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Prepared by Haq Nawaz (M.Phil, MSc (UK), Applied chemistry with distinction UK)
Chemistry Chapter No : (10)
Class 10th
Acid:
An acid is a substance which provides hydrogen (H+ ) ions in aqueous solution.
Base:
Base is a substance which provides hydroxyl (OH- ) ions in aqueous solution
Example: NaOH, KOH
DIFFERENT CONCEPTS OF ACID AND BASES.
ARHENIUS CONCEPT OF ACID AND BASE
According to Arrhenius, acids are those substances which give hydrogen ions (
H+ ) in aqueous solution and bases are those substances which give hydroxyl
ions (OH-) in aqueous solution.
Explanation: In 1884, Arhenius studied different compounds and found that
some compounds separate into their individual ions when dissolved in water.
According to him, those compounds which contain hydrogen and when
dissolved in water release hydrogen ions are acids.
Example: HCl is an acid because gives hydrogen ions when dissolved in water
HCl + H2O ------ H3O+ + ClSimilarly, those compounds which give hydroxide ions when dissolved in water
are bases.
Example: NaOH and KOH are bases because give hydroxide ions when dissolved
in water
NaOH -- Na+ + OHKOH-- K+ + OHLOWRY AND BRONSTED CONCEPT
According to Lowry & Bronsted concept, an acid is a compound or species
which donates or tends to donate protons while base is a compound or species,
which accepts, tends to accept protons in aqueous solution. In short words,
acids are protons donor and bases are protons acceptor where proton means H+
ions.
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Example: HCl , HNO3 , H2SO4 etc.
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For example:
CH3COOH + H2O -CH3COO- + H3O+
Acid 1
Base 1
BaseII
acid II
In the above example, acetic acid donates a proton(H+) to water so acetic acid
accept proton so base and hydronium ion can donate proton so acid.
Conjugate acid and base.
When a base accepts proton, it forms positive charged species which can act as
an acid (proton donar) and called conjugate acid of the corresponding base.
Similarly when an acid donates proton, it forms negative charged species
which can act as a base (proton acceptor) and called conjugate base of the
corresponding acid.
For example:
CH3COOH + H2O -CH3COO- + H3O+
Acid
Base
conjugate base
conjugate acid
In the above example, acetic acid donates a proton(H+) and forms acetate
ions (CH3COO- ) which can accept proton and acts as a base so conjugate base
of acetic acid. Similarly water accepts proton and form hydronium ions (H3O+)
which can donate proton and acts as an acid so conjugate acid of water.
Monoprotic acids. Those acids which can donate only one proton (H+)
are
called monoprotic acids.
For example, HCl, HNO3, HBr, HCN etc
Polyprotic acids. Those acids which can donate two or more than two protons
(H+)
are called polyprotic acids.
For example, H2SO4, H3PO4
Amphoteric substances: Those substances which can act both as an acid and a
base are called amphoteric substances.
For example, Water is an amphoteric substance because when treated with an
acid it acts as a base but when treated with a base it acts as an acid.
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is an acid and water is a base but on the right hand side acetate ions can
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The Lewis Concept:
According to Lewis, Acids are those substances which can accept a pair of
electrons, while bases are those substances which can donate a pair of
electrons.In other words, acids are electron pair acceptors while bases are
electron pair donars.

Substances having positive ions or less than eight electrons in the valence
shell of central atoms are acids. Similarly thoses substances having
negative ions or lone pair electron in the valence shell of central atoms

Co-ordinate covalent bond is formed between donar (base) and acceptor
(acid)
For example,
Strength of acids: The ability of acids to give hydrogen ions (H+) in water is
called strength of acids.
It depends upon the concentration of hydrogen ions in water due to which it
has two types.
1. Strong acids: Those acids which completely ionize in water and give
higher concentration of hydrogen ions (H+) are called strong acids.
For example, HCl is a strong acid because when dissolved in water it completely
dissociates in water.
Other examples are HNO3, H2SO4
2. Weak acids: Those acids which do not completely ionize in water and
give lower concentration of hydrogen ions (H+) are called weak acids.
For example, acetic acid (CH3COOH)
is a weak acid because when dissolved in
water only a few molecules dissociates in water to give hydrogen ions.
Other examples are H2CO3, H3PO4
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are bases.
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Strength of bases: The ability of bases to give hydroxyl ions (OH-) in water is
called strength of bases.
