Acids have a sour taste, vinegar gives the taste of
acetic acid and lemons and other citrous fruits contain
They change the colour of blue litmus to red.
Acids react with certain metals, such as (Zn, Mg and Fe)
to produce (H2) gas, for example, when dilute
(HCL) reacts with (Zn) metal, .produces (H2)
Zn(s) + 2HCl(aq)
------à ZnCl2(s) + H2(g)
Acids react with carbonates and bicarbonates, such as Na2
CO3, CaCO3 and NaHCO3
to produce CO2 gas.
CaCO3(s) + 2HCl(aq)
CaCl2(aq) +H2O(l) + CO2(g)
NaHCO3(s) + HCl(aq)
NaCl(aq) +H2O(l) + CO2(g)
Acids react with oxides and hydroxide of metals, forming
salt and water.
FeO(s) + 2HCl(aq)
------à FeCl2(aq) +
Fe(OH)3(s) + 3HCl(aq)
FeCl3(aq) + 3H2O(l)
Aqueous acid solutions, conduct electricity.
They react with bases to form salts and water.
Bases have bitter taste.
Bases have slippery touch.
They change the colour of red litmus to blue
Aqueous solutions of bases, conduct electricity.
They react with acids to form salts. When acids and
bases are mixed in right proportions, the acidic and
basic properties disappear and new substances salt and
water are formed and the reaction is neutralization
9.2.1 Acids and Bases (Arrhenius
Svante-Arrhenius, a Swedish chemist in (1887) first gave
the clue to chemical nature of acids in his theory of
ionization. It states, that an acid can be defined as a
substance that yields hydrogen (H+) ions when
dissolved in water. A base can be defined as a substance
that yields hydroxide (OH-) ions, when
dissolved in water. Thus (HCL) and (H2SO4)
are acids and (NaOH) and (KOH) are bases.
< ----- > H+(aq) + CI (aq)
< ------ > Na+(aq) + OH (aq)
Bronsted Lowry Theory (Proton-Donor and Acceptor
Arrhenius's definitions of acids and bases are limited
in that, they apply only to water (aqueous) solutions
and it also does not account for the basicity of ammonia
(NH3), that doesn't contain (OH) group.
Broader definitions, which were proposed by the Danish
Chemist Johannes and English Chemist Thomas Lowry, in
1923, describe, that An acid is a substance having a
tendency to donate one or more protons and is substance,
having a tendency to accept (add) protons.
Bronsted-Lowry acid = A substance that can donate (H+)
Bronsted-Lowry base = A substance that can accept (H+)
In the above examples, the (HCL) and water (H2O),
are proton donors and act as Bronsted-Lowry acids,
whereas (H2O) and ammonia (NH3)
are proton acceptors and are known as Bronsted Lowry
LEWIS CONCEPT OF ACIDS AND BASES
Electronic explanation of Acids and
In 1923 G. N. Lewis proposed a more general concept of
acids and bases. It explains the behaviour of acids in
terms of electrons rather than protons, since electrons
are responsible for chemical bonding. Lewis acids
include not only (H+) protons, but also other
cations and neutral molecules, having vacant valence
orbitals that can accept a pair of electrons donated by
According to the Lewis theory, an acid is any species
(molecule or ion) which can accept a pair of electrons,
and base is any species (molecule or ion) which can
donate a pair of electrons. An acid-base reaction, in
which electron pair donor is base and electron-pair
acceptor is acid, they form a co-ordinate covalent bond
between the two.
Lewis Acid = An electron pair acceptor.
Lewis Base = An electron pair donor.
For example: when ammonia (NH3)
reacts with proton (H+) to form
ammonium ion (NH+4), in
which the nitrogen of (NH3) donates a pair of
electrons whereas the (H+) accepts that pair
of electrons for bond formation, this is shown by curved
Another example is provided by the reaction of ammonia
(NH3) with boron tri-fluoride (BF3),
in which nitrogen of (NH3.) donate an
electron pair and (B) of BF3, which lacks a
pair of electrons to complete its outer most shell
(octet), accepts that pair of electrons and form a
co-ordinate covalent bond.
If we add an acid to a base drop by drop, the acidic
character of the acid decreases gradually. A stage will
come when the resultant solution will become neutral to
litmus. This stage is called neutralization. In other
words when equivalent quantities of an acid and a base
are mixed, salt and water are formed. A common example
is the reaction between HCL and NaOH.
