Many metals form amphoteric oxides or hydroxides, including copper, zinc, tin, lead, aluminum, and beryllium. The amphoteric characteristic of these oxides depends on the oxidation states of the oxide in question. Examples of these substances are included at the end of the article.
Metal oxides that can react with acids and bases to produce salts and water are known as amphoteric oxides. Lead and zinc oxides are very good examples, among other compounds.
What are amphoteric?
According to the acid-base theory of Bronsted and Lowry, acids are those substances that donate protons, while bases are those that accept or take protons.
An important and widely recognized case is that of the universal solvent, water (H₂O). This substance reacts easily with acids, for example, in reaction with hydrochloric acid:
H 2 O + HCl → H 3 O + + Cl –
But at the same time, it also has no problem reacting with a base, as in the case of ammonia:
H 2 O + NH 3 → NH 4 + OH –
With these examples it can be seen that water fully acts as an amphoteric substance.
Types of amphoteric
Even though amphoteric substances can be molecules or ions, there are molecules that best demonstrate amphoteric characteristics and help to better study this behavior: amphiprotic substances. These are molecules that can specifically donate or accept a proton to act as an acid or base.
It should be clarified that all amphiprotic substances are amphoteric, but not all amphoteric substances are amphoteric; there are amphoters that do not possess protons, but can behave like acids or bases in other ways (as per Lewis’s theory).
Acidic protogenic or amphiprotic substances
They are those that have a greater tendency to give up a proton than to accept one. Among these are sulfuric acid (H 2 SO 4 ) and acetic acid (CH 3 COOH), among others.
Basic protophilic or amphiprotic substances
They are those for which accepting a proton is more common than giving it up. These substances include ammonia (NH 3 ) and ethylenediamide [C 2 H 4 (NH 2 ) 2 ].
They have the same facility or capacity to accept a proton as to give it up. Among these are water (H 2 O) and minor alcohols (-ROH), mainly.
Examples of amphoteric substances
Now, having already described the amphoteric substances, it is necessary to denote the examples of reactions in which these characteristics occur.
The carbonic acid ion presents a basic case of an amphiprotic substance; below are its reactions when it acts as an acid:
HCO 3 – + OH – → CO 3 2- + H 2 O
The following reaction occurs when it acts as a base:
HCO 3 – + H 3 O + → H 2 CO 3
There are also many other substances. Of these there are the following examples:
Zinc oxide, as already mentioned, is an amphoteric but not an amphiprotic substance. The following shows why.
Behaving like acid:
ZnO + H 2 SO 4 → ZnSO 4 + H 2 O
Behaving as a base:
ZnO + 2NaOH + H 2 O → Na 2 [Zn (OH) 4 ]
Lead oxide (PbO), aluminum (Al 2 O 3 ) and tin (SnO) also have their own amphoteric characteristics:
Behaving like acids:
PbO + 2HCl → PbCl 2 + H 2 O
Al 2 O 3 + 6HCl → 2AlCl 3 + 3H 2 O
SnO + HCl ↔ SnCl + H 2 O
And as bases:
PbO + 2NaOH + H 2 O → Na 2 [Pb (OH) 4 ]
Al 2 O 3 + 2NaOH + 3H 2 O → 2Na [Al (OH) 4 ]
SnO + 4NaOH + H 2 O ↔ Na 4 [Sn (OH) 6 ]
Amphoteric oxides also exist from gallium, indium, scandium, titanium, zirconium, vanadium, chromium, iron, cobalt, copper, silver, gold, germanium, antimony, bismuth and tellurium.
Hydroxides can also have amphoteric characteristics, as in the cases of aluminum hydroxide and beryllium hydroxide. Below are both examples:
Aluminum hydroxide as acid:
Al (OH) 3 + 3HCl → AlCl 3 + 3H 2 O
Aluminum hydroxide as a base:
Al (OH) 3 + NaOH → Na [Al (OH) 4 ]
Beryllium hydroxide as acid:
Be (OH) 2 + 2HCl → BeCl 2 + H 2 O
Beryllium hydroxide as a base:
Be (OH) 2 + 2NaOH → Na 2 [Be (OH) 4 ]
Differences between amphoteric, amphiprotic, ampholytic and aprotic
It is necessary to know how to differentiate the concept of each term, since their similarity can become confusing.
Amphoters are known to be substances that behave like acids or bases in a reaction that produces a salt and water. They can do this by donating or capturing a proton, or simply by accepting an electronic pair (or giving it away) according to Lewis’s theory.
In contrast, amphiprotic substances are those amphoteric that act as acids or bases with the donation or uptake of a proton, according to the Bronsted-Lowry law. All amphiprotic substances are amphoteric, but not all amphoteric substances are amphiprotic.
Ampholyte compounds are amphoteric molecules that exist as zwitterions and possess zwitterions at certain pH ranges. They are used as buffering agents in buffer solutions.
Finally, aprotic solvents are those that do not have protons to give up and cannot accept them either.