It depends upon the concentration of hydroxyl ions (OH-) in water due to
which it has two types.
1. Strong bases: Those bases which completely ionize in water and give
higher concentration of hydroxyl ions (OH-) are called strong bases.
completely dissociates in water.
Other examples are KOH, RbOH
2. Weak bases: Those bases which do not completely ionize in water and give
lower concentration of hydroxyl ions (OH-) are called weak bases.
For example,NH4OH
is a weak base because when dissolved in water only
partially dissociates in water to give hydroxyl ions (OH-).
PH : The logarithm of the reciprocal of molar hydrogen ions concentration is
called PH.
PH
=
log
1
[𝐻 + ]
OR
PH
=
log 1 - log [𝐻 + ]
(as log 1= 0)
= 0- log [𝐻 + ] = -log [𝐻 + ]
PH = -log [𝐻 + ]
On the basis of above equation PH can also be defined as “ The negative
logarithm of molar hydrogen ions concentration is called PH.
POH : The logarithm of the reciprocal of molar hydroxyl ions (OH-)
concentration is called POH.
POH
=
log
1
[𝑂𝐻 − ]
OR
POH
=
log 1 - log [𝑂𝐻 − ]
(as log 1= 0)
= 0- log [𝑂𝐻 − ]= - log [𝑂𝐻 − ]
POH = - log [𝑂𝐻 − ]
On the basis of above equation POH can also be defined as “ The negative
logarithm of molar hydroxyl ions (OH-) concentration is called POH.
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For example, NaOH is a strong base because when dissolved in water it
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PH scale or Hydrogen ion concentration or prove that Kw = 10-14
The acidity of aqueous solution depends upon the relative number of hydrogen
ions (H+) present in it. Similarly the basicity of aqueous solution depends upon
the relative number of hydroxyl ions (OH-) present in it. Pure water contains
Applying the law of mass action
Kc =
[𝐻 + ][𝑂𝐻 − ]
[𝐻2 𝑂]
Kc x [𝐻2 𝑂]= [𝐻 + ] [𝑂𝐻 − ]
As the concentration of water remains constant at constant temperature so Kc
x [𝐻2 𝑂]= Kw
Kw = [𝐻 ∓ ] [𝑂𝐻 − ] eq-1
It has been observed that water molecules are very stable and only one
molecule of water out of 107 molecules dissociates to form H+ and OH- at 250C.
Therefore
1
Concentration of hydrogen ions [𝐻 + ] =
107
Concentration of hydroxyl ions [𝑂𝐻 − ] =
107
1
= 10-7
= 10-7
Put the values of hydrogen ions and hydroxyl ions in equation 1.
Kw = [𝐻 − ] [𝑂𝐻 − ] =10-7 x 10-7 = 10-14
So
Kw = 10-14
From above value it is clear that in water the product of hydronium ions
concentration and hydroxide ions concentration is always 10-14 at 25C and
called water dissociation constant Kw.
Q. Prove that PH + POH = 14
As we know that
Kw = [𝐻 − ] [𝑂𝐻 − ] = 10-14
or
[𝐻 − ] [𝑂𝐻 − ] = 10-14
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equal number of hydrogen and hydroxyl ions.
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Take log both sides
log [𝐻 − ] [𝑂𝐻 − ] = log 10-14
log [𝐻 − ] + 𝑙𝑜𝑔[𝑂𝐻 − ] = -14 log10
as log10=1
log [𝐻 − ] + 𝑙𝑜𝑔[𝑂𝐻 − ] = -14
Multiply both sides by minus sign
-log [𝐻 − ] + (−𝑙𝑜𝑔[𝑂𝐻 − ] )= 14
but PH = -log [𝐻 − ] and POH =−𝑙𝑜𝑔[𝑂𝐻 − ] by putting values in above equation
PH + POH = 14
For neutral substances the sum of PH and POH is 14 so PH = 7 and POH = 7
When the PH of the solution is below 7, the solution is acidic but when above 7
the solution is basic. The PH of whole number below 7 is ten times more acidic
than the next higher value. For example, a PH of 3 is ten times more acidic
than PH 4 and 100 times more acidic than PH 5. Similarly, a PH of whole
number above 7 is ten times more alkaline than the next lower value. For
example, a PH of 9 is ten times more alkaline than PH 8.
