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nitric acid

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nitric acid
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{{Chembox| Watchedfields = changed| verifiedrevid = 476996354| Name = | ImageFile = | ImageFile1 = Nitric-acid-resonance-A.png| ImageName1 = Resonance description of the bonding in the nitric acid molecule| ImageFileL2 = Nitric-acid-3D-balls-B.png| ImageNameL2 = Ball-and-stick model of nitric acid| ImageFileR2 = Nitric-acid-3D-vdW-A.png| ImageNameR2 = Resonance space-filling model of nitric acid| IUPACName = Nitric acid
Aqua fortis, Spirit of niter, Etching>Eau forte, Hydrogen nitrate, Acidum nitricum| SystematicName = | Section1 = {{Chembox Identifiers| CASNo = 7697-37-2correct|CAS}}| PubChem = 944| ChemSpiderID = 919correct|chemspider}}| UNII = 411VRN1TV4correct|FDA}}| EINECS = 231-714-2| UNNumber = 2031correct|kegg}}| KEGG = D02313| MeSHName = Nitric+acidcorrect|EBI}}| ChEBI = 48107| RTECS = QU5775000| Gmelin = 1576| 3DMet = B00068| SMILES = [N+](=O)(O)[O-]| SMILES1 = ON(=O)=Ocorrect|EBI}}| ChEMBL = 1352| StdInChI = 1S/HNO3/c2-1(3)4/h(H,2,3,4)correct|chemspider}}| InChI = 1/HNO3/c2-1(3)4/h(H,2,3,4)| StdInChIKey = GRYLNZFGIOXLOG-UHFFFAOYSA-Ncorrect|chemspider}}| InChIKey = GRYLNZFGIOXLOG-UHFFFAOYAO}}| Section2 = {{Chembox Properties N=1 | O=3| Appearance = Colorless, yellow or red fuming liquid| Odor = acrid, suffocating| Density = 1.51 g cm−3, 1.41 g cm−3 [68% w/w]| Solubility = Completely miscible| MeltingPtC = -42| BoilingPtC = 83121F K}}| ConjugateBase = Nitratelast=Bell title=The Proton in Chemistry publisher=Cornell University Press year=1973 |isbn=}}| RefractIndex = 1.397 (16.5 Â°C)| Dipole = 2.17 ± 0.02 D| VaporPressure = 48 mmHg (20 Â°C)-1.99e-5|u=cm3/mol}} }}| Section3 = | Section4 = {{Chembox Thermochemistry PUBLISHER = HOUGHTON MIFFLIN COMPANY ISBN = 978-0-618-94690-7, A22, | Entropy = 146 J·mol−1·K−1
}}| Section5 = | Section6 = | Section7 = {{Chembox Hazards| ExternalSDS = ICSC 0183PCTL Safety Website| GHSPictograms = {{GHS03}} {{GHS06}} {{GHS08}} {{GHS09}} {{GHS corrosion}}| GHSSignalWord = DANGER
272310373|411}}210260310|370+378}}| NFPA-H = 4| NFPA-R = 2| NFPA-F = 0| NFPA-S = OX| FlashPt = Non-flammable0447}}| IDLH = 25 ppm| LC50 = 138 ppm (rat, 30 min)| REL = TWA 2 ppm (5 mg/m3)ST 4 ppm (10 mg/m3)
}}| Section8 = {hide}Chembox Related| OtherAnions = Nitrous acid| OtherCations = Sodium nitratePotassium nitrateAmmonium nitrate| OtherCompounds = Dinitrogen pentoxide
{edih}}}{{Distinguish|nitrous acid}}Nitric acid ({{Chemical formula|H||NO3}}), also known as aqua fortis (Latin for "strong water") and spirit of niter, is a highly corrosive mineral acid.The pure compound is colorless, but older samples tend to acquire a yellow cast due to decomposition into oxides of nitrogen and water. Most commercially available nitric acid has a concentration of 68% in water. When the solution contains more than 86% HNO3, it is referred to as fuming nitric acid. Depending on the amount of nitrogen dioxide present, fuming nitric acid is further characterized as white fuming nitric acid at concentrations above 95%, or red fuming nitric acid at concentrations above 86%.Nitric acid is the primary reagent used for nitration – the addition of a nitro group, typically to an organic molecule. While some resulting nitro compounds are shock- and thermally-sensitive explosives, a few are stable enough to be used in munitions and demolition, while others are still more stable and used as pigments in inks and dyes. Nitric acid is also commonly used as a strong oxidizing agent.

