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ACID & BASE THEORY
The theory of acids and bases, like many other chemical theories, has undergone numerous changes in recent times. Always the changes have been such as to make the theory more general.
The three main theories in use today are:
The Water or Arrhenius Theory;
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The Proton or Bronsted- Lowry Theory; The Electronic or Lewis Theory.
Arrhenius theory of acid & base:
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Svante Arrhenius prosposed this theory of acid and base. (Noble prize in 1903)
Acid: Substance that produces hydrogen ion (H+ ion) in an aqueous solution.
e.g.- HCl H+ + Cl-
+ 2- H2SO4 2H + SO4
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Base: Substance that produces hydroxide ion (OH– ion) in an aqueous solution.
e.g.- NaOH Na+ + OH-
KOH K+ + OH-
Multiple Choice
Q. Which acid and base are not defined by Arrhenius theory?
A. HCl B. H2SO4
C. BCl3 D. NH3
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Ans. C,D
Limitations of Arrhenius theory:
can’t explain the acidic nature of substance which don’t contain hydrogen. We know generally metal oxides
are acidic in nature, CO2, SO2
can’t explain the basic nature of substance basic in nature, CaO,
K2O
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Brønsted-Lowry Theory:
J.N Brønsted & T.M Lowry prosposed this theory of acid and base.
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(Broder concept of acid & base in order to overcome draback of Arrhenius theory)
Note: Hudrogen ion (H+ ion) also termed as proton.
Acid: Substance that release H+ ion or simply proton donars
e.g.- HCl H+ + Cl-
+ 2- H2SO4 2H + SO4
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Base: Substance that accepts H+ ion or proton acceptors.
+ + e.g.- NH3 + H NH4
Note:
+ + NH3 + H NH4 Base Conguate acid
HCl H+ + Cl-
Base Conjugate base
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Q. Write conjugate base of HBr
Ans. Br–
Q. Write conjugate acid of H2O
+ Ans. H3O
Amphoteric species : An amphoteric substance is one that can act as either an acid or a base.
Eaxmple : Zinc, Al etc.
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Q. How H2O is an amphoteric species ?
Ans.
H234 ONHNHOH acidconjugatebase conjugate acid base
H23 O HCl H O Cl base acidconjugate conjugate acid base
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Lewis Theory:
A c
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id: electron pair acceptor
+ + e.g.- BCl3, H , NH4 etc
Base: electron pair donars.
e.g.- NH3, CLASSIFICATION OF ACID
1. Classification based on the source: Organic acid: acid obtained from organic materials such as plants and animals. For e.g. Citric acid (Citrus
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fruits), Acetic acid (Vinegar), Oleic acid (Olive oil), Acitic acid, Propionic acid, Butyric acid etc..
All members of (-COOH) group are acids
Mineral acid/inorganic acid: Mineral acid is procured from minerals. They are also known as inorganic acids. They do not contain
carbon. For e.g. H2SO4, HCl. HNO3, etc.
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2. Classification based on presence of oxygen: Oxy acid: Acid that contain both hydrogen and oxygen
e.g- HCl, H2SO4, H3PO4,
Hydra acid: Acid that contains hydrogen and other nonmetallic elements except oxygen. e.g- HCl, HCN, HBr, HI etc.
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Q. Which is not a hydra acid ?
A. HCl
B. HI
C. HBr
D. None
3. Classification on the basis of concentration: Concentrated acid: aqueous solution that has high % of acid
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Dilute acid: aqueous solution that has low % of acid
4. Classification on the basis of basicity: Number of H+ ions produced by ionization of one molecule of an acid in aqueous solution is called its basicity.
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a. Monobasic acid:
Acid on dissociation in aqueous medium produce one H+ ion per molecule of the acid.
e.g- HCl
b. Dibasic acid:
Acid on dissociation in aqueous medium produce otwo H+ ion per molecule of the acid.
e.g- H2SO4
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c. Tribasic acid:
Acid on dissociation in aqueous medium produce three H+ ion per molecule of the acid.
e.g- H3PO4
d. Tetrbasic acid:
Acid on dissociation in aqueous medium produce four H+ ion per molecule of the acid.
e.g- H4SiO4
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Q. Which one is monobasic acid ?
A. HCl
B. H2SO4
C. H3PO4
D. H3PO3
Q. Which is dibasic acid ?
A. H3PO3
B. H3PO4
C. HCl
D. None
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5. Classification on the basis Strong acid, weak acid and Strong base, weak base. Strong Acid: Acid that ionise
completely. e.g- HCl, HNO3, H2SO4
Weak Acid: Acid that ionise
partially. e.g- CH3COOH, HCOOH,
H2C2O4
Strong Base: Base that ionise completely. e.g- NaOH, KOH
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Weak Base: Base that ionise
partially. e.g- NH4OH
Acid:
Cation : H+
2- 2- 2- - - Anion: CO3 , SO4 ,CO3 ,NO3 , HCO3 ,
- 2- - - - HSO4 ,SO3 ,HSO3 , CH3COO , HCOO ,
- - - - - 2- 2- Cl , Br , I , F , CN , CrO4 , MnO4 etc.
Q. Which is not a acid ? A. KCN B. HCl
C. HBr D. HNO3
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Q. Which is not a base ?
A. NH3 B. NaOH C. H+ D. NaCl
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