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52-547: C-Stoff ( [t͡seː ʃtɔf] ; "substance C") was a reductant used in bipropellant rocket fuels (as a fuel itself) developed by Hellmuth Walter Kommanditgesellschaft in Germany during World War II . It was developed for use with T-Stoff (a high-test peroxide ) as an oxidizer, which together with C-Stoff as the fuel, forms a hypergolic mixture . The proportions of the components in C-Stoff were developed to catalyse

104-426: A hydride ion . Reductants in chemistry are very diverse. Electropositive elemental metals , such as lithium , sodium , magnesium , iron , zinc , and aluminium , are good reducing agents. These metals donate electrons relatively readily. Hydride transfer reagents , such as NaBH 4 and LiAlH 4 , reduce by atom transfer: they transfer the equivalent of hydride or H . These reagents are widely used in

156-411: A gas. Later, scientists realized that the metal atom gains electrons in this process. The meaning of reduction then became generalized to include all processes involving a gain of electrons. Reducing equivalent refers to chemical species which transfer the equivalent of one electron in redox reactions. The term is common in biochemistry . A reducing equivalent can be an electron or a hydrogen atom as

208-589: A liquid at room temperature. Pedro Paulet , a Peruvian polymath , reported in 1927 that he had experimented in the 1890s with a rocket engine that used spring-loaded nozzles that periodically introduced vaporized nitrogen tetroxide and a petroleum benzine to a spark plug for ignition, with the engine putting out 300 pulsating explosions per minute. Paulet would go on to visit the German rocket association Verein für Raumschiffahrt (VfR) and on March 15, 1928, Valier applauded Paulet's liquid-propelled rocket design in

260-459: A more easily corroded " sacrificial anode " to act as the anode . The sacrificial metal, instead of the protected metal, then corrodes. A common application of cathodic protection is in galvanized steel, in which a sacrificial zinc coating on steel parts protects them from rust. Oxidation is used in a wide variety of industries, such as in the production of cleaning products and oxidizing ammonia to produce nitric acid . Redox reactions are

312-413: A redox reaction that takes place in a cell, the potential difference is: However, the potential of the reaction at the anode is sometimes expressed as an oxidation potential : The oxidation potential is a measure of the tendency of the reducing agent to be oxidized but does not represent the physical potential at an electrode. With this notation, the cell voltage equation is written with a plus sign In

364-425: A so-called dissociating gas. "Cool" dinitrogen tetroxide is compressed and heated, causing it to dissociate into nitrogen dioxide at half the molecular weight. This hot nitrogen dioxide is expanded through a turbine, cooling it and lowering the pressure, and then cooled further in a heat sink, causing it to recombine into nitrogen tetroxide at the original molecular weight. It is then much easier to compress to start

416-557: A very limited extent as an additive for S-Stoff (fuming nitric acid). It became the storable oxidizer of choice for many rockets in both the United States and USSR by the late 1950s. It is a hypergolic propellant in combination with a hydrazine -based rocket fuel . One of the earliest uses of this combination was on the Titan family of rockets used originally as ICBMs and then as launch vehicles for many spacecraft. Used on

468-471: A weak bond, since it is significantly longer than the average N-N single bond length of 1.45   Å. This exceptionally weak σ bond (amounting to overlapping of the sp hybrid orbitals of the two NO 2 units ) results from the simultaneous delocalization of the bonding electron pair across the whole N 2 O 4 molecule, and the considerable electrostatic repulsion of the doubly occupied molecular orbitals of each NO 2 unit. Unlike NO 2 , N 2 O 4

520-408: A whole reaction. In electrochemical reactions the oxidation and reduction processes do occur simultaneously but are separated in space. Oxidation originally implied a reaction with oxygen to form an oxide. Later, the term was expanded to encompass substances that accomplished chemical reactions similar to those of oxygen. Ultimately, the meaning was generalized to include all processes involving

572-444: Is diamagnetic since it has no unpaired electrons. The liquid is also colorless but can appear as a brownish yellow liquid due to the presence of NO 2 according to the following equilibrium: Higher temperatures push the equilibrium towards nitrogen dioxide. Inevitably, some dinitrogen tetroxide is a component of smog containing nitrogen dioxide. Solid N 2 O 4 is white, and melts at −11.2 °C. Nitrogen tetroxide

