INDUSTRIAL CHEMISTRY NITRIC ACID Vincent & Humza
Structure, Physical & Chemical Properties: Acid Nitric acid is a strong monoprotic acid and is almost completely ionised in aqueous solution. The structure follows the figure to the right. The hydrogen (proton) attaches itself to an oxygen atom, and the other two oxygen atoms attaches off the nitrogen atom with an alternating bond. Nitric acid is a resonance-stabilized acid allowing it to share its electrons among its own bonds . Chemical Properties: Molar mass: 63g/mol Colour: Pure nitric acid is a colourless liquid. But commercial nitric acid may be yellowish brown, due to the presence of dissolved nitrogen dioxide. Odour: Nitric acid is a fuming, hygroscopic liquid, the fumes of which give it a choking smell.
Acidity: Nitric acid is a very strong acid and dissociates very highly, and is very corrosive. Nitric Acid’s Dissociation Constant is 28 (Ka value) Taste: It is sour in taste due to acidic nature Density: Its density is 1.513 g cm-3 at 20oC. Thus pure nitric acid is about 1½ times as dense as water while commercial nitric acid has a lower density. Boiling point: Pure nitric acid boils at 86oC. However it undergoes partial decomposition at this temperature. Melting point: When cooled below 0oC nitric acid freezes to a white solid, which melts at - 42oC. Solubility: It is soluble in water in all proportions.
Concentration Nitric acid can be concentrated up to 68%. It cannot be concentrated beyond this percentage by simply boiling because the aqueous solution of this concentration of nitric acid forms a constant boiling mixture at 121oC. However, the acid can be further concentrated by one of the following methods: 1) By passing a mixture of nitrogen dioxide and air through the 68% aqueous solution. This solution can be further concentrated by distilling it with concentrated sulphuric acid under reduced pressure. By this method nitric acid of 98% concentration can be obtained. 2) If the 98% acid is cooled to -42oC, then pure nitric acid of 100% concentration crystallises out as colourless crystals. If the solution contains more than 86% nitric acid, it is referred to as fuming nitric acid . Fuming nitric acid is characterized as white fuming nitric acid and red fuming nitric acid, depending on the amount of nitrogen dioxide present. At concentrations above 95%, it tends to develop a slight yellow colour due to its tendency to adsorb water from the atmosphere into its crystal structure. Pure anhydrous nitric acid (100%) is a colourless liquid with a density of 1.522 g/cm³ which solidifies at -42 °C to form white crystals and boils at 83 °C. When boiling in light, even at room temperature, there is a partial decomposition with the formation of nitrogen dioxide following the reaction: 4 HNO3 2 H2O + 4 NO2 + O2 (72°C)
Redox Nature Nitric Acid has two properties in redox nature. It is dependant on how concentrated the acid solution is. (It is a very strong oxidant) Nitric acid (conc): NO3- +2H+ +e -> NO2 + H2O Nitric acid (dilute): NO3- + 4H+ +3e -> NO + 2H2O
Manufacture: Manufacture of nitric acid The manufacture of nitric acid is a three-stage process. The first stage is the oxidation of ammonia and it uses a platinum/rhodium catalyst. INDUSTRIAL PREPARATION OF NITRIC ACID On industrial scale, nitric acid can be prepared by the following methods. (a) CHILLI-SALT PETER's METHOD: By NaNO3 (b) BRIKLAND-EYDE's METHOD: By using air (c) OSTWALD' s METHOD : By ammonia
OSTWALD'S METHOD Materials used: - Ammonia gas - Water - Oxygen gas CATALYST - Platinum DETAILS OF PROCESS First step PRIMARY OXIDATION (formation of nitric acid) Oxidation of ammonia is carried out in a catalyst chamber in which one part of ammonia and eight parts of oxygen by volume are introduced. The temperature of chamber is about 600 o C. This chamber contain a platinum gauze which serves as catalyst. CHEMISTRY OF PRIMARY OXIDATION Oxidization of ammonia is reversible and exothermic process. Therefore according to Le-Chatelier's principle., a decrease in temperature favour reaction in forward direction. In primary oxidization 95% of ammonia is converted into nitric oxide (NO).
Second step SECONDARY OXIDATION (formation of nitrogen dioxide) Nitric oxide gas obtain by the oxidation of ammonia is very hot. In order to reduce its temperature , it is passed through a heat exchanger where the temperature of nitric oxide is reduce to 150 o C. Nitric oxide after cooling is transferred to another oxidizing tower where at about 50 o c it is oxidizing to NO2.
Third step ABSORPTION OF NO2 (formation of HNO3) Nitrogen dioxide from secondary oxidation chamber is introduced into a special absorption tower. NO2 gas passed through the tower and water is showered over it. By the absorption, nitric acid is obtained. 3 NO 2 + H 2 O 2HNO 3 + NO Nitric acid so obtain is very dilute. It is recycled in absorption tower so that more and more NO2 get absorbed. HNO3 after recycle becomes about 68% concentrated.
Fourth step CONCENTRATION In order to increase the concentrated of HNO3 , vapour of HNO3 are passed over concentrated H2SO4. Being a dehydrating agent H2SO4,absorbs water from HNO3 and concentrated HNO3 is obtained
Industrial Use: The Important Uses of Nitric Acid Are: Manufacture of Various Products such as: Explosives eg. Trinitrotoluene (TNT), Nitro-glycerine, Gun Cotton, Ammonal Fertilisers such as calcium nitrate, Ammonium Nitrate Nitrate Salts such as calcium nitrate, silver nitrate, ammonium nitrate Dyes, Perfumes, Drugs Synthetic fibres such as Nylon Can make Sulphuric acid from Nitric acid by Lead Chamber process Used in Purification of silver, gold, platinum Used for carving designs on copper, brass, bronze Used to make “Aqua Regia” which dissolves the noble elements (aqua regia is a mixture of more than one type of acid) Used as a Laboratory reagent
HEALTH AND ENVIRONMENTAL ISSUES Nitric acid is a powerful oxidizing agent (accepting electrons, and very highly electronegative) and the reactions of nitric acid with compounds such as cyanides, carbides, and metallic powders can be explosive. Reactions of nitric acid with many organic compounds, such as turpentine, are violent and hypergolic (i.e., self-igniting). Concentrated nitric acid dyes human skin yellow due to a reaction with the keratin. These yellow stains turn orange when neutralized. Furthermore many reactions of Nitric acid involve the release of various N Ox gases which is causing global warming.
Metal + Nitric Acid Metal Sulfate + Water + Nitric Oxide
With concentrated nitric acid (cold or hot):
Metal + Nitric Acid Metal Sulfate + Water + Nitrogen Di-Oxide
With Turpentine (A Cool Prac) Turpentine is a combustible hydrocarbon. When a few drops of turpentine are added to fuming or very concentrated nitric acid, it bursts into flames forming water, nitrogen dioxide and carbon dioxide. The overall reaction is: