• Is a solution where the pH remains
approximately constant when small amounts of acid or bases are added.
• Common examples:– blood– sea water
Acid buffers• Consists of a weak acid with one of
its salts (of a strong alkali)– e.g. ethanoic acid + sodium ethanoate
• The acid is partially dissociated and equilibrium with its ions.
• The salt is fully ionised.
• Addition of alkali:
• Addition of acid:
CH3COONa(s) → Na+(aq) + CH3OO-(aq)
The conjugate base removes any added H+(aq)
Supplies H3O+(aq) ions if any removed in reacting with an added base.
pH of buffer solutions
Because the by diluting a buffer the concentration of acid and salt will decrease in proportion, dilution will not affect the pH of a buffer solution.