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Amphoterism
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Amphoterism
In chemistry, an amphoteric compound (from Greek amphoteros 'both') is a molecule or ion that can react both as an acid and as a base. What exactly this can mean depends on which definitions of acids and bases are being used.
Amphoteric is derived from the Greek word amphoteroi (ἀμφότεροι) meaning "both". Related words in acid-base chemistry are amphichromatic and amphichroic, both describing substances such as acid-base indicators which give one colour on reaction with an acid and another colour on reaction with a base.
Amphiprotism is exhibited by compounds with both Brønsted acidic and basic properties. A prime example is H2O. Amphiprotic molecules can either donate or accept a proton (H+). Amino acids (and proteins) are amphiprotic molecules because of their amine (−NH2) and carboxylic acid (−COOH) groups.
Ampholytes are zwitterions ‒ molecules or ions that contain both acidic and basic functional groups. Amino acids H2N−RCH−CO2H have both a basic group −NH2 and an acidic group −COOH. Often such species exists as several structures in chemical equilibrium:
In approximately neutral aqueous solution (pH ≅ 7), the basic amino group is mostly protonated and the carboxylic acid is mostly deprotonated, so that the predominant species is the zwitterion H3N+−RCH−COO−. The pH at which the average charge is zero is known as the molecule's isoelectric point. Ampholytes are used to establish a stable pH gradient for use in isoelectric focusing.
Metal oxides which react with both acids as well as bases to produce salts and water are known as amphoteric oxides. Many metals (such as zinc, tin, lead, aluminium, and beryllium) form amphoteric oxides or hydroxides. Aluminium oxide (Al2O3) is an example of an amphoteric oxide. Amphoterism depends on the oxidation states of the oxide. Amphoteric oxides include lead(II) oxide and zinc oxide, among many others.
According to the Brønsted-Lowry theory of acids and bases, acids are proton donors and bases are proton acceptors. An amphiprotic molecule (or ion) can either donate or accept a proton, thus acting either as an acid or a base. Water, amino acids, hydrogencarbonate ion (or bicarbonate ion) HCO−3, dihydrogen phosphate ion H2PO−4, and hydrogensulfate ion (or bisulfate ion) HSO−4 are common examples of amphiprotic species. Since they can donate a proton, all amphiprotic substances contain a hydrogen atom. Also, since they can act like an acid or a base, they are amphoteric.
The water molecule is amphoteric in aqueous solution. It can either gain a proton to form a hydronium ion H3O+, or else lose a proton to form a hydroxide ion OH−.
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Amphoterism
In chemistry, an amphoteric compound (from Greek amphoteros 'both') is a molecule or ion that can react both as an acid and as a base. What exactly this can mean depends on which definitions of acids and bases are being used.
Amphoteric is derived from the Greek word amphoteroi (ἀμφότεροι) meaning "both". Related words in acid-base chemistry are amphichromatic and amphichroic, both describing substances such as acid-base indicators which give one colour on reaction with an acid and another colour on reaction with a base.
Amphiprotism is exhibited by compounds with both Brønsted acidic and basic properties. A prime example is H2O. Amphiprotic molecules can either donate or accept a proton (H+). Amino acids (and proteins) are amphiprotic molecules because of their amine (−NH2) and carboxylic acid (−COOH) groups.
Ampholytes are zwitterions ‒ molecules or ions that contain both acidic and basic functional groups. Amino acids H2N−RCH−CO2H have both a basic group −NH2 and an acidic group −COOH. Often such species exists as several structures in chemical equilibrium:
In approximately neutral aqueous solution (pH ≅ 7), the basic amino group is mostly protonated and the carboxylic acid is mostly deprotonated, so that the predominant species is the zwitterion H3N+−RCH−COO−. The pH at which the average charge is zero is known as the molecule's isoelectric point. Ampholytes are used to establish a stable pH gradient for use in isoelectric focusing.
Metal oxides which react with both acids as well as bases to produce salts and water are known as amphoteric oxides. Many metals (such as zinc, tin, lead, aluminium, and beryllium) form amphoteric oxides or hydroxides. Aluminium oxide (Al2O3) is an example of an amphoteric oxide. Amphoterism depends on the oxidation states of the oxide. Amphoteric oxides include lead(II) oxide and zinc oxide, among many others.
According to the Brønsted-Lowry theory of acids and bases, acids are proton donors and bases are proton acceptors. An amphiprotic molecule (or ion) can either donate or accept a proton, thus acting either as an acid or a base. Water, amino acids, hydrogencarbonate ion (or bicarbonate ion) HCO−3, dihydrogen phosphate ion H2PO−4, and hydrogensulfate ion (or bisulfate ion) HSO−4 are common examples of amphiprotic species. Since they can donate a proton, all amphiprotic substances contain a hydrogen atom. Also, since they can act like an acid or a base, they are amphoteric.
The water molecule is amphoteric in aqueous solution. It can either gain a proton to form a hydronium ion H3O+, or else lose a proton to form a hydroxide ion OH−.