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624-429: Is a complex reaction forming various nitrogen oxides of varying stability which depends on the concentration of the nitric acid, presence of oxygen, and other factors. The unstable species further react to form nitrogen dioxide which is then purified and condensed to form dinitrogen tetroxide. Nitrogen tetroxide is used as an oxidizing agent in one of the most important rocket propellant systems because it can be stored as

676-463: Is a powerful oxidizer that is hypergolic (spontaneously reacts) upon contact with various forms of hydrazine , which has made the pair a common bipropellant for rockets. Dinitrogen tetroxide could be regarded as two nitro groups (-NO 2 ) bonded together. It forms an equilibrium mixture with nitrogen dioxide . The molecule is planar with an N-N bond distance of 1.78   Å and N-O distances of 1.19   Å. The N-N distance corresponds to

728-408: Is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a decrease in the oxidation state. The oxidation and reduction processes occur simultaneously in the chemical reaction. There are two classes of redox reactions: "Redox" is a portmanteau of

780-429: Is also called an electron acceptor . Oxidants are usually chemical substances with elements in high oxidation states (e.g., N 2 O 4 , MnO 4 , CrO 3 , Cr 2 O 7 , OsO 4 ), or else highly electronegative elements (e.g. O 2 , F 2 , Cl 2 , Br 2 , I 2 ) that can gain extra electrons by oxidizing another substance. Oxidizers are oxidants, but

832-447: Is also known as its reduction potential ( E red ), or potential when the half-reaction takes place at a cathode. The reduction potential is a measure of the tendency of the oxidizing agent to be reduced. Its value is zero for H + e → 1 ⁄ 2 H 2 by definition, positive for oxidizing agents stronger than H (e.g., +2.866 V for F 2 ) and negative for oxidizing agents that are weaker than H (e.g., −0.763V for Zn ). For

884-469: Is dependent on these ratios. Redox mechanisms also control some cellular processes. Redox proteins and their genes must be co-located for redox regulation according to the CoRR hypothesis for the function of DNA in mitochondria and chloroplasts . Wide varieties of aromatic compounds are enzymatically reduced to form free radicals that contain one more electron than their parent compounds. In general,

936-456: Is made by the catalytic oxidation of ammonia (the Ostwald process ): steam is used as a diluent to reduce the combustion temperature. In the first step, the ammonia is oxidized into nitric oxide : Most of the water is condensed out, and the gases are further cooled; the nitric oxide that was produced is oxidized to nitrogen dioxide, which is then dimerized into nitrogen tetroxide: and

988-494: Is manufactured on a large scale via N 2 O 4 . This species reacts with water to give both nitrous acid and nitric acid : The coproduct HNO 2 upon heating disproportionates to NO and more nitric acid. When exposed to oxygen, NO is converted back into nitrogen dioxide: The resulting NO 2 and N 2 O 4 can be returned to the cycle to give the mixture of nitrous and nitric acids again. N 2 O 4 undergoes molecular autoionization to give [NO ] [NO 3 ], with

1040-432: Is mined as its magnetite (Fe 3 O 4 ). Titanium is mined as its dioxide, usually in the form of rutile (TiO 2 ). These oxides must be reduced to obtain the corresponding metals, often achieved by heating these oxides with carbon or carbon monoxide as reducing agents. Blast furnaces are the reactors where iron oxides and coke (a form of carbon) are combined to produce molten iron. The main chemical reaction producing

1092-505: Is not the reverse of the redox reaction in cellular respiration: Biological energy is frequently stored and released using redox reactions. Photosynthesis involves the reduction of carbon dioxide into sugars and the oxidation of water into molecular oxygen. The reverse reaction, respiration, oxidizes sugars to produce carbon dioxide and water. As intermediate steps, the reduced carbon compounds are used to reduce nicotinamide adenine dinucleotide (NAD ) to NADH, which then contributes to

