Acids
Prepared by Alrick Moodie
February 2007
Objectives
The students should be able to :
a) define an acid
b) recall the properties of an acid
c) distinguish between the concentration
and strength of an acid
d) recall the major reactions of acids
e) know the formula and names of major
mineral and organic acids
Definition
An acid is a solution which has
hydrogen ions , H+ as its’ only
positive ion .
An acid anhydride is a substance
that when dissolved in water forms
an acid .
The word anhydride means without water .
H2SO4 is an acid
H2SO4 2H+ + SO4 2-
Note that H+ are the only positive ions
formed .
Properties of Acids
Physical properties
• Acids are sour
• They are clear , colourless liquids
• They are corrosive
Properties of Acids [cont’d]
Chemical Properties
• They react with alkalis/bases to form
a salt and water
• They react with reactive metals to
form a salt and hydrogen gas
• They react with carbonates to form a
salt , water and carbon dioxide
Properties of Acids [cont’d]
Chemical Properties
• They turn moist litmus red
Acid Strength
The concentration of an acid is the number
of moles of the acid in solution . Dilute
acids have a low molar concentration .
The strength of an acid is the extent to
which the acids separate into ions .
An acid in solution dissociates into H+ ions
and anions .
Some acids completely break down into ions
, these are strong acids .
e.g. if you have 100 molecules of HCl (aq) all
100 molecules will break down into H+ and
Cl- .
Weak acids only break down partially
e.g. if you had 100 molecules of vinegar only
15 molecules will break down .
CH3COOH (aq) CH3COO- + H+
85 mol 15 mol 15 mol
Common Acids
• Mineral Acids
sulphuric acid H2SO4
hydrochloric acid HCl
nitric acid HNO3
phosphoric acid H3PO4
carbonic acid H2CO3
Common Acids
• Organic Acids
methanoic acid HCOOH
ethanoic acid CH3COOH
also called vinegar , acetic acid
Common Acids
lactic acid - found in spoilt milk
formic acid - found in ants sting
salicylic acid - found in aspirin
gastric acid - found in the stomach
(mostly hydrochloric acid)
Basicity of acids
The basicity of an acid is the number of
hydrogen ions liberated per molecule of
acid .
What is the basicity of the following acids ?
H2SO4
HCl
HNO3
H3PO4
H2CO3
H2SO4 =2
HCl =1
HNO3 =1
H3PO4 =3
H2CO3 =2
Preparation of acids
Acids can be prepared by
dissolving a non-metal oxide in
water .
Sulphur dioxide forms sulphurous acid
SO2 + H2O H2SO3
Sulphur trioxide forms sulphuric acid
SO3 + H2O H2SO4
Nitrogen dioxide forms nitric acid and nitrous acid
NO2 + H2O HNO3 + HNO2
Phosphorous (V) oxide forms phosphoric acid
P 2O 5 + H 2 O H3PO4
Uses of Acids
Sulphuric
• Manufacture of fertilizers
• Manufacture of detergents and soaps
• Manufacture of paint pigments
• Vulcanization of rubber
Uses of Acids
Nitric
• Manufacture of fertilizers
• Manufacture of explosives
• Manufacture of dyes
• Cleaning jewellery
Uses of Acids
Hydrochloric
• Manufacture of plastics