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

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

13. Calculating pH of weak acids

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

15. Indicators

16. Universal indicator

17. Methyl orange

18. Phenolphthalein

19. Indicators
Are weak acids

20. 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

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

22. 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

23. 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?

24. Methyl red

25. Phenolphthalein

26. 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.

27. Strong acid / strong base
Addition of 0.01M NaOH to 50cm3 0.01M HCl

29. Phenolphthalein

31. Examples

32. Practice

33. Buffers

34. 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

35. 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.

36. 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)

38. 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.

39. Practice

40. Basic buffers
Consist of a weak base with one of its salts (of a strong acid).