Transcript Equilibria involving ions: acids and bases

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.
Calculating pH of weak acids
Dissociation of bases
This is a measure of the strength of a base
- the larger the value of Ka, the weaker the base.
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 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?
Methyl red
Phenolphthalein
• 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.
Strong acid / strong base
Phenolphthalein
Examples
Practice
Buffers
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:
Supplies H3O+(aq) ions if any removed in reacting with an added base.
• Addition of acid:
CH3COONa(s) → Na+(aq) + CH3OO-(aq)
The conjugate base removes any added H+(aq)
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.
Practice
Basic buffers
• Consist of a weak base with one of its
salts (of a strong acid).