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The Ostwald process is a
chemical process In a scientific sense, a chemical process is a method or means of somehow changing one or more chemicals or chemical compounds. Such a chemical process can occur by itself or be caused by an outside force, and involves a chemical reaction of some ...
used for making
nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...
(HNO3). Wilhelm Ostwald developed the process, and he patented it in 1902. The Ostwald process is a mainstay of the modern
chemical industry The chemical industry comprises the companies that produce industrial chemicals. Central to the modern world economy, it converts raw materials ( oil, natural gas, air, water, metals, and minerals) into more than 70,000 different products. ...
, and it provides the main raw material for the most common type of fertilizer production. Historically and practically, the Ostwald process is closely associated with the
Haber process The Haber process, also called the Haber–Bosch process, is an artificial nitrogen fixation process and is the main industrial procedure for the production of ammonia today. It is named after its inventors, the German chemists Fritz Haber and ...
, which provides the requisite raw material,
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...
(NH3).


Description


Stage 1

Ammonia is converted to nitric acid in 2 stages. It is
oxidized Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
by heating with
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
in the presence of a
catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
such as
platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...
with 10%
rhodium Rhodium is a chemical element with the symbol Rh and atomic number 45. It is a very rare, silvery-white, hard, corrosion-resistant transition metal. It is a noble metal and a member of the platinum group. It has only one naturally occurring i ...
, platinum metal on fused silica wool, copper or nickel, to form
nitric oxide Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its ...
(nitrogen(II) oxide) and
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
(as steam). This reaction is strongly
exothermic In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity ...
, making it a useful heat source once initiated: :4 NH3 (g) + 5 O2 (g) -> 4 NO (g) + 6 H2O (g) (Δ''H'' = −905.2 kJ/mol)


Stage 2

Stage two encompasses two reactions and is carried out in an
absorption Absorption may refer to: Chemistry and biology *Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which s ...
apparatus containing water. Initially nitric oxide is oxidized again to yield
nitrogen dioxide Nitrogen dioxide is a chemical compound with the formula . It is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year for use primarily in the productio ...
(nitrogen(IV) oxide). This gas is then readily absorbed by the water, yielding the desired product (nitric acid, albeit in a dilute form), while reducing a portion of it back to nitric oxide: :2 NO (g) + O2 (g) -> 2 NO2 (g) (Δ''H'' = −114 kJ/mol) :3 NO2 (g) + H2O (l) -> 2 HNO3 (aq) + NO (g) (Δ''H'' = −117 kJ/mol) The NO is recycled, and the acid is concentrated to the required strength by
distillation Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the he ...
. And, if the last step is carried out in air: :4 NO2 (g) + O2 (g) + 2 H2O (l) -> 4 HNO3 (aq) (Δ''H'' = −348 kJ/mol).
n Absorption Tower N, or n, is the fourteenth letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''en'' (pronounced ), plural ''ens''. History ...
Typical conditions for the first stage, which contribute to an overall yield of about 98%, are: *
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
is between and *
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
is about . A complication that needs to be taken into consideration is a side-reaction in the first step that reverts the nitric oxide back to nitrogen: :4 NH3 + 6 NO -> 5 N2 + 6 H2O This is a secondary reaction that is minimised by reducing the time the gas mixtures are in contact with the catalyst.


Overall reaction

The overall reaction is the sum of the first equation, 3 times the second equation, and 2 times the last equation; all divided by 2: :2 NH3 (g) + 4 O2 (g) + H2O (l) -> 3 H2O (g) + 2 HNO3 (aq) (Δ''H'' = −740.6 kJ/mol) Alternatively, if the last step is carried out in air, the overall reaction is the sum of equation 1, 2 times the equation 2, and equation 4; all divided by 2. Without considering the state of water, :NH3 (g) + 2 O2 (g) -> H2O + HNO3 (aq) (Δ''H'' = −370.3 kJ/mol)


References

{{reflist, 30em


External links


Nitrogen & Phosphorus
(General Chemistry course)
Purdue University
* Drake, G
"Processes for the Manufacture of Nitric Acid"
(1963)
International Fertiliser Society
(paysite/password)

Carlton Comprehensive High School; Prince Albert; Saskatchewan, Canada. Chemical processes Industrial processes Catalysis German inventions 1902 in science 1902 in Germany