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