Physical properties and significance in medicine
Ammonia occupies a completely exceptional position in chemistry, acting as the main link connecting atmospheric nitrogen with numerous industrial compounds. It is a colorless gas with an extremely pungent, easily recognizable odor. It liquefies already at a pressure of about 7 atmospheres. One of the most important physical characteristics is its strikingly high solubility in water.
In medical practice, a 10% (mass/mass) aqueous solution, widely known as ammonia solution (spirit of hartshorn), occupies a special place. Its pharmacological effect is based on the ability of vapors to excite the nervous system: inhaling air with a small admixture of the gas causes reflex stimulation of the respiratory center. This effect is universally used by physicians to revive patients from a fainting state.
Spatial structure of the molecule
The spatial structure of the molecule has a pronounced pyramidal shape. Nitrogen is in the sp³ hybridization state. Of the four available hybrid orbitals, three are spent on forming strong covalent bonds with hydrogen atoms. The fourth orbital remains occupied by a lone electron pair.
It is this lone pair that determines the chemical activity and basic properties of the substance in aqueous solutions. The nitrogen atom acts as a donor, accepting a proton from water. As a result, ammonium ions and hydroxide ions are formed, but only about 1% of the total dissolved substance passes into the ionized form.
Chemical reactions and preparation
Under laboratory conditions, the gas is synthesized either by heating its concentrated aqueous solution or by the action of strong alkalis on ammonium salts. Free ammonia is capable of entering into various types of chemical interactions:
- Redox processes: The substance can burn in oxygen with the release of free nitrogen. When the gas is passed over heated black copper(II) oxide, reduction occurs: red metallic copper is formed.
- Catalytic oxidation: A crucial industrial process in which nitric oxide (nitrogen(II) oxide) is formed with the participation of a platinum-rhodium alloy catalyst.
- Substitution reactions: Hydrogen atoms can be substituted. The reaction with molten sodium yields sodium amide, and the interaction with haloalkanes allows obtaining important organic amines (for example, ethylamine).
- Complex formation: Molecules easily form complex coordination compounds with transition metal cations, such as zinc ions.
Properties and decomposition of ammonium salts
The interaction of ammonia with acids leads to the formation of salts. A classic example is the appearance of dense "white smoke" upon mixing with gaseous hydrogen chloride, when crystals of ammonium chloride (sal ammoniac) form in the air.
The radius of the ammonium ion is close to that of the potassium ion, so salts (such as sulfate and nitrate, used as fertilizers) are highly soluble in water. Solutions undergo hydrolysis and have a slightly acidic medium (pH around 5). Upon the addition of alkalis, salts decompose with the release of free ammonia.
Heating crystalline salts leads to their thermal decomposition:
- Salts of non-oxidizing acids (chlorides) decompose reversibly.
- Ammonium carbonate slowly decomposes even at ordinary temperatures, releasing carbon dioxide, which is actively used in baking.
- Salts with an oxidizing anion (dichromates) undergo irreversible intramolecular redox reactions with the release of molecular nitrogen.
Briefly: answers to topical questions
Why do aqueous solutions of ammonium salts have a slightly acidic medium?
Aqueous solutions of ammonium salts have a slightly acidic medium because these salts undergo hydrolysis. Unlike alkali metal salts, the ammonium ion acts as an acid and interacts with water. For example, during hydrolysis, the cation donates a proton to water, forming ammonia and hydronium ions. As a result of this process, an acidic medium with a pH of about 5 is established in the solution.
What complex compounds (ammine complexes) are formed upon the interaction of ammonia with silver and copper salts?
The sources indicate the following silver and copper ammonia complex compounds:
- diamminesilver(I) nitrate — $[Ag(NH_3)_2]NO_3$;
- diamminesilver(I) hydroxide — $[Ag(NH_3)_2]OH$;
- diamminecopper(I) chloride — $[Cu(NH_3)_2]Cl$.
Ammoniacal solutions of silver and copper compounds are used in qualitative reactions for alkynes with a terminal triple bond, during which acetylide precipitates are formed.
In which biochemical cycle does the detoxification of toxic ammonia occur in the human body?
The detoxification of toxic ammonia in the human body occurs in the urea cycle (ornithine cycle). This biochemical process takes place exclusively in the liver. During the reactions of the ornithine cycle, toxic ammonia is converted into low-toxicity urea, which is then excreted from the body. In case of disruptions in this cycle, for example, due to liver diseases or inherited enzyme defects, ammonia is not converted into urea. This leads to an increase in its concentration in the blood and a toxic effect on the central nervous system.
Which classes of amines are formed during the sequential alkylation of ammonia with haloalkanes?
During the sequential alkylation of ammonia with haloalkanes, a mixture of amines of varying degrees of substitution is formed, as well as quaternary salts.
In the course of nucleophilic substitution reactions (polyalkylation), the following are formed sequentially:
- Primary amines — 1 hydrogen atom is substituted.
- Secondary amines — 2 hydrogen atoms are substituted.
- Tertiary amines — 3 hydrogen atoms are substituted.
Further interaction of the formed tertiary amines with halogen derivatives of hydrocarbons leads to addition reactions, resulting in the formation of tetrasubstituted (quaternary) ammonium salts.
To easily remember hydrolysis, recall that ammonia is a weak base. A salt of a weak base and a strong acid (for example, ammonium chloride) always gives an acidic medium to the solution.
Class and exam questions
Why does an ammonia solution exhibit basic properties?
In the ammonia molecule, a lone electron pair is present on the nitrogen atom, capable of acting as a donor for a water proton, releasing hydroxide ions.
How to distinguish ammonium salts from other soluble salts?
When any ammonium salt is heated with a strong alkali, gaseous ammonia is abundantly released, which is easily identified by its pungent odor.
What is ammonia solution used for in medicine?
The ten percent solution is used to revive patients from a fainting state due to reflex stimulation of the nervous system and the respiratory center.
What else to study on this topic
- Mechanism of catalytic oxidation over a platinum-rhodium catalyst
- Synthesis of organic amines using haloalkanes
- Features of complex formation with transition metal ions
- Industrial application of ammonium nitrate and sulfate
- Substitution reactions yielding alkali metal amides
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