The reaction of hydrogen burning in oxygen. Hydrogen combustion temperature: description and reaction conditions, application in machinery
One of the current problems is environmental pollution and the limited energy resources of organic origin. A promising way to solve these problems is to use hydrogen as a source of energy. In the article, consider the problem of hydrogen burning, temperature and chemistry of this process.
What is hydrogen?
Before considering the question of which temperature of the combustion of hydrogen, it is necessary to remember what this substance is.
Hydrogen is the easiest chemical element consisting of only one proton and one electron. Under normal conditions (pressure 1 atm., Temperature 0 o C) It is present in a gaseous state. Its molecule (H 2) is formed by 2 atoms of this chemical element. Hydrogen is the 3rd prevalence in the element on our planet, and 1st in the Universe (about 90% of the whole matter).
Hydrogen gas (H 2) does not smell, taste and colors. It is not toxic, however, when the content of it in the atmospheric air is a few percent, then a person may be suffocating, due to lack of oxygen.
It is curious to note that although from a chemical point of view, all H 2 molecule is identical, their physical properties are somewhat different. The point is all in the orientation of the spins of electrons (they are responsible for the appearance of the magnetic moment), which can be parallel and anti-parallel, such a molecule is called ortho- and parachododitoes, respectively.
Chemical combustion reaction
Considering the question, the combustion temperatures of hydrogen with oxygen, we present a chemical reaction that describes this process: 2H 2 + O 2 \u003d\u003e 2H 2 O. That is, 3 molecules (two hydrogen and one oxygen) are involved in the reaction, and the product is two water molecules . This reaction describes combustion from a chemical point of view, and it can be judged according to it that after its passage it remains only clean water, which does not pollute the environment, as it happens when combustion of organic fuel (gasoline, alcohol).
On the other hand, this reaction is exothermic, that is, in addition to the water, it highlights some amount of heat that can be used to drive machines and rockets, as well as to transfer it to other sources of energy, for example, electricity.
Mechanism of the process of hydrogen burning
The chemical response described in the previous paragraph is known to any high school student, but it is a very rough description of the process that occurs in reality. It should be noted that until the middle of the last century, humanity did not know how the burning of hydrogen in the air occurs, and in 1956, the Nobel Prize in Chemistry was awarded for its study.
In fact, if you step by O 2 and H 2 molecules, then no reaction will occur. Both molecules are sufficiently stable. In order for the burning occurred, water is formed, the existence of free radicals is necessary. In particular, H, O atoms and OH groups. Below is a sequence of reactions that occur in reality when hydrogen burning:
- H + O 2 \u003d\u003e OH + O;
- OH + H 2 \u003d\u003e H 2 O + H;
- O + H 2 \u003d OH + H.
What can be seen from these reactions? With the burning of hydrogen, water is formed, yes, right, but it only happens when the group of two OH atoms occurs with the H 2 molecule. In addition, all reactions occur with the formation of free radicals, which means that the process of self-suggestion of burning is launched.
Thus, the key point in the start of this reaction is to form radicals. They appear if you bring a burning match to the oxygen-hydrogen mixture, or if you heat this mixture above a certain temperature.
Reaction initiation
As noted, it can be done in two ways:
- With the help of a spark that should provide only 0.02 MJ heat. This is a very small energy value, for comparison, let's say that a similar value for the gasoline mixture is 0.24 MJ, and for methane - 0.29 MJ. With a decrease in pressure, the reaction initiation energy is growing. So, at 2 kPa, it is already 0.56 mJ. In any case, these are very small values, so hydrogen-oxygen mixture is considered to be easily flammable.
- Using the temperature. That is, the oxygen-hydrogen mixture can be simply heated, and above some temperature it will flame itself. When this happens depends on the pressure and the percentage ratio of gases. In a wide range of concentrations at atmospheric pressure, the self-burning reaction occurs at temperatures above 773-850 K, that is, above 500-577 o C. This is quite high values \u200b\u200bcompared to a gasoline mixture, which begins to self-propagate at temperatures below 300 o C.
