Bronsted - Lowry Theory
According to this theory,
Acid: a substance or an ion which acts as proton donor, because it can donate a hydrogen ion (proton) to a base.
Base: a substance or an ion which acts as proton acceptor, because it can accept a hydrogen ion (proton) from an acid.
Remember, a chemical equation of acid solution, based on Arrhenius theory.
HCl(g) + water --> HCl(aq)
HCl(aq) --> H+(aq) + Cl-(aq),
Arrhenius stated that pure hydrogen chloride, HCl(g) isn't an acid; however HCl(aq) is an acid. Because when this substance is dissolved in water, its polar covalent bond is broken by water molecules, forms H+ ions and Cl- ions.
Based on Bronsted - Lowry theory, when HCl(g) is dissolved into water, its covalent bond is broken and its proton ( H+ ion) is attracted by water molecule, forms hydronium ion, H3O+ ion. This can be said that HCl(g) acts as proton donor. On the other hand, water molecule, H2O(l) is a base, because it acts as proton acceptor. The equation can be written as follow,
HCl(g) + H2O(l) == Cl-(aq) + H3O+(aq)
Acid-1 Base-2 Base-1 Acid-2
From the above equation, you can see that reaction of acid and base forms acid and base too. Its mean, this reaction is reversible. When Cl-(aq) collides with H3O+(aq), Cl-(aq) attracts a proton (H+) from H3O+(aq); as a result, Cl-(aq) becomes HCl(g) and H3O+(aq) becomes H2O(l). Bronsted - Lowry stated that,
HCl(g) is an acid and Cl-(aq) is a conjugate base, so HCl and Cl- are a conjugate pair.
H2O(l) is a base and H3O+(aq) is conjugate acid, so H2O and H3O+ are a conjugate pair.
Please compare the acid base theories of Arrhenius and Bronsted - Lowry. Actually, both theories have a similarity, that is HCl(g) in water is acid, according to both theories. However, there is a difference between the two theories. The important thing is Arrhenius stated that H+ in water is solvated by water molecules, so he wrote it as H+(aq). This is a simple way in writing the symbol of hydrogen ion. The sign of (aq) is really very important, because it indicates the solvation process of hydrogen ion. This solvation process is also mentioned by BL. Because of this process, BL wrote H+ ion as H3O+(aq).
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