Difference Between Amphiprotic and Amphoteric

Amphiprotic vs Amphoteric
 

As amphiprotic and amphoteric are quite similar, the difference between amphiprotic and amphoteric too is quite confusing. Both of the terms, amphiprotic and amphoteric, are both related to acid – base chemistry. Amphoteric substances behaves as an acid and as a base. All amphiprotic substances are able to donate and accept protons and can show both acid and base properties. Therefore, they are amphoteric too. This article describes the difference between amphiprotic substances and amphoteric substances in detail. Moreover, it gives examples and reactions to show their properties.

What are Amphiprotic Substances?

The term amphiprotic refers to the substances that can both accept and donate a proton; it can be either ionic or covalent. Therefore, an amphoteric substance should have two main properties.

- The molecule must contain at least one hydrogen atom and it can be donated to another molecule.

- The molecule must contain a lone pair of electrons (electrons that are not involved in chemical bonding) to accept a proton.

Water (H2O) is of the most common of amphiprotic substances; a water molecule satisfies both requirements needed for an amphiprotic substance.

Difference Between Amphiprotic and Amphoteric

In addition to water, most of conjugated bases of diprotic acids can act as amphiprotic substances.

Diprotic Acid                         Conjugate Base

H2SO4                                                  HSO4-
H2CO3                                                  HCO3-
H2S                                                        HS-
H2CrO3                                                 HCrO3-

Example: Carbonic acid (H2CO3) is a weak diprotic acid, bicarbonate (HCO3-) is its conjugate base. In aqueous solutions, bicarbonate shows two types of reactions.

(1) Donating a proton to water (as a bronsted – Lowry acid)

HCO3- (aq) + H2O —> H3O+ (aq) + CO32- (aq)

(2) Accepting a proton from water (as a bronsted – Lowry base)

HCO3- (aq) + H2O —> H2CO3 (aq) + OH- (aq)

Therefore, bicarbonate (HCO3-) is an amphiprotic species.

What are Amphoteric Substances?

Substances that can act as both acid and base are called amphoteric substances. This definition is quite similar to amphiprotic substances. Because, all amphiprotic substances show acidic properties by donating a proton and similarly, they show basic properties by accepting a proton. Therefore, all the amphiprotic substances can be considered as amphoteric. However, the reverse statement is not always true.

We have three theories for acids and bases:

Theory                          Acid                                           Base
Arrhenius                    H+ producer                              OH- producer
Bronsted-Lowry         H+ donor                                    H+ accepter
Lewis                            electron pair acceptor             electron pair donor

Example: Al2O3 is a Lewis acid and a Lewis base. Therefore, it is an amphoteric substance, since it does not contain protons (H+), it is not an amphiprotic substance.

Al2O3 as a base:

Al2O3 + 6 HCl  —>   2 AlCl3 + 3 H2O

Al2O3 as an acid:

Al2O3 + 2NaOH + 3 H2O   —>  NaAl(OH)4

What is the difference between Amphiprotic and Amphoteric?

• An amphiprotic substance behaves as an acid and as a base. An amphoteric substance can accept or donate a proton (H+ ion).

• All amphoteric substances are amphiprotic, but all amphiprotic substances are not amphoteric.

• Amphiprotic species consider the ability to donate or to accept a proton. However, amphoteric species consider the ability to act as an acid and as a base. Acid-base properties depend on three factors including the ability to donate or accept a proton.

If a substance possesses an electron pair to donate and it has the ability to accept an electron pair is considered as amphoteric.

If a substance has the ability to produce both H+ ion and OH- ion, it is considered as amphoteric.

Summary:

Amphiprotic vs Amphoteric

Amphoteric and amphiprotic substances are related to the acid-base chemistry. Both of these substances show acid and base properties. In other words, they can react as an acid and as a base depending on the other reactants. Amphiprotic substances can donate and accept a proton. Water is the most common example for an amphiprotic species. Most of the conjugated bases of diprotic acids also amphiprotic. Amphoteric substances can behave as an acid and as a base.