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DARRELL H. BEACH The Culver Academies CuIver. Indiana 46511
Nitric Acid Sardul S. Pannu 3HN02
University of the District of Columbia Washington, DC 20008
Old Names: Latin: Aoua Fortis. Acidum Nitri, Aqua Dissolutiva French: E&Lde Depart, Acide ~ i t r i q u k German: Starkwasser, Salt Petersaure
Alchemist's Symbol:
7
History The earliest reference to nitric acid is in the writing of the eighth century Arab scholar Geber, who prepared nitric acid by "distilling a pound of Cyprus vitriol, a pound and half of saltpeter and a quarter of a pound of alum." In 1658 Glauber obtained concentrated fuming nitric acid by distilling niter with oil of vitriol (sulfuric acid). In 1776 Lavoisier proved that nitric acid contained oxygen. The complete chemical composition was established in 1816 by Gay-Lussac and Bertholet. Occurrence A considerable amount of nitric acid is produced by lightning and in the combustion of fossil fuel causing "acid rains" which contain mainly sulfuric acid and nitric acid. Oxides of nitrogen are also formed when air is irradiated in a nuclear reactor. In the combined state it occurs in form of sodium nitrate (Chile saltpeter) and potassium nitrate.
-
2N0
+ HN03 + Hz0
Ostwald's Method Catalytic Oxidation of Ammonia. In this method NH3 is oxidized a t 700°C by air or oxygen gas to form NO in the presence of either platinum gauze or platinum-rhodium catalyst. The reaction is rapid and goes almost to completion (959~98%).
+
4NH3 502
-
4N0
+ 6H20
AH = -229.4
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Physical Properties Nitric acid is a colorless mobile liquid which fumes strongly in moist air. It has a pungent, choking odor. Nitric acid melts a t -41.6'C and boils at 82.6"C. Its density is 1.51 g cm-%at 25°C. I t is very soluble in water. I t forms a maximum constant-boiling mixture which boils at 120.5% containing 68.5% HN03, and the solution has a density of 1.41 g ~ m at- 20°C. ~ "Fuming nitric acid" is the concentrated nitric containing dissolved nitrogen dioxide. The color of fuming nitric acid ranges from yellow to red depending upon the concentration of NO? present. The following are some h p o r t a n t standard electrode potentials of nitric acid solution.
Methods of Preparation
Until 1924, nitric acid was obtained from the reaction of Chile saltpeter and concentrated sulfuric acid. Since then Ostwald's method of catalytic oxidation of ammonia has been used in the manufacture of nitric acid.
Nitric acid is a planar molecule containing an sp2nitrogen atom. The predominant resonating structures of the acid are
From Chile Saltpeter and Concentrated Sulfuric Acid
Rirkeland-Eyde Arc Process In this process air is passed through an electric arc at a temperature of about 3000°C. N2(g)
+ O2(g)
f
2NO(g)
AH = +43.2 kcal
According to Le Chatelier's Principle, the amount of NO formed will be greater at higher temperatures and will not be affected by pressure. The equilibrium concentration of NO is 0.4% at 1500°C and 5% at 3000°C. This process is not used currently because of the high cost of electricity. Another variation of the above method was developed at the University of Wisconsin. It consists of passing air through a preheated bed of magnesia and then heating by combustion of fuel gases. After the gases containing NO have been chilled, nitric oxide reacts with oxygen to form nitrogen dioxide which is suhsequently absorbed in water. 174
Journal of Chemical Education
The bond distance and angles in nitric acid are given in the structural formula:
The structure, properties, and uses of a variety of chemicals are highlighted in this feature which is aimed at increasingthe use of descriptive chemistry.
Chemical Properties
Decomposition Nitric acid is fairly stable a t room temperature. However, i t is decomposed by heat, especially ahove 68°C. T h e familiar yellow color of concentrated nitric acid in reagent bottles is due to its decomposition to NO*. The following seems to he the most plausible mechanism for the decomposition of nitric acid a t room temperature.
-
-
+0.80V to +O.96V. Therefore, it can oxidize V2+ V3+, V3+ to VO2+, Cr2+ Cr3+, Ti3+ to Ti02+, Fez+ to Fe3+, and Nh3+ to NhO2+.
+ 4Ht + NOsFez++ NO
-
3Fe3++ 2H20 +NO Fe(N0)2+ brawn These reactions form the basis of detecting nitric acid or nitrate ion by the familiar ring-test method. 3Fezt
Reaction with Nitrogen ( I I ) Oxide When gaseous nitric oxide is bubbled through an aqueous nitric acid solution nitrous acid is formed. The ahove reaction is catalyzed by hydrochloric acid and small amounts (10-8 to 10-9M) of silver (I) and mercury (I) ions.
Acidic Properties Nitric acid is hizhlv ionized (93% in 0.10 M ) in water solutions and, therefore,;t is a strong monoprotic acid. It reacts with bases, metallic oxides, and carbonates to form salts and water unless the metallic ion concerned is a reducing agent. CdO + 2HN03 3Fe0 + lOHN03
-
-
Cd(N03h + Hz0
+
3Fe(NOh 5H20 + NO
Oxidation of Metals Nitric acid solutions are strong oxidizing agents. Nearly all metals are attacked by nitric acid except Au, Pt, Ir, Ta, and Rh. Metals like Fe, Cr, and Al readily dissolve in dilute nitric acid hut are rendered passive by concentrated nitric acid solution due to the formation of an oxide layer on the metallic surface. This feature makes it possible to store and ship concentrated nitric in steel containers. The metalloids Si, Ge, Sn, As, and S b along with the transition metals Mo and W are converted to oxides by hot concentrated nitric acid. The remaining metals are attacked by nitric acid to form nitrates. The other reaction products depend upon the nature of the metal, temperature, and concentration of the acid. The reaction product may contain one or more of the following substances: NzO, NO, NO*, Hz, NH20H, Np, and NH3. In general, metals which are ahove hydrogen on the electromotive series yield H2, N2, NH3, NH20H, or NO when treated with nitric acid. Metals which are below hydrogen in the electromotive series tend to give NO or NO*. 2H+ + ZnZ++ Hz? Zn (
