Amphoteres are elements that can act as acids or bases.
Brønsted-Lowry theory
According to the so-called Brønsted-Lowry theory (named after those who proposed it independently, the Danish Johannes Nicolaus Brønsted and the Englishman Thomas Martin Lowry ), the reaction between an acid and a base leads to the fact that, from From the exchange of a proton , the acid forms its conjugate base and the base forms its conjugate acid. In this way, acids give up protons and bases accept them.
This definition expands the acid-base concept beyond dissociation in water, allowing consideration of acid-base reactions in other solvents and in non-aqueous media. An important aspect of Brønsted-Lowry theory is the concept of acid-base equilibrium .
Acids and bases have relative strengths to donate or accept protons, which is reflected in their equilibrium constant. Stronger acids have a greater tendency to donate protons, while stronger bases have a greater capacity to accept them. This theory also allows the classification of substances in terms of their acid-base behavior .
Amphiprotic molecules
Amphiprotic molecules are capable of donating or receiving a proton, so they are amphoteric elements. Proteins and amino acids are amphoteric of this class.
These amphiprotic molecules have an acid radical and a base. That is why they act in one way or another according to the environment. Amphoteric substances modify the electrical charge of their hydrophilic sector taking into account the pH of the medium: to counteract said pH , they are positioned as acids if they are in a basic medium, while if they are in an acidic medium they are positioned as bases.
The amphiprotic property of a molecule is related to the presence of specific functional groups in its molecular structure. These have the ability to ionize or exchange protons in response to environmental conditions. We must point out that they play an important role in various chemical and biological reactions . Their ability to donate or accept protons allows them to participate in processes such as pH regulation in biological systems, the formation of chemical bonds and the catalysis of enzymatic reactions.
Examples
Some examples of amphoteric are the following:
- Water (H2O) : Water can behave as an acid or a base in different situations. For example, it can donate a proton to form the hydroxide ion (OH-) in an acidic solution, and it can accept a proton to form the hydronium ion (H3O+) in a basic solution;
- Amino acids : Amino acids, which are the building blocks of proteins, are amphoteric. They have carboxyl groups (-COOH) that can donate protons and amino groups (-NH2) that can accept protons. Depending on the pH of the medium, amino acids can act as acids by donating protons or as bases by accepting protons;
- aluminum hydroxide (Al(OH)3) – Aluminum hydroxide is an amphoteric substance that can react as both an acid and a base. In the presence of a strong base, it can accept protons and act as a base. On the other hand, in the presence of a strong acid, it can donate protons and act like an acid;
- zinc sulfate (ZnSO4) – Zinc sulfate is another example of an amphoteric compound. In acidic solutions, the sulfate ion can accept protons and act as a base. In basic solutions, the zinc ion can donate protons and act as an acid;
- phosphate ion (HPO42-) : in an acidic medium, the phosphate ion can accept a proton (H+) and act as a base, forming phosphoric acid (H3PO4) . For example, in phosphoric acid solution, the phosphate ion captures protons and is converted to phosphoric acid.
Water is a well-known example of amphoteric.