HCl(aq) + NaOH(aq)
< ----- > NaCl(aq) + H2O(l)
The hydrogen ion (H+) which is responsible
for acidic properties reacts with the hydroxide ion (OH-),
which is responsible for the basic properties, producing
neutral water (H-OH). Because the only change that takes
place is the reaction of the hydrogen (H+)
ion and hydroxide ion (OH-), the
neutralization may be expressed simply as:
H+ + OH-
The neutralization is an exothermic reaction.
E.g. HCl (aq) + NaOH (aq)
NaCl (aq) + H2O(l) + 13700 cal/mol
In case that either acid or base is not completely
ionized, i.e. (weak acid or weak base) the
neutralization reaction may not go to completion and the
heat of neutralization may be less than 13700 cal. For
example, when strong acid (HCl) reacts with weak base Ca
(OH)2, the amount of heat evolved is 24700
2 HCl + Ca (OH)2 ------à
CaCl2 + 2H2O + 24700 cals
Fig. 9.1 shows when solutions of hydrochloric acid and
sodium hydroxide are mixed, only hydrogen ions and
hydroxide ions react forming water. Therefore we may
write the equation: H+(aq) + OH-(aq)
A reaction in
which an acid and a base form an ionic compound
(salt and water) is called neutralization reaction.
If water is formed, the reaction can be classified
as a double-replacement reaction.
9.2.3 Mono and Poly Acids and Bases (Mono Basic and Poly
Basic Acids) (Basicity of Acids)
As we have seen that an acid yields the protons.
Different acids have different number of protons
(acidic-hydrogen) per molecule and yield different
number of (H3O+) ion in a
The common acids like (HCl) (HNO3) and (CH3COOH)
contain only one acidic hydrogen atom per molecule. When
dissolved in water 1 mole of each of these acids is
capable of producing 1 mole of hydrated FT = (H3O+)
ions, and in order to neutralize this solution 1 mole of
(OH-) ions is required. Consequently these
acids are called mono-basic acids more commonly called
E.g. H+δ-Cl-δ(aq) + H+δ
– OH-δ(l) ----- >H3O+ (aq)
The number of replaceable or ionizable hydrogen atoms
present in molecule of an acid is called 'Basicity" of
the acid. Sulphuric acid (H2SO4)
contains two acidic hydrogen atoms per molecule. It can
neutralize two molecules of hydroxide (OH) ions.
Consequently sulphuric acid (H2SO4)
is called dibasic acid (Diprotic acid). Sulphuric acid
(H2SO4) dissociates in two steps:
H2SO4 (aq) + H2O (l)
< ----- > H3O+ (aq) + HSO-4(aq)
HSO-4 (aq) + H2O (l)
< ----- > H3O+ (aq) + SO-24(aq)
The acid like phosphoric acid (H3PO4),
which contains three acidic hydrogen atoms per molecule
can neutralize three molecules of (OH-)ions,
so phosphoric acid (H3PO4) is
called tri basic acid or (Tri-protic acid).
H3PO4 (aq) + H2O (l)
< ----- > H3O+ (aq) + H2PO-14(aq)
H2PO-14(aq) + H2O
< ----- > H3O+ (aq) +
H2PO-24(aq) + H2O
< ----- > H3O+ (aq) + PO-34(aq)
Acids that contain two, three or more acidic hydrogen
per molecule are called poly-basic acids, or more
commonly poly-protic acids.
9.2.4 Mono-Acid and Poly-Acid Bases (Acidity of Bases):
Similarly, bases that produce 1 mole of (OH") ions per
mole of base (such as NaOH and KOH) are called mono-acid
bases. Bases that produce 2 moles of (OH) ions per one
mole of base (such as Ca(OH)2 and Ba(OH)2
) are called tri-acid bases, and bases that produce 3
moles of (OH-) ions per 1 mole of base (such
as Al (OH)3 and Cr (OH)3 are
called tri-acid bases. The number of ionizable or
replaceable (-OH-) ions, present in a
molecule of base is called acidity of the base. Bases
that contain two three or more hydroxide (OH-)
ions per molecule are called poly-acid bases.