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:

• 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.

Practice

Basic buffers• Consist of a weak base with one of its

salts (of a strong acid).