- Nitric acid structure
- Resonance structures
- Physical and chemical properties
- Chemical names
- Molecular weight
- Physical appearance
- Odor
- Boiling point
- Melting point
- Water solubility
- Density
- Relative density
- Relative vapor density
- Vapor pressure
- Decomposition
- Viscosity
- Corrosion
- Molar Enthalpy of Vaporization
- Standard molar enthalpy
- Standard molar entropy
- Surface tension
- Odor threshold
- Dissociation constant
- Refractive index (η / D)
- Chemical reactions
- Hydration
- Dissociation in water
- Salt formation
- Protonation
- Autoprotolysis
- Metal oxidation
- Other
- Synthesis
- Industrial
- Stage 1: Oxidation of ammonium to nitric oxide
- Stage 2. Oxidation of nitric oxide to nitrogen dioxide
- Stage 3. Dissolution of nitrogen dioxide in water
- At the laboratory
- Applications
- Fertilizer production
- Industrial
- Metal Purifier
- Royal water
- Furniture
- Cleaning
- Photography
- Others
- Toxicity
- References
The nitric acid is an inorganic compound consisting of an oxoacid of nitrogen. It is considered a strong acid, although its pKa (-1.4) is similar to the pKa of the hydronium ion (-1.74). From this point on, it is perhaps the "weakest" of many known strong acids.
Its physical appearance consists of a colorless liquid that changes to a yellowish color upon storage, due to the formation of nitrogen gases. Its chemical formula is HNO 3.
Source: Aleksander Sobolewski via Wikimedia Commons
It is somewhat unstable, undergoing slight decomposition from exposure to sunlight. Furthermore, it can be completely decomposed by heating, giving rise to nitrogen dioxide, water and oxygen.
The image above shows some nitric acid contained in a volumetric flask. Its yellow coloration can be noticed, indicative of a partial decomposition.
It is used in the manufacture of inorganic and organic nitrates, as well as in nitroso compounds that are used in the manufacture of fertilizers, explosives, intermediate agents for dyes, and different organic chemical compounds.
This acid was already known to the alchemists of the 8th century, which they called “agua fortis”. The German chemist Johan Rudolf Glauber (1648) designed a method for its preparation, which consisted of heating potassium nitrate with sulfuric acid.
It is prepared industrially following the method designed by Wilhelm Oswald (1901). The method, in general lines, consists of the catalytic oxidation of ammonium, with the successive generation of nitric oxide and nitrogen dioxide to form nitric acid.
In the atmosphere, NO 2 produced by human activity reacts with water in clouds, forming HNO 3. Then, during acid rains, it precipitates along with drops of water, eating away, for example, the statues in public squares.
Nitric acid is a very toxic compound, and continuous exposure to its vapors can cause chronic bronchitis and chemical pneumonia.
Nitric acid structure
Source: Ben Mills, from Wikimedia Commons
The upper image shows the structure of an HNO 3 molecule with a spheres and bars model. The nitrogen atom, the blue sphere, is located in the center, surrounded by a trigonal plane geometry; however, the triangle is distorted by one of its longest vertices.
Nitric acid molecules are then flat. The N = O, NO and N-OH bonds make up the vertices of the flat triangle. If you look closely, the N-OH bond is more elongated than the other two (where the white sphere representing the H atom is found).
Resonance structures
There are two links that are the same length: N = 0 and NO. This fact goes against the valence bond theory, where double bonds are predicted to be shorter than single bonds. The explanation for this lies in the phenomenon of resonance, as seen in the image below.
Source: Ben Mills, from Wikimedia Commons
Both bonds, N = O and NO, are therefore equivalent in terms of resonance. This is represented graphically in the structure model by using a dashed line between two O atoms (see structure).
When HNO 3 is deprotonated, the stable nitrate anion NO 3 - is formed. In it, the resonance now involves all three O atoms. This is the reason why HNO 3 has a high Bronsted-Lowry acidity (H + ion donor species).
Physical and chemical properties
Chemical names
-Nitric acid
-Azotic acid
-Hydrogen nitrate
-Agua fortis.
Molecular weight
63.012 g / mol.
Physical appearance
Colorless or pale yellow liquid, which may turn reddish brown.
Odor
Pungent, suffocating characteristic.
Boiling point
181 ° F to 760 mmHg (83 ° C).
Melting point
-41.6 ° C.
Water solubility
Very soluble and miscible with water.
Density
1.513 g / cm 3 at 20 ° C.
Relative density
1.50 (in relation to water = 1).
Relative vapor density
2 or 3 times estimated (in relation to air = 1).
Vapor pressure
63.1 mmHg at 25 ° C.