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1144-504: Is often used with the addition of a small percentage of nitric oxide , which inhibits stress-corrosion cracking of titanium alloys, and in this form, propellant-grade NTO is referred to as mixed oxides of nitrogen ( MON ). Most spacecraft now use MON instead of NTO; for example, the Space Shuttle reaction control system used MON3 (NTO containing 3% NO by weight). On 24 July 1975, NTO poisoning affected three U.S. astronauts on

1196-441: Is oxidized, and the oxidant or oxidizing agent gains electrons and is reduced. The pair of an oxidizing and reducing agent that is involved in a particular reaction is called a redox pair. A redox couple is a reducing species and its corresponding oxidizing form, e.g., Fe / Fe .The oxidation alone and the reduction alone are each called a half-reaction because two half-reactions always occur together to form

1248-594: The Messerschmitt Me 163 aircraft that employed this fuel system. Another hazard was toxicity to humans of each of the propellants. After the war, Allied studies into rocket propellants continued with engines such as the Armstrong Siddeley Beta , under the name " C-fuel ". Redox Redox ( / ˈ r ɛ d ɒ k s / RED -oks , / ˈ r iː d ɒ k s / REE -doks , reduction–oxidation or oxidation–reduction )

1300-485: The U.S. Gemini and Apollo spacecraft and also on the Space Shuttle , it continues to be used as station-keeping propellant on most geo-stationary satellites, and many deep-space probes. It is also the primary oxidizer for Russia's Proton rocket . When used as a propellant, dinitrogen tetroxide is usually referred to simply as nitrogen tetroxide and the abbreviation NTO is extensively used. Additionally, NTO

1352-518: The VfR publication Die Rakete , saying the engine had "amazing power". Paulet would soon be approached by Nazi Germany to help develop rocket technology, though he refused to assist and never shared the formula for his propellant. In early 1944, research on the usability of dinitrogen tetroxide as an oxidizing agent for rocket fuel was conducted by German scientists, although the Germans only used it to

1404-568: The anhydrous transition metal nitrates have striking colours. This branch of chemistry was developed by Cliff Addison and Norman Logan at the University of Nottingham in the UK during the 1960s and 1970s when highly efficient desiccants and dry boxes started to become available. In even slightly basic solvents, N 2 O 4 adds to alkenes radically, giving mixtures of nitro compounds and nitrite esters . Pure or in entirely nonbasic solvents,

1456-443: The copper sulfate solution, thus liberating free copper metal. The reaction is spontaneous and releases 213 kJ per 65 g of zinc. The ionic equation for this reaction is: As two half-reactions , it is seen that the zinc is oxidized: And the copper is reduced: A disproportionation reaction is one in which a single substance is both oxidized and reduced. For example, thiosulfate ion with sulfur in oxidation state +2 can react in

1508-479: The creation of a proton gradient , which drives the synthesis of adenosine triphosphate (ATP) and is maintained by the reduction of oxygen. In animal cells, mitochondria perform similar functions. Free radical reactions are redox reactions that occur as part of homeostasis and killing microorganisms . In these reactions, an electron detaches from a molecule and then re-attaches almost instantly. Free radicals are part of redox molecules and can become harmful to

1560-540: The decomposition of T-Stoff, promote combustion with the oxygen released by the decomposition, and sustain uniform combustion through sufficient quantity of the highly reactive hydrazine. The combination of the C-Stoff, used as a rocket fuel, with the T-Stoff used as the oxidizer, often resulted in spontaneous explosion from their combined nature as a hypergolic fuel combination, necessitating strict hygiene in fueling operations; there were numerous catastrophic explosions of

1612-476: The electron donor is any of a wide variety of flavoenzymes and their coenzymes . Once formed, these anion free radicals reduce molecular oxygen to superoxide and regenerate the unchanged parent compound. The net reaction is the oxidation of the flavoenzyme's coenzymes and the reduction of molecular oxygen to form superoxide. This catalytic behavior has been described as a futile cycle or redox cycling. Minerals are generally oxidized derivatives of metals. Iron

