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Equilibria involving ions: acids and bases
AH Unit 2(b)(iii)
Key question
• What is are acids and bases?
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)
Key question• What are the limitations of these
definitions?
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
Hydronium ion
Conjugate acids and bases
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.
pH scale
Dissociation of acids
This is a measure of the strength of an acid
- the larger the value of Ka, the stronger the acid.
Relative strengths of acids
Calculating pH of weak acids
Ionisation of water• The ionisation of water is endothermic.
• What will happen to the extent of ionisation of water as temperature increases?
Dissociation of bases
This is a measure of the strength of a base
- the larger the value of Ka, the weaker the base.
Buffers
Buffer solutions• Is a solution where the pH remains
approximately constant when small amounts of acid or bases are added (or when diluted).
• Common examples:– blood– sea water– contact lens solution– swimming pools
Acid buffers• Consists of a weak acid with one of its
salts (of a strong alkali)– e.g. ethanoic acid + sodium ethanoate
• This is a weak acid and its conjugate base.
– The acid is partially dissociated and equilibrium with its ions.
– The salt (conjugate base) is fully ionised.
HA(aq) + H2O(l) H3O+(aq) + A-(aq)Plenty of Plenty of
Source of H3O+(aq) to replace any removed by addition of extra OH-(aq)
Sink that removes extra H3O+(aq)
• 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 by diluting a buffer the concentration of acid and salt will decrease in proportion, dilution will not affect the pH of a buffer solution.
Practise
Basic buffers• Consist of a weak base with one of its
salts (of a strong acid).
• Suggest two chemical that could be used to prepare a basic buffer– ammonia– ammonium chloride
Indicators
Universal indicator
Methyl orange
Phenolphthalein
Indicators
• Are weak acids
• 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 usually has to differ by a factor of 10 in order to see it
– i.e. when [H+] = KIn ± 10
– OR when pH = pKIn ± 1
ExampleFor an indicator which is red in the unionised form (HIn) and blue in the ionic form (In-), it will appear:
– Completely red when [HIn] > 10 x [In-]– Completely blue when [In-] > 10 x [HIn]
This means that a colour change takes place typically over 2 pH units.
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?
• Ideally, the equivalence point will equal the end point.
Rapid initial rise
• The pH at which an indicator changes colour depends on the value of the pKa for that indicator.
• A good indicator will change colour on addition of one or two drops of the acid or base, corresponding to the steepest part of the titration curve.
Methyl red
Phenolphthalein
Practise