Physical and chemical properties

Commercially available nitric acid is an azeotrope with water at a concentration of 68% HNO3. This solution has a boiling temperature of 120.5 Â°C at 1 atm. It is known as "concentrated nitric acid". Pure concentrated nitric acid is a colourless liquid at room temperature. Two solid hydrates are known; the monohydrate (HNO3·H2O or [H3O]NO3) and the trihydrate (HNO3·3H2O). An older density scale is occasionally seen, with concentrated nitric acid specified as 42° Baumé.BOOK, Dean, John, Lange's Handbook of Chemistry, 14, McGraw-Hill, 1992, 2.79–2.80, 978-0-07-016194-8,weblink

Contamination with nitrogen dioxide

missing image!
- Fuming nitric acid 40ml.jpg -
Fuming nitric acid contaminated with yellow nitrogen dioxide.
Nitric acid is subject to thermal or light decomposition and for this reason it was often stored in brown glass bottles:
4 HNO3 → 2 H2O + 4 NO2 + O2
This reaction may give rise to some non-negligible variations in the vapor pressure above the liquid because the nitrogen oxides produced dissolve partly or completely in the acid.The nitrogen dioxide (NO2) remains dissolved in the nitric acid coloring it yellow or even red at higher temperatures. While the pure acid tends to give off white fumes when exposed to air, acid with dissolved nitrogen dioxide gives off reddish-brown vapors, leading to the common name "red fuming acid" or "fuming nitric acid" – the most concentrated form of nitric acid at Standard Temperature and Pressure (STP). Nitrogen oxides (NOx) are soluble in nitric acid.

Fuming nitric acid

A commercial grade of fuming nitric acid contains 98% HNO3 and has a density of 1.50 g/cm3. This grade is often used in the explosives industry. It is not as volatile nor as corrosive as the anhydrous acid and has the approximate concentration of 21.4 M.Red fuming nitric acid, or RFNA, contains substantial quantities of dissolved nitrogen dioxide (NO2) leaving the solution with a reddish-brown color. Due to the dissolved nitrogen dioxide, the density of red fuming nitric acid is lower at 1.490 g/cm3.An inhibited fuming nitric acid (either IWFNA, or IRFNA) can be made by the addition of 0.6 to 0.7% hydrogen fluoride (HF). This fluoride is added for corrosion resistance in metal tanks. The fluoride creates a metal fluoride layer that protects the metal.

Anhydrous nitric acid

White fuming nitric acid, pure nitric acid or WFNA, is very close to anhydrous nitric acid. It is available as 99.9% nitric acid by assay. One specification for white fuming nitric acid is that it has a maximum of 2% water and a maximum of 0.5% dissolved NO2. Anhydrous nitric acid has a density of 1.513 g/cm3 and has the approximate concentration of 24 molar. Anhydrous nitric acid is a colorless mobile liquid with a density of 1.512 g/cm3 that solidifies at −42 Â°C to form white crystals. As it decomposes to NO2 and water, it obtains a yellow tint. It boils at 83 Â°C. It is usually stored in a glass shatterproof amber bottle with twice the volume of head space to allow for pressure build up, but even with those precautions the bottle must be vented monthly to release pressure.

