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The Arrhenius theory
ACID: Substance that produces H+1 in water.
BASE: Substance that produces OH-1 in water.
Acids are substances which produce hydrogen ions in solution.
Bases are substances which produce hydroxide ions in solution.
Neutralisation happens because hydrogen ions and hydroxide ions react to produce water.
The Bronsted-Lowry theory
ACID: Substance that can donate proton (H+1).
BASE: Substance that can accept proton (must contain lone pair of electrons).
An acid is a proton (hydrogen ion) donor.
A base is a proton (hydrogen ion) acceptor.
The Bronsted-Lowry theory doesn't go against the Arrhenius theory in any way - it just adds
to it. Hydroxide ions are still bases because they accept hydrogen ions from acids and form
water. An acid produces hydrogen ions in solution because it reacts with the water
molecules by giving a proton to them. When hydrogen chloride gas dissolves in water to
produce hydrochloric acid, the hydrogen chloride molecule gives a proton (a hydrogen ion)
to a water molecule. A co-ordinate covalent (two atoms sharing a pair of electrons, held
together because the electron pair is attracted by both of the nuclei) bond is formed
between one of the lone pairs on the oxygen and the hydrogen from the HCl. Hydroxonium
ions, H3O+, are produced.
When an acid in solution reacts with a base, what is actually functioning as the acid is the
hydroxonium ion. For example, a proton is transferred from a hydroxonium ion to a
hydroxide ion to make water.
The Lewis theory
ACID: Substance that can accept a pair of electrons from another atom to form a new bond.
BASE: Substance that can donate a pair of electrons to another atom to form a new bond.
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An acid is an electron pair acceptor.
A base is an electron pair donor.
The relationship between the Lewis theory and the Bronsted-Lowry theory is easiest to see
the relationship by looking at exactly what Bronsted-Lowry bases do when they accept
hydrogen ions. These Bronsted-Lowry bases of hydroxide ions and water are typical of all the
The Bronsted-Lowry theory says that they are acting as bases because they are combining
with hydrogen ions.…read more