In older conventions scientists called those substances acids :
1) Which had sour teste.
2) Which turned blue litmus red.
3) Yield Hydrogen on reacting with metals.
4) Turn basics neutral.
5) Evolve carbon dioxide on reacting with carbohydrates.
Materials turned bases were those :
1) Which had bitter teste.
2) Which turned red litmus to blue.
3) Which reacts with acids to make them neutral.
Arrhenius Theory of acid and base :
Acids are materials which release hydrogen ion [ H+ ] on dissolving in water and bases are material which release hydroxide ion [ OH ¯ ] on dissolving in water. According to this theory the neutralization of acid and base is the reaction between H+and OH ¯ ions.
This theory could explain only aqueous solution. It can not explain the properties of insoluble acid and bases.
Concept of acid base Brønsted–Lowry :
Bronsted and Lowry gave this concept in 1923. According to it acid is the material which can provide proton [ H+] to other material i.e. acid is proton donor.
HCl ⟶ H+ + Cl ¯
(Acid) (proton)
HNO3 ⟶ H+ + NO3 ¯
(Acid) (proton)
Base is the material which can accept proton from acid i.e. base is proton acceptor.
OH ¯ + H+ ➝ H₂O
(Base) (proton)
NH3 + H+ ➝ NH4+
(Base) (proton)
Conjugate Acid and Base :
The base formed by rejection of a proton by the base is called the conjugate acid of that base. Conjugate acid and base differ by one proton.
In this reaction HCl donates one proton and becomes Cl ¯ . H₂O obtain one proton and convert into H3O+ . According to Bronsted-Lowry concept HCl is acid and H2O is a base. The base Cl- formed by rejection of proton by HCl is conjugate base and H3O+ acid formed by acceptance of proton by H2O is a Conjugate acid.
Lewis Concept of acid and base :
According to the Lewis, acids are substances which have a tendency to gain alone pair of electron and bases are the substances which have a tendency to donate a lone pair of electron.
Based on the concept of material which could not be classified as acids or bases were classified using other concept.
Example : The reaction of Ammonia and Boron tri fluoride is and acid-base reaction.
In the above example ammonia gives a lone pair of electrons and boron tri fluoride accepts the election pair. So ammonia is Lewis base and boron tri fluoride is Lewis acid. Similarly hydroxyl ion [ OH ] is Lewis base because here oxygen can donate it's lone election pair to proton H+ .
H+ + OH ¯ ➝ H - O - H
Some example of Lewis base and Lewis acid are ➝
Lewis acid : NH4 + , SO3H+ , Br+ , NO2+ , NO+ , AlCl3 , FeCl3 , SO3 etc.
Lewis base : OH ¯ , OR ¯ , NH2 ¯ , Cl ¯ , HOH , ROH , R-O-R , RNH2 , R2N H etc.
Written by : Jayanta Kumar Meher (BSc, BEd)
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