1. Equilibria involving acids and bases
Key knowledge
pH as a measure of strength of acids and bases: Kw Ka for weak acids

2. Remember back to Unit 2 Lowry-Bronsted theory
Defines an acid as a proton donor and a base as a proton acceptor
An acid-base reaction involves the transfer of one or more protons from an acid to a base
Amphiprotic substances have the ability to accept or donate protons. Example water

3. Ionisation constant of water
In Unit 2 Remember that water can self-ionisation:
Kw = [H3O+][OH-]= 10-7 x 10-7 = 1.0 x 10-14M2 at 25C

4. Acidic and basic solutions
In solutions of acidic substance, H3O+ ions are formed by reaction of acid with water
In pure water and neutral solutions: [H3O+]=[OH-]= 10-7 M
In acidic solutions: [H3O+]>10-7 M and [OH-]< 10-7 M
In basic solutions: [H3O+]<10-7 M and [OH-]> 10-7 M

5. pH Remember pH = -log10 [H3O+] For pure water at 25C pH=7
Acidic solution pH < 7
Basic solution pH >7
Don’t forget
[H3O+][OH-]= 10-14M2 at 25C

8. How is pH affected by temperature?
Earlier, we defined the ionisation constant of water as: Kw = [H3O+][OH-]= 10-7 x 10-7 = 1.0 x 10-14M2 at 25C However as temperature increases the Kw decreases

9. Question 3
Complete the following table

10. Acidity constant
Most acid-base reactions in water can be considered as equilibrium reactions.
Hydrochloric acid, for example, ionises in water according to the equation:
The expression for the equilibrium constant

11. In this reaction, the Cl- ion can be described as the conjugate base of HCl
Similarly, HCl is referred to as the conjugate base of Cl-
So this is for a strong acid

12. Weak acid
Ethanoic acid is classified as a weak acid because at equilibrium only a small proportion of the acid is been ionised

13. Weak acids

14. Acid strength

17. How the equilibrium between a weak acid, HA, and its conjugate base, A–, keeps the pH constant?

18. Buffers: using equilibrium to resist change
Buffers are solutions that can absorb the addition of acids or bases with little change of pH
They can be made most easily by mixing a weak acid and a salt of its conjugate base
The important feature of this solution is that it contains significant amounts of both the weak acid and its conjugate base

19. The addition of HCl solution disturbs the equilibrium
If a solution of a strong acid such as HCl is added to the equilibrium mixture of ethanoic acid/ethanoate ion, the pH will decrease (not much)
The addition of HCl solution disturbs the equilibrium
Le Chatelier’s principle tell us that the system will respond to oppose the change and restore equilibrium
More reactants = net forward reaction
Less reactants = net backward reaction
More products = net backward reaction
Less products = net forward reaction

20. Buffering action of the oceans

21. Questions

22. Answers