Percentage of gases in a combustible mixture
Speaking about the temperature of hydrogen burning in the air, it should be noted that not every mixture of these gases will join the process under consideration. It is experimentally established that if the amount of oxygen is less than 6% by volume, or if the amount of hydrogen is less than 4% by volume, then no reaction will be. Nevertheless, the limits of the existence of a combustible mixture are wide enough. For air, the percentage of hydrogen can be from 4.1% to 74.8%. Note that the upper value just corresponds to the required minimum of oxygen.
If pure oxygen is considered, then here the limits are even wider: 4.1-94%.
Reducing the pressure of gases leads to a reduction in the specified limits (the lower boundary rises, the upper is lowered).
It is also important to understand that in the process of burning hydrogen in air (oxygen), the reaction products arising (water) lead to a decrease in the concentration of reagents, which can lead to the cessation of the chemical process.
Safety burning
This is an important characteristic of the flammable mixture, since it allows you to judge that the reaction occurs calmly, and it can be controlled, or the process has an explosive character. What does burning speed depend on? Of course, on the concentration of reagents, from pressure, as well as on the amount of energy "seedings".
Unfortunately, hydrogen in a wide concentration interval is capable of explosive burning. The following figures are given in the literature: 18.5-59% hydrogen in the air mixture. Moreover, at the edges of this limit, the largest amount of energy per unit volume is released as a result of detonation.
The marked nature of the combustion represents a greater problem for using this reaction as a controlled energy source.
The temperature of the combustion reaction
Now we approached directly to the answer to the question of which lowest temperature of the combustion of hydrogen. It is 2321 to or 2048 o C for a mixture with 19.6% H 2. That is, the combustion temperature of hydrogen in the air is higher than 2000 o C (for other concentrations it can reach 2500 o C), and in comparison with the gasoline mixture, it is a huge number (for gasoline about 800 o C). If you burn hydrogen in pure oxygen, then the flame temperature will be even higher (up to 2800 o C).
Such a high flame temperature represents another problem in the use of this reaction as a source of energy, since there are no alloys currently, which could work for a long time in such extreme conditions.
Of course, this problem is solved if using a well-thought-out camera cooling system, where hydrogen burning occurs.
Number of heat released
As part of the temperature of hydrogen combustion, it is curious to also lead data on the amount of energy that is allocated during this reaction. For different conditions and compositions of the combustible mixture, the values \u200b\u200bwere obtained from 119 mJ / kg to 141 MJ / kg. To understand how much it is, we note that a similar value for gasoline mixture is about 40 mJ / kg.
The energy yield of the hydrogen mixture is much higher than for gasoline, which is a huge plus for its use as fuel for internal combustion engines. However, and here is not so simple. It's all about hydrogen density, it is too low at atmospheric pressure. So, 1 m 3 of this gas weighs only 90 grams. If it burns this 1 m 3 H 2, then about 10-11 MJ heat out is available, which is already 4 times less than when burning 1 kg of gasoline (a little more than 1 liter).
The above figures suggest that to use the reaction of hydrogen burning, it is necessary to learn how to store this gas in high-pressure cylinders, which creates additional difficulties, both in the technological question and in terms of security.
The use of hydrogen combustible mixture in the technique: problems
Immediately it is necessary to say that at present the hydrogen combustible mixture is already used in some areas of human activity. For example, as an additional fuel for space missiles, as sources for generating electrical energy, as well as in the experimental models of modern cars. However, the scale of this application is meager, compared with those for organic fuel and, as a rule, are experimental. The reason for this is not only difficulties in controlling the reaction itself, but also in storage, transportation and production H 2.
Hydrogen on Earth practically does not exist in its pure form, so it must be obtained from various compounds. For example, out of water. This is a fairly popular method currently, which is carried out by passing an electric current through H 2 O. The whole problem is that it consumes more energy than then can be obtained by burning H 2.
