Proof of amphotericity using hydroxides as an example
Chemical duality is visually confirmed by comparing insoluble bases. For the experiment, suspensions of magnesium hydroxide and zinc hydroxide are prepared. This is achieved by precipitation: an alkali is added to soluble salts of these metals (for example, sulfates).
Magnesium hydroxide ($Mg(OH)_2$) acts as a typical base. It easily enters into a neutralization reaction with hydrochloric acid, forming water and magnesium chloride, but it does not interact at all with alkalis such as sodium hydroxide.
Zinc hydroxide ($Zn(OH)_2$), on the contrary, behaves as an amphoteric compound:
- With an acid, it exhibits classic basic properties: $Zn(OH)_2 + 2HCl \rightarrow ZnCl_2 + 2H_2O$.
- With an alkali, it behaves as an acid, binding additional $OH^-$ ions. As a result, a soluble complex (sodium tetrahydroxozincate) is formed: $Zn(OH)_2 + 2NaOH \rightarrow Na_2[Zn(OH)_4]$.
Ionization and classification of insoluble hydroxides
In an aqueous medium, amphoteric zinc hydroxide is capable of ionizing in two ways:
- Basic type: a hydroxide ion ($OH^-$) is cleaved off, and the $Zn(OH)(H_2O)^+$ cation is formed.
- Acidic type: a proton ($H^+$) is cleaved off, and the $Zn(OH)_3^-$ anion is formed.
The second stages of dissociation in both directions proceed extremely weakly.
According to the degree of expression of dual properties, insoluble hydroxides are divided into two groups:
- Typical amphoteric hydroxides: these include compounds of aluminum ($Al(OH)_3$), chromium ($Cr(OH)_3$), tin ($Sn(OH)_2$), and lead ($Pb(OH)_2$). They readily dissolve in dilute solutions of both acids and alkalis.
- Compounds with weak amphotericity: iron (II) and copper (II) hydroxides. Basic properties dominate in them (neutralization with dilute acids occurs), and acidic features can be detected only upon treatment with concentrated alkalis.
Amphoteric oxides
The oxides corresponding to the metals listed above also exhibit an amphoteric character. When they are fused with solid alkalis, a chemical reaction occurs with the formation of oxygen-containing salts, the structure of which is similar to the salts of traditional acids.
A classic example is the interaction of aluminum oxide with caustic soda at high temperatures: $Al_2O_3 + 2NaOH \xrightarrow{t} 2NaAlO_2 + H_2O\uparrow$. In this reaction, aluminum oxide acts as an acid anhydride, forming solid sodium aluminate and water, which evaporates.
Acid-base properties of salts
From a theoretical point of view, any salt possesses amphotericity, since its cation is an acid and its anion is a base. In practice, this phenomenon is noticeable in salts formed by a combination of a weak base and a weak acid. An ideal example is ammonium acetate ($CH_3COONH_4$). The strength of its constituent parts (the $NH_4^+$ cation and the $CH_3COO^-$ anion) is absolutely equal. During hydrolysis, the concentrations of hydronium ions ($H_3O^3^+$ / $H_3O^+$) and hydroxide ions ($OH^-$) coincide and amount to $1 \cdot 10^{-7}$ mol/L, which gives a strictly neutral medium ($pH = 7$).
Dual properties are also characteristic of acid salts, for example, those containing the bicarbonate ion ($HCO_3^-$). In solution, it simultaneously undergoes hydrolysis (as a base, forming $OH^-$) and dissociation (as an acid, forming $H_3O^+$).
In practice, the amphotericity of sodium bicarbonate is confirmed by the reactions:
- With sodium hydroxide, a neutral salt (sodium carbonate) and water are formed.
- With citric acid, carbon dioxide is released.
If solid $NaOH$ pellets are added to a saturated solution of baking soda (with a mass fraction of 8.8%) containing undissolved precipitate and stirred, the precipitate will disappear. This directly proves the manifestation of acidic properties. However, in an aqueous solution of sodium bicarbonate, basic properties predominate over acidic ones, so the medium turns out to be weakly alkaline.
Organic compounds
In organic chemistry, amphotericity is of colossal importance for biological systems. Vivid representatives of compounds with dual properties are amino acids, the structural monomers of polypeptide chains. Proteins themselves also retain the ability to behave both as acids and as bases, which determines their buffer properties in blood plasma and tissues.
Briefly: answers to topical questions
Which functional groups determine the amphoteric properties of amino acids?
The amphoteric properties of amino acids are due to the presence of two types of functional groups in their molecules.
- Carboxyl group ($-COOH$) — determines acidic properties.
- Amino group ($-NH_2$) — determines basic properties.
In aqueous solution, amino acids exist as zwitterions due to internal neutralization. However, this process does not eliminate the chemical activity of the acidic and basic groups, so they retain the ability to react with both acids and bases.
Due to which amino acid residues do blood plasma proteins perform a buffer function?
The protein buffer system of the blood functions due to free carboxyl and amino groups that are part of the radicals of acidic and basic amino acids.
- Acidic amino acids (glutamate, aspartate) — contain an additional carboxyl group, which gives a negative charge.
- Basic amino acids (lysine, arginine) — contain an additional amino group, which gives a positive charge.
When acid enters the blood, amino groups bind excess protons, and when bases enter, carboxyl groups donate protons to neutralize them. An anionic protein buffer operates predominantly in the blood.
To memorize metals that form typical amphoteric hydroxides, use the phrase: "Chromium, Aluminum, Lead, Tin, Zinc — CrAlPbSnZn". And copper and iron (II) are just a "weak shadow" of amphotericity.
Class and exam questions
Why does zinc hydroxide dissolve in excess alkali?
In an alkaline medium, zinc hydroxide exhibits acidic properties. It adds extra hydroxide ions, resulting in the formation of a complex water-soluble salt—sodium tetrahydroxozincate.
Is baking soda solution neutral?
No. In a sodium bicarbonate solution, hydrolysis and ionization of the acid residue occur in parallel. Since the basic properties of the bicarbonate ion are more pronounced, the resulting pH of the medium becomes weakly alkaline.
Is it possible to detect acidic properties in iron (II) hydroxide?
Yes, but this is a hydroxide with weak amphotericity. Its acidic properties can be detected only upon exposure to concentrated alkali solutions.
What else to study on this topic
- Mechanism of hydroxo complex formation in reactions with alkalis
- Stepwise ionization of amphoteric hydroxides in aqueous solutions
- Competitive hydrolysis and dissociation of acid salts
- Fusion of amphoteric oxides with solid alkalis: conditions and products
- Properties of hydroxides with weak amphotericity (copper, iron)
Master the entire topic in the "Chemistry" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Amphoteric compounds" and 3,009 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 95 video lessons for the subject
Other topics in "Chapter 7. Classification of chemical reactions by electronic processes"
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.