ACID AND BASES
Definition and properties of Acid:
Acid is defined as a substance whose aqueous
solution possesses the following characteristic
properties:
1.
It conducts electricity
2.
It reacts with active metal like zinc,
magnesium etc to give hydrogen.
3.
It turns blue litmus red.
4.
It has sour taste.
5.
Its acidic properties disappear on reaction
with a base.
Definition and properties of Base: Base is defined
as a substance whose aqueous solution possess the
following characteristic properties:
1.
It conducts electricity
2.
It turns red litmus blue.
3.
It has bitter taste.
4.
It has soapy(slippery) touch.
5.
Its basic properties disappear on reaction
with an acid.
Arrhenius concept of acid and bases:
proposed by Arrhenius in 1884.
It was
Arrhenius Concept of Acid :
1.
An acid is defined as a substance which contains
hydrogen ion.
2.
When dissolved in water it gives hydrogen ion.
3.
Example HC l(hydrochloric acid), HNO3 ( nitric
acid).
4. The acid which dissociate completely into H + and
the negative ions are called strong acid. Example
HCl, HNO3 .
HCl water
H+(aq) + Cl- (aq)
5. Some acid which do not dissociate
completely into H+ ion and negative ion are
called weak acid. Example CH3COOH(acetic
acid),H2CO3(carbonic acid).
6. There is an equilibrium between ions and
undissociated molecules.
CH3COOH Water
CH3COO- + H+
Arrhenius Concept of Base :
1. A base is defined as a substance which contains hydroxyl
group(OH-).
2. When dissolved in water it gives hydroxyl (OH-)ion.
Example NaOH (sodium hydroxide), NH4OH (ammonium
hydroxide).
3. The base which dissociate completely into OH -and the
positive ions are called strong base. Example NaOH, KOH .
NaOH
water
Na+(aq) + OH- (aq)
4. Some base which do not dissociate completely into OHion and positive ion are called weak base. Example NH4OH
and Al(OH)3.
5. There is an equilibrium between ions and undissociated
molecules.
NH4OH Water NH4+(aq) + OH-(aq)
Neutralization process:
1. It is the process in which hydrogen ions
and hydroxide ions join to form unionized
molecules of water.
2. As a result of this properties of acids and
base are destroyed.
HCl ( water)
H+(aq) + Cl- (aq)
NaOH
water Na+(aq) + OH- (aq)
Therefore
Na+(aq) + OH- (aq) + H+(aq) + Cl- (aq)
Na+(aq) + Cl- (aq) + H2O
Amphoteric substance :
1. The substance which acts as both acid and
base are called amphoteric or amphiprotic
substance.
Example: Water is proton acceptor
HCl + H2O
H3O+
+ ClExample: Water is proton donor.
NH3 + H2O
NH4+ + OH-
Conjugate acid base pair
1. A conjugate pair of acid and a base differs by a proton
(H+) only.
Conjugate acid
Conjugate base + H+
Acid 1
Base2
Acid 2
Base1
HCl + H2O
H3O+
+ ClH2O + NH3
NH4+
+ OH
2. Acid 1 and Base1 are conjugate pair I ( for forward
reaction).
3. Acid 2 and Base2 are conjugate pair II ( backward
reaction).
Lewis concept of acid and base:
1. It was proposed by G.N.Lewis in 1923.
2. It is also called electronic concept.
Lewis acid
1.A Lewis acid is defined as a substance( an
atom, ion or molecule) which is capable of
accepting a pair of electrons .
2.A Lewis acid is electron pair acceptor.
Examples : BF3 , AlCl3 ( neutral molecules)
Examples : Ag+ , Cu+2,Fe+3 ( cations)
Lewis base
1. A Lewis base is defined as a substance( an
atom,ion or molecule) which is capable of
donating a unshared pair of electrons .
2. A Lewis base is electron pair donor.
Examples : NH3 , ROH,H2O, RNH2 ( neutral
molecules).
Examples : F- , Cl-,CN- ,OH-,I-, Br - ( anions).
Reaction between Lewis acid and Lewis base
NH3 is a Lewis base (electron donor) and BF3 is a
Lewis acid (electron acceptor). Reaction between
NH3 a Lewis base (electron donor) and BF3 a
Lewis acid ( electron acceptor) can be shown as :
Strength of Acid and Bases : pH
The pH and pOH scales provide a convenient way
to express the acidity and basicity of dilute
aqueous solutions. The pH and pOH of a
solution are defined as as the negative
logarithm of hydronium(H3O+) concentration in
moles per litre.
Where [ H3O+] represent the number of moles of [ H3O+]
per litre.
Example: If in a solution [ H3O+] = 10 –x moles per litre.
Now according the equation
pH
=
=
=
=
=
- log [ H3O+]
- log 10 -x
- ( - x log 10)
- (- x ) (as log 10 =1)
x
Calculate the pH value of 10-2 molar HNO3solution.
HNO3 solution completely ionizes as
HNO3 + H2O
H3O+ + NO3Now given [ H3O+] = [ HNO3] = 10-2 M
Therefore, pH= -log [ H3O+] = -log(10-2) = (-2 x log 10) = 2
pH scale:
1 pH scale is from 0 to 14.
2 Neutral solution has pH =7.
3 pH of acid solution is always less than 7.
4 pH of basic solution is always greater than 7.
The self-ionization of water:
Water ionizes weakly to produce OH- and H+ ions, where Kw is the
ionization constant of water, according to :
H2O(l) 
H+ (aq) + OH-(aq)
Kw = 10-14
This means that the product of the concentration (molarity) of [H+] and
[OH-] must always be 10-14.
Kw = 10-14 = [H+] [OH-]
pH and pOH
The concentration of [H+] and [OH-] can be expressed as the pH
and pOH: (p in pH or pOH means ‘potency’, and means that any
constant ‘K’ or concentration with a ‘p’ in front of it is
represented as pK = -log K , or pH = -log [H+])
pOH = - log [OH-]
pH = - log [H+]
We can show that:
pH
+
pOH
=
14.0 = pKw