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Theories of Acids and Bases

Theories of Acids and Bases

Define acids and bases according to the Bronsted-Lowry and Lewis theories

 There are two main theories that exist for classifying acids and bases: that of Bronsted and Lowry, and the Lewis theory. Both are used.

The Bronsted-Lowry Theory is that:

An acid is a substance that can donate a proton, or hydrogen ion

A base is a substance that can accept a proton, or hydrogen ion

In the equation above, Cl- is the conjugate base of HCl and H3O+ is the conjugate acid of H2O. That is to say, it is what remains when the molecule has gained or lost a proton.

Interestingly, water (H2O) is able to act as both an acid and a base in Bronsted-Lowry theory, called amphiprotic. Other examples include HCO3- and HCO4-. The substances are able to accept or donate a proton. Alternatively, a substance that can act as both an acid and a base is called amphoteric, such as Al2O3

The Lewis Theory is that:

 

An acid is an electron pair acceptor

 

A base is an electron pair donator

 

Therefore, all Lewis acids are deficient in an electron pair (such as H+, BF3 and SO3), whilst a Lewis base must contain a non-bonding pair of electrons (such as OH-, F- and H2O).

BF3 is a Lewis acid because it does not have any non-bonding pairs of electrons and only three bonding pairs.

Deduce whether or not a species could act as a Bronsted-Lowry and/or a Lewis acid or base

Many acids or bases that do not fit the Bronsted-Lowry definition fit the Lewis definition, along with their reactions. Hence, substances like BF3, SO3, H+ and SbF5 are not Bronsted- Lowry acids, but are Lewis acids. Also, H2O, OH- and F- are not Bronsted-Lowry bases, but are Lewis bases. Many other substances can be classified as both.

 

The reaction below could be classified as both:

From the perspective of a Bronsted-Lowry acid-base reaction, the H+ ion is being donated to the NH3 molecule to form the new complex. As for the Lewis theory, the NH3 molecule has a non-bonding pair of electrons, which are able to bond with the H+ ion.

When classifying a reaction, the Lewis theory is usually only used if the Bronsted-Lowry theory does not apply.

Deduce the formula of the conjugate acid (or base) of any Bronsted-Lowry base (or acid)

Remember that a conjugate acid is the complex formed after a Bronsted-Lowry base has received a proton. On the other hand, a conjugate base is the complex formed after a Bronsted-Lowry acid has donated a proton. Therefore, an acid will always have one more proton than its conjugate base, and vice versa.

When you write the formula of the conjugate acid or base, you must always indicate where the proton is located. For example:

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