1. Acid / Base Theories
Peter Jackson

2. Acids
Have a sour taste
React with metals to to form a salt plus hydrogen H2SO4 + Zn = ZnSO4 + H2
React with carbonates to form a salt plus water plus carbon dioxide
CaCO HCl = CaCl2 + H2O + CO2
Turn blue litmus red
Peter Jackson

3. Arrhenius Theory
In 1887 Svante Arrhenius [a Swedish chemist] developed a theory of acids and bases based on two major observations.
Peter Jackson

4. Acids contain at least one hydrogen atom per molecule.
Pure acids such as glacial acetic acid and anhydrous sulphuric acid are non-conductors while their aqueous solutions are conductors [of electricity].
Peter Jackson

5. A substance that dissociates in water to produce H+ ions
HCl → H+ + Cl-
HNO3 → H+ + NO3-
H2SO4 → 2 H+ + SO42-
H3PO4 → 3 H+ + PO43-
Hydrochloric and nitric acids are monobasic
Sulphuric acid is dibasic
Phosphoric acid is tribasic
The above acids dissociate fully in water and are said to be strong acids
Peter Jackson

6. Weak acids do not dissociate fully in water Carbonic acid
Ethanoic acid
Citric acid
Ascorbic acid [ vitamin C]
Are examples of weak acids
Not all acids are dangerous
Conc. Acids tend to be dangerous e.g. sulphuric acid in a car battery,
formic acid in an ant bite or nettle sting is not dangerous but in a bottle they are
Peter Jackson

7. Bases A base dissociates in water to produce OH- ions [hydroxide ions]
Turn red litmus blue
NaOH and KOH are strong bases - they dissociate fully in water
Calcium hydroxide and Magnesium hydroxide are weak bases – they do not dissociate fully in water
Bases that dissolve in water are called Alkalis
Peter Jackson

8. Limitations of Arrhenius
It is H3O+ not H+ that exists in water
Restricted to aqueous solutions
Not all acid base reactions require water
HCl + NH3 = NH4Cl
Peter Jackson

9. Brønsted
Lowry
Peter Jackson

10. Brønsted-Lowry Theory (1923)
Both had same idea at the same time
Acid is a proton [H+] donor
Base is a proton [H+] acceptor
acid-base reactions involve the transfer of a proton from the acid to the base.
Stronger the acid - the more readily it donates the proton
Stronger the base the more readily it accepts the proton
Peter Jackson

11. Conjugate acid/base Pairs
HCl + H2O = H3O+ + Cl-
HCl gives away a proton [H+] to become Cl- so it is an acid
H2O accepts a proton to become H3O+ so it is a base
The reaction is reversible
H3O+ gives away a proton [H+] to become H2O so it is an acid
Cl- accepts a proton to become HCl so it is a base
Peter Jackson

12. Conjugate acid/base Pairs
Let us look at the water in the last equation
[going right]
accepts H+
H2O
Base
H3O+
Acid
donates H+
[going left]
Species which differ by H+ are called
conjugate acid-base pairs
Peter Jackson

13. Conjugate acid/base Pairs
NH3 + H2O = NH OH-
NH3 accepts a proton to become NH4+ so it is a base
H2O donates a proton to become OH- so it is an acid
In reverse
NH4+ donates a proton to become NH3 so it is an acid
OH- accepts a proton to become H2O so it is a base
Peter Jackson

14. Strong Weak Acid Conjugate base Base Conjugate acid HClO4  ClO4-
H2SO4  HSO4-
HCl  Cl-
HNO3  NO3-
H3O+  H2O
HSO4-  SO42-
HF  F-
CH3COOH  CH3COO-
H2S  HS-
NH4+  NH3
HCO3-  CO32-
H2O  OH-
HS-  S2-
NH3  NH2-
OH-  O2-
O2-  OH-
OH-  H2O
CO32-  HCO3-
NH3  NH4+
HS-  H2S
CH3COO-  CH3COOH
F-  HF
H2O  H3O+
Cl-  HCl
Weak
Peter Jackson

15. Acid - Base reactions #1
These lead preferentially to the formation of the weaker acid and base HNO3 + H2O = H3O NO3-
H3O+ is a weaker acid than HNO3
NO3- is a weaker base than H2O
so the reaction to the right is favoured
Peter Jackson

16. Acid - Base reactions #2
These lead preferentially to the formation of the weaker acid and base CH3COOH + H2O = CH3COO H3O+
CH3COOH is a weaker acid than H3O+
H2O is a weaker base than CH3COO-
so the reaction to the left is favoured
Peter Jackson

17. Levelling Effect of Water
HCl, HNO3, H2SO4 and HClO4
all fully dissociate in water to form H3O+
all appear equally strong
they are NOT equally strong
this is called the Levelling Effect of Water
Their true strengths can be shown in ethanoic acid [less basic - more acidic]
This makes it harder for them to dissociate
Peter Jackson

18. Amphoteric behaviour Amphoteric = can be both acidic and basic
Self Ionisation of Water
H2O H2O = H3O OH-
Base Acid Acid Base Another Example
NH NH3 = NH NH2-
Base Acid Acid Base
This only happens to a very limited extent
Peter Jackson

19. Neutralisation Reactions
HCl + NaOH = NaCl + H2O
HCl dissociates in water to form H3O+ + Cl-
H3O+ + Cl- + Na+ + OH- = 2 H2O + Na+ + Cl-
the Cl- and the Na+ do not change
therefore they have not reacted
We can leave them out of the equation
H3O+ + OH- = 2H2O
this is called the Ionic or Half Equation
The omitted ions are “spectator ions”
Peter Jackson

20. Advantages of Bronsted-Lowry
Not limited to reactions in water
Allows more species to be regarded as acids and bases
Explains the amphoteric nature of many substances -
even relatively strong acids such as ethanoic acid
CH3COOH + HCl = CH3COOH2+ + Cl- Base Acid Acid Base
Peter Jackson