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?

Reaction taking place out-with solution No hydroxide ions produced

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 Addition of 0.01M NaOH to 50cm3 0.01M HCl

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.

CH3COONa(s) → Na+(aq) + CH3OO-(aq) 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).