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1664-415: The electrons cancel: The protons and fluoride combine to form hydrogen fluoride in a non-redox reaction: The overall reaction is: In this type of reaction, a metal atom in a compound or solution is replaced by an atom of another metal. For example, copper is deposited when zinc metal is placed in a copper(II) sulfate solution: In the above reaction, zinc metal displaces the copper(II) ion from

1716-474: The entire cycle again. Such dissociative gas Brayton cycles have the potential to considerably increase efficiencies of power conversion equipment. The high molecular weight and smaller volumetric expansion ratio of nitrogen dioxide compared to steam allows the turbines to be more compact. N 2 O 4 was the main component of the "nitrin" working fluid in the decommissioned Pamir-630D portable nuclear reactor which operated from 1985 to 1987. Nitric acid

1768-446: The environment. Cellular respiration , for instance, is the oxidation of glucose (C 6 H 12 O 6 ) to CO 2 and the reduction of oxygen to water . The summary equation for cellular respiration is: The process of cellular respiration also depends heavily on the reduction of NAD to NADH and the reverse reaction (the oxidation of NADH to NAD ). Photosynthesis and cellular respiration are complementary, but photosynthesis

1820-645: The final descent to Earth after the Apollo-Soyuz Test Project flight. This was due to a switch accidentally left in the wrong position, which allowed the attitude control thrusters to fire after the cabin fresh air intake was opened, allowing NTO fumes to enter the cabin. One crew member lost consciousness during descent. Upon landing, the crew was hospitalized for five days for chemical-induced pneumonia and edema . The tendency of N 2 O 4 to reversibly break into NO 2 has led to research into its use in advanced power generation systems as

1872-412: The former nitrosonium ion being a strong oxidant. Various anhydrous transition metal nitrate complexes can be prepared from N 2 O 4 and base metal. where M = Cu , Zn , or Sn . If metal nitrates are prepared from N 2 O 4 in completely anhydrous conditions, a range of covalent metal nitrates can be formed with many transition metals. This is because there is a thermodynamic preference for

1924-660: The foundation of electrochemical cells, which can generate electrical energy or support electrosynthesis . Metal ores often contain metals in oxidized states, such as oxides or sulfides, from which the pure metals are extracted by smelting at high temperatures in the presence of a reducing agent. The process of electroplating uses redox reactions to coat objects with a thin layer of a material, as in chrome-plated automotive parts, silver plating cutlery , galvanization and gold-plated jewelry . Many essential biological processes involve redox reactions. Before some of these processes can begin, iron must be assimilated from

1976-413: The human body if they do not reattach to the redox molecule or an antioxidant . The term redox state is often used to describe the balance of GSH/GSSG , NAD /NADH and NADP /NADPH in a biological system such as a cell or organ . The redox state is reflected in the balance of several sets of metabolites (e.g., lactate and pyruvate , beta-hydroxybutyrate and acetoacetate ), whose interconversion

2028-413: The loss of electrons or the increase in the oxidation state of a chemical species. Substances that have the ability to oxidize other substances (cause them to lose electrons) are said to be oxidative or oxidizing, and are known as oxidizing agents , oxidants, or oxidizers. The oxidant removes electrons from another substance, and is thus itself reduced. Because it "accepts" electrons, the oxidizing agent

2080-492: The molten iron is: Electron transfer reactions are central to myriad processes and properties in soils, and redox potential , quantified as Eh (platinum electrode potential ( voltage ) relative to the standard hydrogen electrode) or pe (analogous to pH as -log electron activity), is a master variable, along with pH, that controls and is governed by chemical reactions and biological processes. Early theoretical research with applications to flooded soils and paddy rice production

2132-494: The nitrate ion to bond covalently with such metals rather than form an ionic structure. Such compounds must be prepared in anhydrous conditions, since the nitrate ion is a much weaker ligand than water, and if water is present the simple nitrate of the hydrated metal ion will form. The anhydrous nitrates concerned are themselves covalent, and many, e.g. anhydrous copper nitrate , are volatile at room temperature. Anhydrous titanium nitrate sublimes in vacuum at only 40 °C. Many of