Another important problem is the transportation and storage of hydrogen. The fact is that this gas, due to the small sizes of its molecules, is able to "depart" from any containers. In addition, falling into the metal lattice of alloys, it causes their embrittlement. Therefore, the most effective method of storage H 2 is the use of carbon atoms that can firmly associate "elusive" gas.
Thus, the use of hydrogen as fuel at a more or less broad scale is possible only if it is used as a "saving" of electricity (for example, translate wind and solar energy into hydrogen using electrolysis of water), or if you learn to deliver H 2 from space (where there is a lot of it) to the ground.
Than curse the darkness
better to light at least
one small candle.
Confucius
At the beginning
The first attempts to understand the combustion mechanism are associated with the names of the Englishman Robert Boyl, the Frenchman Antoine Laurent Lavauzier and Russian Mikhail Vasilyevich Lomonosov. It turned out that when burning, the substance does not "disappear", as naively believed once, and turns into other substances, mainly gaseous and therefore invisible. Lavoisier in 1774 first showed that when combustion from the air is about fifth part of it. During the XIX century, scientists investigated physical and chemical processes accompanying the combustion. The need for such work was caused primarily by fires and explosions in mines.
But only in the last quarter of the twentieth century, basic chemical reactions accompanying burning were identified, and to this day a lot of dark spots remained in the chemistry. They are investigated by the most modern methods in many laboratories. These studies have several goals. On the one hand, it is necessary to optimize the combustion processes in the HTP furnaces and in the cylinders of internal combustion engines, prevent explosive burning (detonation) when compressing in the car cylinder of the air-gasoline mixture. On the other hand, it is necessary to reduce the amount of harmful substances generated during the combustion process, and at the same time - to look for more effective means of extinguishing fire.
There are two types of flames. Fuel and oxidizing agent (most often oxygen) can be forcibly or spontaneously pushed to the burning zone of the apodium and mix already in the flame. And may be mixed in advance - such mixes are able to burn or even explode in the absence of air, such as, for example, powder, pyrotechnic mixtures for fireworks, rocket fuels. The combustion can occur both with the participation of oxygen entering the combustion zone with air and with the help of oxygen concluded in the oxidizer substance. One of these substances - bertolet salt (potassium chlorate KCLO 3); This substance easily gives oxygen. Strong oxidizing agent - Nitric acid HNO 3: in its pure form it ignites many organic matter. Nitrates, nitric acid salts (for example, in the form of fertilizer - potash or ammonium nitrate), are easily flammable if mixed with combustible substances. Another powerful oxidant, nitroxide nitrogen N 2 O 4 - component of rocket fuels. Oxygen can replace such strong oxidizing agents, such as chlorine, in which many substances are burning, or fluorine. Pure fluorine is one of the strongest oxidants, water is burning in his jet.
Chain reactions
The foundations of the theory of combustion and spread of the flame were laid at the end of the 20s of the last century. As a result of these studies, branching chain reactions were discovered. For this discovery, the domestic physicochemist Nikolai Nikolayevich Semenov and the English researcher Syril Khinchelwood were awarded the Nobel Chemistry Prize in 1956. More simple unbranched chain reactions opened in 1913 the German chemist Max Bodenstein on the example of the hydrogen reaction with chlorine. Total reaction is expressed by a simple equation H 2 + Cl 2 \u003d 2HCl. In fact, it comes with the participation of very active fragments of molecules - the so-called free radicals. Under the action of light in the ultraviolet and blue areas of the spectrum or at high temperatures, the chlorine molecule is disintegrated into atoms, which begin the long (sometimes up to a million units) chain of transformations; Each of these transformations is called an elementary reaction:
CL + H 2 → HCl + H,
H + Cl 2 → HCl + Cl, etc.
