Topic 8: Acids and Bases
• Theories of acids and bases
• Properties of acids and bases
• Strong and weak acids and bases
• The pH-scale

8.1 Brönsted-Lowry
Acid - base Theory
• The protolytic reaction:
Reaction of an acid with water:
H Cl (g) + H
2
O H
3
O + + Cl -
Oxonium ion
Reaction of a base with water:
NH
3
(g) + H
2
O ↔ OH + N H
4
+
Hydroxide ion

Draw the reaction formulas when these substances react with water as acids:
H NO
3
H
2
SO
4
CH
3
COO H
Draw the reaction formulas when thise substances react with water as bases:
CH
3
CO
3
NH
2
2-

Nitric acid
H NO
3
+ H
2
O H
Sulphuric acid
3
O + + NO
3
-
H
H
2
SO
SO
4
4
–
+ H
2
O H
3
+ H
2
O H
3
O + + H SO
O + + SO
4
4
–
2-
Etanoic acid
CH
3
COO H + H
2
O ↔ H
3
O + + CH
3
COO -
Methylamine
CH
3
NH
2
+ H
2
O ↔ OH + N H
Carbonate ion
4
+
CO
3
2+ H
2
O ↔ OH + H CO
3
-

Brönsted-Lowry
Acid - base Theory
• Acid: proton donor
• Base: proton acceptor

Conjugate acid-base pair
CH
3
COOH + H
2
O  CH
3
COO + H
3
O +
Acid
1
Base
2
Base
1
Acid
2

State for each reaction which reactant is acid and which is base a) HSO
4
b) NH
3
+ H
2
+ H
2
O ↔ SO
O ↔ NH
4
+
4
2+ H
3
O +
+ OH c) HCO
3 d) HCO
3
-
+ H
2
O ↔ CO
3
2-
+ H
2
O ↔ H
2
CO
3
+ H
3
O +
+ OH e) H
3
O + + OH ↔ 2 H
2
O

State for each reaction which reactant is acid and which is base a) HSO
4
+ H
2
O ↔ SO acid base
4
2b) NH
3
+ H
2
O ↔ NH base acid
4
+
+ H
+ OH -
3
O + c) HCO
3
+ H
2
O ↔ CO acid base
3
2+ H
3
O + d) HCO
3
+ H
2
O ↔ H base acid
2
CO
3 e) H
3
O + + OH ↔ 2 H
2
O base acid
+ OH -

Which of these are conjugated acid/base-pairs?
a) HSO
4
/SO
4
2b) H
2
SO
4
/SO
4
2c) NH
3
/NH
4
+ d) HCO
3
/CO
3
2e) CO
3
2/H
2
CO
3 f) H
3
O + /OH -

Which of these are conjugated acid/base-pairs?
a) HSO
4
/SO
4
2c) NH
3
/NH
4
+ d) HCO
3
/CO
3
2-
What is the rule?

• Water can act both as an acid and as a base;
H
3
O +  H
2
O  OH -
• Such compounds are said to be amphiprotic
(ampholytic).


• Monoprotic: CH
3
Acetic acid
COOH  CH
3
COO -
• Diprotic:
Oxalic acid
HOOC-COOH  OOC-COO -
• Triprotic: H
3
Phosphoric acid
PO
4
 PO
4
3-
• Polyprotic

Arrhenius
Acid-Base Theory
• Acid: H + Hydrogen ion / Proton
– Acidic solutions contain H + / H
3
O +
(oxonium, hydroxonium or hydronium ion)
• Base: OH Hydroxide ion
– Alkaline solutions contain OH -
Alkaline = Water soluble base

Lewis
Acid-Base Theory
• Lewis acid: electron pair acceptor,
– e.g. H + , AlCl
3
, BF
3
• Lewis base: electron pair donor,
– e.g. OH , NH
3

• A Lewis acid-base reaction involves the formation of a covalent bond. The Lewis base provides the electrons in that bond. This kind of covalent bond is called dative covalent bonds (see topic 13) or co-ordinate covalent bond.
• Its no difference between a normal covalent bond and a dative covalent bond except the origin of the electrons. Sometimes an arrow is used instead of a line to show that it's a dative bond e.g. H
3
N  BF
3
.

