1. Acids and Bases
Chapter 12

2. Arrhenius’ theory of Acids and Bases
Acid- An acid is a substance that dissociates in water to form H + ions.
Base – A base is a substance which
dissociates in water to form OH- ions.

3. Limitations of the Arrhenius Definitions
The Arrhenius definitions are limited.
(i)Acid- Base reactions are considered only in aqueous solutions
(ii)The OH- ion is singled out as a source of the basicity and therefore certain insoluble oxides cannot be classed as Arrhenius bases e.g. CuO
(iii)The action of amphoteric species cannot be explained.

4. Examples Examples of acids
Hydrochloric acid (HCl) – monobasic acid- donates 1 H+ ion
Nitric acid (HNO3)-monobasic acid-donates 1 H+ ion
Sulphuric acid (H2SO4) – dibasic acid- donates 2H+ ion
Phosphoric acid (H3PO4) – tribasic acid- donates 3H+ ion

5. Strong Acids and Weak Acids
H2SO4 and HCl are strong acids.
A strong acid is one that dissociates (break up) fully in aqueous solution.
A weak acid is one that does not dissociate fully in aqueous solution.
Examples include Ethanoic Acid and Methanoic acid (Formic acid).

6. A strong acid is one which dissociates fully in water
Example: HCl, H2SO4, HNO3
HCl H2O H3O Cl-
A weak acid is one which does not fully dissociate in water
Example: CH3COOH (ethanoic acid)
CH3COOH H2O H3O CH3COO-

7. Brønsted-Lowry Theory
In 1923, Johannes Brønsted (a Danish chemist) and Thomas Lowry (an English chemist) proposed new definitions of acids and bases.
Brønsted
Lowry

8. Bronsted-Lowry theory of acids and bases.
An acid is a proton donor.
A base is a proton acceptor.
A substance that can act as an acid or a base is said to be
amphoteric.

9. Acid = Proton Donor HCl + H2O H3O+ + Cl- Donates a Proton
The HCl donates a proton and so is an acid
The H2O, in this case, accepts a proton and so is a base
Remember: Proton = H+
Accepts a Proton

10. Likewise: HNO3 + H2O H3O+ + NO3- H2SO4 + H2O H3O+ + HSO4-
and
H2SO H2O H3O HSO4-
HSO H2O H3O SO4-2

11. Acid – Base Reaction HCl + NH3 Cl- + NH4+ Acid – Donates Protons
Base – Accepts Protons

12. Conjugate Acid/Base Pairs
An acid changes into a conjugate base when it donates a proton.
A base changes into a conjugate acid when it accepts a proton.
A conjugate acid-base pair is any pair consisting of an acid and a base which differs by one proton.

13. Example: What is the (i) conjugate acid (ii) conjugate base of HC2O4-
Answer
(i) H2C2O4
(ii) C2O4 2-

14. Example
Indicate which species are acting as acids and which are acting as bases:
HNO H2F2 = H2NO HF2-
Also indicate the conjugate acid-base pairs

15. Conjugate acid-base pairs Conjugate acid-base pairs
Answer
HNO H2F2 = H2NO HF2-
Base Acid Acid Base
Conjugate acid-base pairs
Conjugate acid-base pairs

16. Neutralisation
Neutralisation is the reaction between an acid and base to form salt and water.
Acid Base = Salt Water
HCl NaOH = NaCl H2O
A salt is a substance formed when the Hydrogen of an acid is replaced by a metal or an ammonium ion.

17. Neutralisation Acid + Base Salt + Water HCl + NaOH NaCl + H2O
but since the acid and base dissociate in water we can write:
H+ + Cl- + Na+ + OH Na+ + Cl- + H2O
we can cancel the Na+ and Cl- on both sides leaving:
H+ + OH H2O

18. Everyday Examples of Neutralisation
Indigestion remedies are bases that neutralise excess stomach acid
Lime is a base that neutralises acid in soil
Toothpaste is a base that neutralises acid in the mouth

19. Nettle, bee and ant stings are acidic
They can be neutralised with baking soda
Wasp stings are basic
They can be neutralised with vinegar or lemon juice