Salts:
A solid substance formed as a result of neutralization reaction of an acid and
base is called salt. A salt contains positive ion from the base and negative ion
from the acid.
For example,
NaOH + HCl ---- NaCl + H2O
In above example, the NaCl is formed by combination of sodium ion from base
with chlorine ion from acid.
Other examples of salt are AgBr, KCl, K2SO4
Generally salts are ionic compound. Soluble salts are called electrolytes.
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-log [𝐻 − ] − 𝑙𝑜𝑔[𝑂𝐻 − ] =-( -14)
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Preparation of salts:
Salts can be prepared by different ways. Some of the methods are given below.
A. By the neutralization of acids and bases.
NaOH + HCl ---- NaCl + H2O
2KOH + H2SO4 - K2SO4 + H2O
Zn + 2HCl -- ZnCl2 + H2
Types of salts:
On the basis of neutralization reactions, salts are divided into the
following types.
A. Normal salt: The salts formed when the hydrogen ions of an acid are
completely replaced by metal ions or a group of atom behaving like
metal ions are called normal salts. It is normally formed by the
complete neutralization reaction of strong acids and bases.
For example, NaOH + HCl ---- NaCl + H2O
Similarly, potassium sulphate, sodium sulphate,
B. Acidic salts: Salts formed when hydrogen ions of an acid are partially
replaced by metal ions or a group of atom behaving like metal ions are
called acidic salts.
KOH + H2SO4 - KHSO4 + H2O
Acidic salts are formed by polybasic (polyprotic) acids only
C. Basic Salts: Salts formed when hydroxyl ions (OH-) of a base are
partially replaced by an acids are called basic salts
Basic salts are produced by poly acid (polyhydroxy) bases.
Pb(OH)2 + HCl - Pb(OH)Cl + H2O
Uses of Salts:
1. Sodium carbonate (Na2CO3) is used as a cleaning agent, water softer,
raw material in glass manufacturing. Similarly it is used in paper
industry, petroleum refining industry and leather industry.
2. Sodium bicarbonate (NaHCO3) also called baking soda is used in
baking, confectionaries, medicine and toothpaste.
3. Copper sulphate (CuSO4.5H2O) is used in electroplating process for
copper plating, killing algae in water reservoir and agriculture spray.
4. Magnesium sulphate (MgSO4.7H2O) also called Epsom salt is used in
medicine and dye industries.
5. Alum or potash alum K2SO4 Al2 (SO4)3.24H2O is used for water
purification to remove suspended impurities, in textile industries,
medicine, blood coagulant in small injuries.
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B. By the action of acids with metal
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Neutralization:
The process in which acids react with bases to form salt and water is
called neutralization reaction.
For example, when hydrochloric acid reacts with sodium hydroxide, it
forms salt and water the reaction is called neutralization reaction.
Common Ion effect:
The process in which the solubility of weak electrolyte in solution is
decreased by the addition of strong electrolyte having one ion common is
called common ion effect.
For example, when KCl (strong electrolyte) is added to the solution of
KClO3(weak electrolyte), the solubility of KClO3 decreases. The decreased
in the solubility is due to common ion effect (K+).
Example 2. When HCl is added to solution of H2S, they produce H+ ions as
common.
The solubility of H2S is depressed because it is a weak acid and HCl is a
strong acid, so less S-2 ions are produced
Exercise
Q1. i.a) proton donar ii. b) base iii. c) 7 iv.
b) stronger v. d) neutral vi.
b. Alkalies vii. b) acidic.
Q.2
i. Double salts: Those salts which contain more than one cation or anion
and obtained by crystallization of two different salts are called double
salts.
Examples: Potash alum ( K2SO4.Al2(SO4)3.24H2O ), chrome alum
K2SO4.Cr2(SO4)3.24H2O
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NaOH + HCl ---- NaCl + H2O
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The main difference between double salts and complex salt is that double
salt when dissolved in water it completely dissociates into simple ions
while complex salts do not dissociate into simple ions.
iii. Difference between lone pair and bond pair of electrons.
Those valence electrons which do not take part in covalent bond
formation are called lone pair electrons which those valence electrons
electrons. In other words, lone pair is unshared pair of valence electrons
and bond pair is shared pair of electrons.
Example. Ammonia has one lone pair and three bond pairs.
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which take part in covalent bond formation are called bond pair