Structure and bonding

missing image!
- Nitric-acid-resonance-A.png -
Two major resonance representations of HNO3
Two of the N–O bonds are equivalent and relatively short (this can be explained by theories of resonance; the canonical forms show double-bond character in these two bonds, causing them to be shorter than typical N–O bonds), and the third N–O bond is elongated because the O atom is also attached to a proton.JOURNAL, V., Luzzati, Structure cristalline de l'acide nitrique anhydre, fr, Acta Crystallographica, 1951, 4, 2, 120–131, 10.1107/S0365110X51000404, JOURNAL, D. R., Allan, W. G., Marshall, D. J., Francis, I. D. H., Oswald, C. R., Pulham, C., Spanswick, The crystal structures of the low-temperature and high-pressure polymorphs of nitric acid, Dalton Trans., 2010, 39, 15, 3736–3743, 10.1039/B923975H, 20354626,weblink Submitted manuscript,

Reactions

Acid-base properties

Nitric acid is normally considered to be a strong acid at ambient temperatures. There is some disagreement over the value of the acid dissociation constant, though the pKa value is usually reported as less than −1. This means that the nitric acid in diluted solution is fully dissociated except in extremely acidic solutions. The pKa value rises to 1 at a temperature of 250 Â°C.IUPAC SC-Database A comprehensive database of published data on equilibrium constants of metal complexes and ligandsNitric acid can act as a base with respect to an acid such as sulfuric acid:
HNO3 + 2 H2SO4 ⇌ {{chem|NO|2|+}} + H3O+ + 2 {{chem|HSO|4|−}};{{pad|3em}}Equilibrium constant: K ≈ 22
The nitronium ion, {{chem|NO|2|+}}, is the active reagent in aromatic nitration reactions. Since nitric acid has both acidic and basic properties, it can undergo an autoprotolysis reaction, similar to the self-ionization of water:
2 HNO3 ⇌ {{chem|NO|2|+}} + {{chem|NO|3|-}} + H2O

Reactions with metals

Nitric acid reacts with most metals, but the details depend on the concentration of the acid and the nature of the metal. Dilute nitric acid behaves as a typical acid in its reaction with most metals. Magnesium, manganese, and zinc liberate H2:
Mg + 2 HNO3 → Mg(NO3)2 + H2 (Magnesium nitrate) Mn + 2 HNO3 → Mn(NO3)2 + H2 (Manganese nitrate)
Nitric acid can oxidize non-active metals such as copper and silver. With these non-active or less electropositive metals the products depend on temperature and the acid concentration. For example, copper reacts with dilute nitric acid at ambient temperatures with a 3:8 stoichiometry:
3 Cu + 8 HNO3 → 3 Cu2+ + 2 NO + 4 H2O + 6 {{chem|NO|3|-}}
The nitric oxide produced may react with atmospheric oxygen to give nitrogen dioxide. With more concentrated nitric acid, nitrogen dioxide is produced directly in a reaction with 1:4 stoichiometry:
Cu + 4 H+ + 2 {{chem|NO|3|-}} → Cu2+ + 2 NO2 + 2 H2O
Upon reaction with nitric acid, most metals give the corresponding nitrates. Some metalloids and metals give the oxides; for instance, Sn, As, Sb, and Ti are oxidized into SnO2, As2O5, Sb2O5, and TiO2 respectively.BOOK, Inorganic Chemistry, 3rd, Pearson, 2008, 978-0-13-175553-6, Chapter 15: The group 15 elements, Catherine E. Housecroft, Alan G. Sharpe, Some precious metals, such as pure gold and platinum-group metals do not react with nitric acid, though pure gold does react with aqua regia, a mixture of concentrated nitric acid and hydrochloric acid. However, some less noble metals (Ag, Cu, ...) present in some gold alloys relatively poor in gold such as colored gold can be easily oxidized and dissolved by nitric acid, leading to colour changes of the gold-alloy surface. Nitric acid is used as a cheap means in jewelry shops to quickly spot low-gold alloys (


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