At each stage (reaction link), the disappearance of one active center (atom of hydrogen or chlorine) occurs and at the same time a new active center appears, which continues the chain. The chains are broken when two active particles are found, for example Cl + Cl → Cl 2. Each chain spreads very quickly, therefore, if you generate "initial" active particles at high speed, the reaction will go so quickly that can lead to an explosion.
N. N. Semenov and Khinchelwood found that the reaction of combustion of phosphorus and hydrogen vapor is different: the slightest spark or an open flame can cause an explosion even at room temperature. These reactions are branched-chains: the active particles during the reaction "multiply", that is, with the disappearance of one active particle, two or three appear. For example, in a mixture of hydrogen and oxygen, which can be calmly stored hundreds of years, if there are no external influences, the appearance of such a reason for active hydrogen atoms is launched by such a process:
H + O 2 → OH + O,
O + H 2 → OH + H.
Thus, for an insignificant period of time, one active particle (atom H) is converted into three (hydrogen atom and two hydroxyl OH radicals), which are already running three chains instead of one. As a result, the number of chains is avalanche-like growing, which instantly leads to an explosion of a mixture of hydrogen and oxygen, since many thermal energy is released in this reaction. Oxygen atoms are present in the flame and with the burning of other substances. They can be found if to direct the stream of compressed air across the top of the burner flame. In this case, the characteristic odor of ozone will be found in the air - these are oxygen atoms "stick" to oxygen molecules to form ozone molecules: O + O 2 \u003d O 3, which were made from a flame with cold air.
The possibility of an explosion of a mixture of oxygen (or air) with many combustible gases - hydrogen carbon monoxide, methane, acetylene depends on the conditions, mainly on temperature, composition and pressure of the mixture. So, as a result of leakage of household gas in the kitchen (it consists mainly of methane) its content in the air will exceed 5%, then the mixture explodes from the flame of matches or lighters and even from a small spark that glued in the switch when light ignition. The explosion will not be if the chains are broken faster than they have time to branch. That is why there was a safe lamp for miners that the English Chemist Humphrey Davy had developed in 1816, nothing know about the chemistry of flame. In this lamp, the open fire was fenced off from the external atmosphere (which could be explosive) frequent metal grid. On the metal surface, the active particles effectively disappear, turning into stable molecules, and therefore cannot penetrate into an external environment.
The complete mechanism of branching chain reactions is very complex and may include more than hundreds of elementary reactions. The branched chain includes many oxidation and combustion reactions of inorganic and organic compounds. This will also be the reaction of dividing the cores of heavy elements, such as plutonium or uranium, under the influence of neutrons, which are the analogues of active particles in chemical reactions. Penetrating the core of the heavy element, neutrons cause its division, which is accompanied by the release of very high energy; At the same time, new neutrons are flying out of the kernel, which cause division of neighboring cores. Chemical and nuclear branch-chain processes are described by similar mathematical models.
What to start
To burn burning, you need to perform a number of conditions. First of all, the temperature of the combustible substance should exceed a certain limit value called the flammability temperature. The famous Roman Ray Bradbury "451 degrees Fahrenheit" is named because of this temperature (233 ° C) paper lights up. This is "flammable temperature", above which solid fuel highlights combustible pairs or gaseous decomposition products in an amount sufficient for their sustainable burning. Approximately the same inflammation temperature and dry pine wood.
The flame temperature depends on the nature of the combustible substance and on the combustion conditions. Thus, the temperature in the flame of methane in air reaches 1900 ° C, and when burning in oxygen - 2700 ° C. An even more hot flame is given when combustion in pure oxygen hydrogen (2800 ° C) and acetylene (3000 ° C). No wonder the flame of acetylene burner easily cuts almost any metal. The most high temperature, about 5000 ° C (it is fixed in the Guinness Book of Records), it gives a light-boiling liquid in oxygen in oxygen - a carbon subnithide C 4 N 2 (this substance has a structure of dicyanoacetylene NC-C \u003d C-CN). And according to some information, when combining it in an ozone atmosphere, the temperature can reach 5700 ° C. If this liquid set fire to the air, it burns with a red smoked flame with a green-purple border. On the other hand, cold flames are also known. So, for example, they burn with low pressures of phosphorus pairs. A relatively cold flame is obtained by oxidation under certain conditions of the carbon and light hydrocarbons; For example, propane gives a cold flame under reduced pressure and temperature from 260-320 ° C.