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2184-404: The presence of acid to form elemental sulfur (oxidation state 0) and sulfur dioxide (oxidation state +4). Thus one sulfur atom is reduced from +2 to 0, while the other is oxidized from +2 to +4. Cathodic protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell . A simple method of protection connects protected metal to

2236-544: The reaction between hydrogen and fluorine , hydrogen is being oxidized and fluorine is being reduced: This spontaneous reaction releases 542 kJ per 2 g of hydrogen because the H-F bond is much stronger than the F-F bond. This reaction can be analyzed as two half-reactions . The oxidation reaction converts hydrogen to protons : The reduction reaction converts fluorine to the fluoride anion: The half-reactions are combined so that

2288-436: The redox status of soils. The key terms involved in redox can be confusing. For example, a reagent that is oxidized loses electrons; however, that reagent is referred to as the reducing agent. Likewise, a reagent that is reduced gains electrons and is referred to as the oxidizing agent. These mnemonics are commonly used by students to help memorise the terminology: Dinitrogen tetroxide Dinitrogen tetroxide

2340-410: The reducing agent is also called an electron donor . Electron donors can also form charge transfer complexes with electron acceptors. The word reduction originally referred to the loss in weight upon heating a metallic ore such as a metal oxide to extract the metal. In other words, ore was "reduced" to metal. Antoine Lavoisier demonstrated that this loss of weight was due to the loss of oxygen as

2392-494: The reduction of carbonyl compounds to alcohols . A related method of reduction involves the use of hydrogen gas (H 2 ) as sources of H atoms. The electrochemist John Bockris proposed the words electronation and de-electronation to describe reduction and oxidation processes, respectively, when they occur at electrodes . These words are analogous to protonation and deprotonation . They have not been widely adopted by chemists worldwide, although IUPAC has recognized

2444-448: The remainder of the water is removed as nitric acid . The gas is essentially pure nitrogen dioxide, which is condensed into dinitrogen tetroxide in a brine-cooled liquefier. Dinitrogen tetroxide can also be made through the reaction of concentrated nitric acid and metallic copper. This synthesis is practical in a laboratory setting. Dinitrogen tetroxide can also be produced by heating metal nitrates. The oxidation of copper by nitric acid

2496-437: The term is mainly reserved for sources of oxygen, particularly in the context of explosions. Nitric acid is a strong oxidizer. Substances that have the ability to reduce other substances (cause them to gain electrons) are said to be reductive or reducing and are known as reducing agents , reductants, or reducers. The reductant transfers electrons to another substance and is thus itself oxidized. Because it donates electrons,

2548-658: The terms electronation and de-electronation. Redox reactions can occur slowly, as in the formation of rust , or rapidly, as in the case of burning fuel . Electron transfer reactions are generally fast, occurring within the time of mixing. The mechanisms of atom-transfer reactions are highly variable because many kinds of atoms can be transferred. Such reactions can also be quite complex, involving many steps. The mechanisms of electron-transfer reactions occur by two distinct pathways, inner sphere electron transfer and outer sphere electron transfer . Analysis of bond energies and ionization energies in water allows calculation of

2600-446: The thermodynamic aspects of redox reactions. Each half-reaction has a standard electrode potential ( E cell ), which is equal to the potential difference or voltage at equilibrium under standard conditions of an electrochemical cell in which the cathode reaction is the half-reaction considered, and the anode is a standard hydrogen electrode where hydrogen is oxidized: The electrode potential of each half-reaction

2652-448: The words "REDuction" and "OXidation." The term "redox" was first used in 1928. Oxidation is a process in which a substance loses electrons. Reduction is a process in which a substance gains electrons. The processes of oxidation and reduction occur simultaneously and cannot occur independently. In redox processes, the reductant transfers electrons to the oxidant. Thus, in the reaction, the reductant or reducing agent loses electrons and

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2704-433: Was seminal for subsequent work on thermodynamic aspects of redox and plant root growth in soils. Later work built on this foundation, and expanded it for understanding redox reactions related to heavy metal oxidation state changes, pedogenesis and morphology, organic compound degradation and formation, free radical chemistry, wetland delineation, soil remediation , and various methodological approaches for characterizing

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