Decomposition
On exposure to atmospheric humidity or heat, it can decompose forming nitrogen peroxide. When heated to decomposition, it emits a highly toxic fume of nitrogen oxide and hydrogen nitrate.
Nitric acid is not stable, being able to decompose in contact with heat and exposure to sunlight, and emitting nitrogen dioxide, oxygen and water.
Viscosity
1,092 mPa at 0 ° C, and 0.617 mPa at 40 ° C.
Corrosion
It is capable of attacking all base metals, except aluminum and chromic steel. Attacks some of the varieties of plastics, rubbers, and coatings. It is a caustic and corrosive substance, so it must be handled with extreme caution.
Molar Enthalpy of Vaporization
39.1 kJ / mol at 25 ° C.
Standard molar enthalpy
-207 kJ / mol (298 ° F).
Standard molar entropy
146 kJ / mol (298 ° F).
Surface tension
-0.04356 N / m at 0 ºC
-0.04115 N / m at 20 ºC
-0.0376 N / m at 40 ºC
Odor threshold
-Low odor: 0.75 mg / m 3
-High odor: 250 mg / m 3
-Irritating concentration: 155 mg / m 3.
Dissociation constant
pKa = -1.38.
Refractive index (η / D)
1.393 (16.5 ° C).
Chemical reactions
Hydration
-It can form solid hydrates, such as HNO 3 ∙ H 2 O and HNO 3 ∙ 3H 2 O: “nitric ice”.
Dissociation in water
Nitric acid is a strong acid that ionizes rapidly in water in the following ways:
HNO 3 (l) + H 2 O (l) => H 3 O + (aq) + NO 3 -
Salt formation
Reacts with basic oxides to form a nitrate salt and water.
CaO (s) + 2 HNO 3 (l) => Ca (NO 3) 2 (aq) + H 2 O (l)
Likewise, it reacts with bases (hydroxides), forming a salt of nitrate and water.
NaOH (aq) + HNO 3 (l) => NaNO 3 (aq) + H 2 O (l)
And also with carbonates and acid carbonates (bicarbonates), also forming carbon dioxide.
Na 2 CO 3 (aq) + HNO 3 (l) => NaNO 3 (aq) + H 2 O (l) + CO 2 (g)
Protonation
Nitric acid can also behave as a base. For this reason, it can react with sulfuric acid.
HNO 3 + 2H 2 SO 4 <=> NO 2 + + H 3 O + + 2HSO 4 -
Autoprotolysis
Nitric acid undergoes autoprotolysis.
2HNO 3 <=> NO 2 + + NO 3 - + H 2 O
Metal oxidation
In the reaction with metals, nitric acid does not behave like strong acids, which react with metals, forming the corresponding salt and releasing hydrogen in gaseous form.
However, magnesium and manganese react hot with nitric acid, just as the other strong acids do.
Mg (s) + 2 HNO 3 (l) => Mg (NO 3) 2 (aq) + H 2 (g)
Other
Nitric acid reacts with metal sulphites to form a nitrate salt, sulfur dioxide and water.
Na 2 SO 3 (s) + 2 HNO 3 (l) => 2 NaNO 3 (aq) + SO 2 (g) + H 2 O (l)
And it also reacts with organic compounds, substituting a hydrogen for a nitro group; thus constituting the basis for the synthesis of explosive compounds such as nitroglycerin and trinitrotoluene (TNT).
Synthesis
Industrial
It is produced at an industrial level by the catalytic oxidation of ammonium, according to the method described by Oswald in 1901. The process consists of three stages or steps.
Stage 1: Oxidation of ammonium to nitric oxide
Ammonium is oxidized by oxygen in the air. The reaction is carried out at 800ºC and at a pressure of 6-7 atm, with the use of platinum as a catalyst. Ammonia is mixed with air in the following ratio: 1 volume of ammonia to 8 volumes of air.
4NH 3 (g) + 5O 2 (g) => 4NO (g) + 6H 2 O (l)
Nitric oxide is produced in the reaction, which is taken to the oxidation chamber for the next stage.
Stage 2. Oxidation of nitric oxide to nitrogen dioxide
The oxidation is carried out by the oxygen present in the air at a temperature below 100ºC.
2NO (g) + O 2 (g) => 2NO 2 (g)
Stage 3. Dissolution of nitrogen dioxide in water
In this stage the formation of nitric acid occurs.
4NO 2 + 2H 2 O + O 2 => 4HNO 3
There are several methods for the absorption of nitrogen dioxide (NO 2) in water.
Among other methods: NO 2 is dimerized to N 2 O 4 at low temperatures and high pressure, in order to increase its solubility in water and produce nitric acid.