Only in the last quarter of the twentieth century began to clear the mechanism of processes occurring in the flame of many combustible substances. This mechanism is very complicated. The starting molecules are usually too high to respond to oxygen directly to turn into reaction products. For example, the burning of octane, one of the components of the gasoline, is expressed by the 2C 8 H 18 + 25O 2 \u003d 16 + 2 + 25O 2 \u003d O. However, all 8 carbon atoms and 18 hydrogen atoms in the octane molecule cannot simultaneously connect with 50 oxygen atoms. : For this, many chemical bonds should be broken and many new ones. The combustion reaction occurs multi-stage so that at each stage only a small number of chemical bonds, and the process consists of a plurality of consistently occurring elementary reactions, the combination of which is also represented as a flame. It is difficult to study elementary reactions primarily because the concentration of reactive intermediate particles in the flame is extremely small.
Inside flames
The optical sensing of different sections of the flame with the help of lasers made it possible to establish a qualitative and quantitative composition of the active particles present there - fragments of the combustible molecules. It turned out that even in a simple prosecution of the hydrogen combustion reaction in oxygen 2N 2 + O 2 \u003d 2N 2 o more than 20 elementary reactions involving molecules O 2, H 2, O 3, H 2 O 2, H 2 O, active particles N, Oh, he, but 2. Here, for example, what wrote the English chemist of Kenneth Bailey about this reaction in 1937: "The equation of the reaction of hydrogen compound with oxygen is the first equation with which most beginners get acquainted to study chemistry. This reaction seems to them very simple. But even professional chemists are somewhat striking, seeing a book in a hundred pages called "oxygen reaction with hydrogen", published by Hinshelwood and Williamson in 1934. " This can be added that in 1948, significantly large in terms of the volume of the monograph A. B. Nalbandyan and V. V. Voevodsky called "The mechanism of oxidation and burning of hydrogen" was published.
Modern methods of research allowed us to study individual stages of such processes, measure the rate at which various active particles react with each other and with stable molecules at different temperatures. Knowing the mechanism of individual stages of the process, you can "collect" and the whole process, that is, simulate the flame. The complexity of such a modeling lies not only in the study of the entire complex of elementary chemical reactions, but also need to take into account the diffusion processes of particles, heat transfer and convection flows in the flame (it is the latter to suit the fascinating game of the burning campfire languages).
Where everything is taken from
The main fuel of the modern industry is hydrocarbons, ranging from the simplest, methane, and ending with heavy hydrocarbons, which are contained in fuel oil. The flame even the simplest hydrocarbon - methane may include up to one hundred elementary reactions. At the same time, not all of them studied in sufficient detail. When severe hydrocarbons are burning, for example, those are contained in paraffin, their molecules cannot reach the burning zone, remaining integer. Even on the approach to the flame, they are split into fragments due to high temperatures. At the same time, groups containing two carbon atoms are usually cleaved from molecules, for example from 8 H 18 → C 2 H 5 + C 6 H 13. The active particles with an odd number of carbon atoms can eliminate hydrogen atoms, forming compounds with double C \u003d C and triple S≡C connections. It was found that in the flame such compounds may enter into reactions that were not previously known to the chemists, because outside the flame they do not go, for example, with 2 H 2 + o → CH 2 + CO, CH 2 + O 2 → CO 2 + N + N.