• The term Lewis acid is often just used for acids that aren’t Brönstedt acids
• The formation of complex ions, topic 13, is usually Lewis acid-base reactions

• They have pH<7
• They taste sour
• They react with bases and metals
• Where can you find:
Hydrochloric acid
Sulphuric acid
Acetic acid
Carbonic acid

• They have pH>7
• They feel ”slippery”
• They react with acids
• Where can you find:
Ammonia
Sodium bicarbonate
Calcium carbonate

8.3 Strong acids
Totally dissociated
Hydrochloric acid: HCl + H
2
O  Cl + H
3 chloride
O +
Nitric acid: HNO
3
+ H
2
O  NO
3 nitrate
+ H
3
O +
Sulphuric acid: H
2
SO
4
+ H
2
O  HSO
4
+ H
3
O hydrogensulphate
+
HSO
4
+ H
2
O  SO
4
2+ H
3
O + sulphate

Start
End
HCl + H
2
O  H
3
O + +Cl -
100% 0%
0% 100%

Weak acids
Partially dissociated
• Ethanoic acid, (Acetic acid)
CH
3
COOH + H
2
O  CH
3
COO + H
3
O + ethanoate ion
(acetate ion)
• Carbonic acid,
H
2
CO
3
+H
2
O  HCO
3
+ H
3
O + hydrogen carbonate ion
HCO
3
+H
2
O  CO
3
2+ H
3
O + carbonate ion

CH
3
CH
2
COOH + H
2
O  H
3
O + + CH
3
CH
2
COO start 100% 0% end 99% 1%

If the concentration is the same for the strong and the weak acid:
– The strong acid is more acidic than the weak acid
– The strong acid has a higher concentration of hydroxonium ions than the weak acid
– The strong acid has higher conductivity

http://phet.colorado.edu/en/simulation/acid-base-solutions

Strong bases
Containing the OH ion
• All group I hydroxides:
NaOH
(s)
+ H
2
O  Na + + OH -
• Group II hydroxides
Ba(OH)
2
+ H
2
O  Ba 2+ + 2 OH -

Weak bases
Partially dissociated
• Ammonia
NH
3
+ H
2
O  NH
4
+ + OH -
• Ethylamine
CH
3
CH
2
-NH
2
+ H
2
O  CH
3
CH
2
-NH
3
+ + OH -

The anions from carbonic acid;
CO
3
2and HCO
3
-
• Alkaline properties
• Often water soluble salts
• H
2
CO
3
+ H
2
O  HCO
Acid Base Base
3
+ H
3
O +
Conjugated acid and base pair
Acid
• HCO
3
-
Acid
+ H
2
O  CO
3
2-
Base Base
+ H
3
O +
Conjugated acid and base pair
Acid

Litmus
BTB
Phenolphthalein
Universal paper
Neutral Acidic red
(red) yellow colourless
(red) green colourless
(green)
Basic blue
Blue cerise
(blue)

Some typical reactions of acids- salt formations
• Neutralisation
• Reactions with metals or metal oxides

exothermic, no equilibrium
Acid + base  salt + water
HCl + NaOH  NaCl + H
2
O
H
2
SO
4
+ KOH 
HNO
3
+ Ba(OH)
2


H
2
SO
4
2 HNO
3
+ 2 KOH  K
2
SO
4
+ Ba(OH)
2
+ 2 H
2
O
 Ba(NO
3
)
2
+ 2 H
2
O http://chem-ilp.net/labTechniques/AcidBaseIdicatorSimulation.htm

Acid + metal oxide  salt + water
CuO + 2 HCl  CuCl
2
+ H
2
O
To synthesise a salt from a noble metal you can’t start with metal + acid (Why?)

Carbonates and hydrogen carbonates
Carbonates+ acids  salt+ carbon dioxide+ water
Na
2
CO
3
+ 2 HCl  2 NaCl + CO
2
+ H
2
O
CaCO
3
+ HCl 
KHCO
3
+ H
2
SO
4


Carbonates and hydrogen carbonates
CaCO
3
+ 2 HCl  2 CaCl
2
+ CO
2
+ H
2
O
2 KHCO
3
+ H
2
SO
4
 2 K
2
SO
4
+ 2 CO
2
+ 2 H
2
O

Acid + metal  salt + hydrogen gas
Mg + 2 HCl  MgCl
2
Al + H
Na + H
2
3
SO
PO
4
4
+ H
 ? ?
2
 ? ?
More noble metals (Cu, Ag, Au) doesn’t react with HCl or H
2
They demand more oxidative acids (HNO
3 other gases than H
2
(N
2
O)
SO
4
) and will then give
.

Acid + metal  salt + hydrogen gas
2 Al + 3 H
2
SO
4
 Al
2
(SO
4
)
3
+ 3 H
2
6 Na + 2 H
3
PO
4
 2 Na
3
PO
4
+ 3 H
2

pH = -log[H
3
O + ]

• pH = -log[H
3
O + ] => change in one pH unit = 10 times difference in [H
3
O + ]
• pH=5  pH= 3 => 100 times more acidic.
• pH=8  pH= 11 => 1000 times more basic.
• pH-meter, pH-paper
• [H
3
O + ] = 10 -pH

http://phet.colorado.edu/en/simulation/ph-scale