3N 2 O 4 + 2H 2 O => 4HNO 3 + 2NO
The nitric acid produced by the oxidation of ammonia has a concentration between 50-70%, which can be brought to 98% by using concentrated sulfuric acid as a dehydrating agent, allowing the concentration of nitric acid to be increased.
At the laboratory
Thermal decomposition of copper (II) nitrate, producing nitrogen dioxide and oxygen gases, which are passed through water to form nitric acid; as in the Oswald method, previously described.
2Cu (NO 3) 2 => 2CuO + 4NO 2 + O 2
Reaction of a nitrate salt with concentrated H 2 SO 4. The nitric acid formed is separated from H 2 SO 4 by distillation at 83 ° C (boiling point of nitric acid).
KNO 3 + H 2 SO 4 => HNO 3 + KHSO 4
Applications
Fertilizer production
60% of nitric acid production is used in the manufacture of fertilizers, especially ammonium nitrate.
This is characterized by its high concentration of nitrogen, one of the three main plant nutrients, the nitrate being used immediately by the plants. Meanwhile, ammonia is oxidized by microorganisms present in the soil, and is used as a long-term fertilizer.
Industrial
-15% of nitric acid production is used in the manufacture of synthetic fibers.
-It is used in the production of nitric acid esters and nitro derivatives; such as nitrocellulose, acrylic paints, nitrobenzene, nitrotoluene, acrylonitriles, etc.
-You can add nitro groups to organic compounds, and this property can be used to make explosives such as nitroglycerin and trinitrotoluene (TNT).
-Adipic acid, a precursor to nylon, is produced on a large scale through the oxidation of cyclohexanone and cyclohexanol by nitric acid.
Metal Purifier
Nitric acid, due to its oxidizing capacity, is very useful in the purification of metals present in minerals. Likewise, it is used to obtain elements such as uranium, manganese, niobium, and zirconium, and in the acidification of phosphoric rocks to obtain phosphoric acid.
Royal water
It is mixed with concentrated hydrochloric acid to form "aqua regia". This solution is capable of dissolving gold and platinum, which allows its use in the purification of these metals.
Furniture
Nitric acid is used to obtain an antique effect in furniture made with pine wood. Treatment with a 10% nitric acid solution produces a gray-gold coloration in the wood of the furniture.
Cleaning
-The mixture of aqueous solutions of nitric acid 5-30% and phosphoric acid 15-40% is used in the cleaning of the equipment used in the milking work, in order to eliminate the residues of the precipitates of the magnesium compounds and calcium.
-It is useful in cleaning glassware used in the laboratory.
Photography
-Nitric acid has been used in photography, specifically as an additive for ferrous sulfate developers in the wet plate process, with the aim of promoting a whiter color in ambrotypes and tintypes.
-It was used to lower the pH of the silver bath of the collodion plates, which allowed to obtain a reduction in the appearance of a mist that interfered with the images.
Others
-Due to its solvent capacity, it is used in the analysis of different metals by flame atomic absorption spectrophotometry techniques, and inductively coupled plasma mass spectrophotometry.
-The combination of nitric acid and sulfuric acid was used for the conversion of common cotton into cellulose nitrate (nitric cotton).
-The drug Salcoderm for external use is used in the treatment of benign neoplasms of the skin (warts, calluses, condylomas and papillomas). It has cauterizing properties, relieving pain, irritation and itching. Nitric acid is the main component of the drug formula.
-Red fuming nitric acid and white fuming nitric acid are used as oxidants for liquid rocket fuels, especially in the BOMARC missile.
Toxicity
-In contact with the skin, it can cause skin burns, severe pain and dermatitis.
-In contact with the eyes can cause intense pain, tearing and in severe cases, damage to the cornea and blindness.
-Inhalation of the vapors can cause coughing, respiratory distress, causing nosebleeds, laryngitis, chronic bronchitis, pneumonia and pulmonary edema in intense or chronic exposures.
-Due to ingestion, there is lesions in the mouth, salivation, intense thirst, pain to swallow, intense pain throughout the digestive tract and risk of perforation of the wall of the same.
References
- Wikipedia. (2018). Nitric acid. Recovered from: en.wikipedia.org
- PubChem. (2018). Nitric acid. Recovered from: pubchem.ncbi.nlm.nih.gov
- The Editors of Encyclopaedia Britannica. (November 23, 2018). Nitric acid. Encyclopædia Britannica. Recovered from: britannica.com
- Shrestha B. (nd). Properties of nitric acid and uses. Chem Guide: tutorials for chemistry learning. Recovered from: chem-guide.blogspot.com
- Chemical Book. (2017). Nitric acid. Recovered from: chemicalbook.com
- Imanol. (10th of September 2013). Nitric acid production. Recovered from: ingenieriaquimica.net