The gradual loss of hydrogen with the initial molecules leads to an increase in the shares of carbon, until the particles C 2 H 2 are formed, with 2 H, from 2. The zone of a blue-blue flame is due to the glow in this zone of excited particles from 2 and CH. If the access of oxygen into the burning zone is limited, these particles are not oxidized, but are assembled into the aggregates - they are polymerized according to the C 2 H + C 2 H 2 (C 4 H 2 + N, C 2 H + C 4 H 2 → C 6 2 + N, etc.
As a result, soot particles consisting almost exclusively from carbon atoms are formed. They have the shape of tiny balls with a diameter of up to 0.1 micrometer, which contain approximately a million carbon atoms. Such particles at high temperatures give a well-luminous yellow flame. At the top of the flame of the candle, these particles are burned, so the candle does not smoke. If the further sticking of these aerosol particles occurs, larger particles of soot are formed. As a result, the flame (for example, burning rubber) gives black smoke. Such smoke appears if the share of carbon is raised in the initial fuel relative to hydrogen. An example is the turpentine - a mixture of hydrocarbons of the composition C 10 H 16 (C n H 2N-4), benzene C 6 H 6 (C n H 2N-6), other combustible fluids with a disadvantage of hydrogen - all when they burn the smoothie. Kohsovy and brightly luminous flame gives the air burning acetylene C 2 H 2 (C n H 2N-2); Once this flame was used in acetylene lanterns mounted on bicycles and cars in mining lamps. Conversely: hydrocarbons with a high content of hydrogen - methane CH 4, ethane C 2 H 6, propane with 3 H 8, butane C 4 H 10 (general formula C n H 2N + 2) - burn with sufficient air access with almost a colorless flame. The mixture of propane and butane in the form of a liquid under low pressure is in lighters, as well as in cylinders that are used by dackets and tourists; The same cylinders are installed in gas operating cars. It was relatively recently found that spherical molecules consisting of 60 carbon atoms are often present in the soot; They were called fullerenes, and the discovery of this new carbon shape was marked by awarded in 1996 of the Nobel Prize in Chemistry.
Hydrogen is approximately 140 mJ / kg (upper) or 120 MJ / kg (lower), which is several times higher than the specific heat of the combustion of hydrocarbon fuels (for methane - about 50 mJ / kg).
Hardening gas is self-proposal at atmospheric pressure and a temperature of 510 ° C. At room temperature in the absence of sources of ignition (spark, open flame), the rattling gas can be stored indefinitely for a long time, but it is able to explode from the weakest source, as it is enough for the initiation of the explosion a spark with energy of 17 microdzhules. Taking into account the fact that hydrogen has the ability to penetrate the walls of the vessels in which it is stored, for example, to diffuse through the metal walls of the gas cylinder, and does not possess any odor, when working with it should be extremely careful.
Obtaining
The curve of the relationship between critical pressure and temperature at which the self-ignition of the mixture occurs, has a characteristic Z-shaped form, as shown in the figure. The lower, medium and the upper branches of this curve are called according to the first, second and third limits of ignition. If only the first two limits are considered, the curve has the shape of the peninsula, and traditionally this figure is called the inflammation peninsula.
Sophisticated theory
In the 1960s, the American engineer William Roads (William Rhodes) allegedly opened the "new form" of water commercialized by Yull Brown, Bulgarian physicist emigrated to Australia. "Brown Gas", that is, in fact, a mixture of oxygen and hydrogen, obtained in the water electrolysis apparatus, was declared capable of cleaning radioactive waste, burn as fuel, relax muscles and stimulate seed ger. Subsequently, the Italian physicist Rujsero Santilli (En: Ruggero Santilli) put forward a hypothesis that argues the existence of a new form of water in the form of "HHO gas", that is, the chemical structure of the form (H × H - O), where "×" represents a hypothetical magnesium connection, and " - "- Ordinary covalent bond. Santili's article, published in the authoritative referee magazine International Journal of Hydrogen Energy, caused his hard criticism from colleagues who called Santili's allegations, but some other scientists were supported by Santilli.
