1. Equilibria involving ions: acids and bases
AH Unit 2(b)(iii)

2. Key question
What is are acids and bases?

3. 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)

4. Key question
What are the limitations of these definitions?

5. Reaction taking place out-with solution
No hydroxide ions produced

6. 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

7. Hydronium ion

8. Conjugate acids and bases

9. Ionisation of water Water is amphoteric.
Can you write an expression for the equilibrium constant?

10. 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.

11. pH scale

14. Dissociation of acids This is a measure of the strength of an acid
- the larger the value of Ka, the stronger the acid.

15. Relative strengths of acids

16. Calculating pH of weak acids

18. Ionisation of water The ionisation of water is endothermic.
What will happen to the extent of ionisation of water as temperature increases?
Kw increases with increasing temperature, so ionisation increases

19. Water remains NEUTRAL at all of these temperatures
Water remains NEUTRAL at all of these temperatures. However, the definition of neutral is changing. At 298K, neutral is This is because [H][OH] is and [H] is With changing temperature, the extent of ionisation changes, changing the [H] and [OH] but because they remain the same, the water remains neutral.

20. Dissociation of bases This is a measure of the strength of a base
- the larger the value of Ka, the weaker the base.

21. Buffers

22. 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
For swimming pools, alkali is added from swimmers via sweat and acid is added via urine. It is necessary to maintain the pH for effective antibaceterial action of the products produced by adding chlorine

23. 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.

24. HA(aq) + H2O(l) H3O+(aq) + A-(aq)
Plenty of
Plenty of
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
Source of H3O+(aq) to replace any removed by addition of extra OH-(aq)
Sink that removes extra H3O+(aq)

25. 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)

27. 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.

28. Practise

29. 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

30. Indicators

31. Universal indicator

32. Methyl orange

33. Phenolphthalein

34. Indicators
Are weak acids

35. 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

36. The theoretical point at which the colour change occurs is when [HIn] = [In-]
Therefore the colour change occurs when KIn = [H3O+]
pKIn = pH

37. 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

38. Example
For 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.

39. 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.

43. Rapid initial rise

47. 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.

48. Methyl red

49. Phenolphthalein

53. Practise