Notes
- , from. 85,196.
- , from. 311.
- Konnov A. A. Remaining UNCERTIAES IN THE KINETIC Mechanism of Hydrogen Combustion // Combustion and Flame. - Elsevier, 2008. - Vol. 152, No. 4. - P. 507-528. - DOI: 10.1016 / J.comBustFlame.2007.10.024.
- Shimizu K., Hibi A., Koshi M., Morii Y., TSuboi N. Updated Kinetic Mechanism for High-Pressure Hydrogen Combustion // Journal of Propulsion and Power. - American Institute of Aeronautics and Astronautics, 2011. - Vol. 27, № 2. - P. 383-395. - DOI: 10.2514 / 1.48553.
- Burke M. P., Chaos M., JU Y., Dryer F. L., Klippenstein S. J. Complensive H 2 / O 2 Kinetic Model for High-Pressure Combustion // International Journal of Chemical Kinetics. - Wiley Periodicals, 2012. - Vol. 44, № 7. - P. 444-474. - DOI: 10.1002 / KIN.20603.
- , from. 35
- Ball, Philip. Nuclear Waste Gets Star Attention (English) // Nature: Journal. - 2006. - ISSN 1744-7933. - DOI: 10.1038 / NEWS060731-13.
- Ruggero Maria Santilli. A New Gaseous and Combustible Form of Water // International Journal of Hydrogen Energy: Journal. - 2006. - Vol. 31, NO. nine . - P. 1113-1128. - DOI: 10.1016 / j.ijhydene.2005.11.006.
- J. M. Calo.
T \u003d 1.5 · 10 7 K. For each reaction, energy release (reaction energy Q) is given.
The first reaction in the chain is the interaction of two hydrogen nuclei with the formation of deuteron, positron and neutrino. This reaction occurs as a result of weak interaction and is determining in the speed of the entire PP chain (T \u003d 5.8 · 10 9 years). At the second stage, as a result of the interaction of the formed deuteron with hydrogen, the formation of 3 he isotope with emission -Quance occurs. Next can be implemented one of two possibilities. With a probability of 69%, the reaction occurs:
giving a stream of high-energy neutrinos available for registration. Complete energy (total reaction energy Q), released as a result of the synthesis of isotope 4 HE of 4 protons, is 24.7 MeV - for PPI chains, PPIII and 25.7 MeV for the PPII chain. The positrons formed during the synthesis are annihilated, increasing energy release for all chains to 26.7 MeV.
In tab. 8 shows the values \u200b\u200bof the coefficient S ij at e \u003d 0 for some PP - cycle reactions and uncertainty of estimates of the values \u200b\u200bof the corresponding coefficients.
Table 8.
The value of the values \u200b\u200bof the S IJ coefficient in the PP-cycle reactions
Values \u200b\u200bs ij, mev · mb |
|||
p + P → D + E + + ν |
|||
| 3 He + 3 He → 4 HE + 2P |
|||
3 he + 4 He → 7 be + γ |
|||
7 be + p → 8 b + γ |
The values \u200b\u200bof the ij and their uncertainty shown in the table allow you to obtain an idea of \u200b\u200bthe complexity of calculating the nuclear reactions in the stars and the accuracy achieved today.
The hydrogen cycle can also begin with the reaction:
In the stars with a lot of greater than that of the Sun, PP - the chain is not the main source of energy.
The substance of second-generation stars, along with hydrogen and helium, contains heavier elements resulting in hydrogen combustion reactions and helium, and, in particular, nitrogen, carbon, oxygen, neon and others. These elements play the role of catalysts in hydrogen burning reactions.
When the temperature in the center of the star is approaching 20 million, a chain of nuclear reactions begins in the stars, during which carbon kernel experience a series of consecutive transformations, and helium is formed from hydrogen. This chain of reactions is called CNO - cycle.
