Endothermic compounds reactions

There are many compounds in existence which have a considerable positive enthalpy of formation. They are not made by direct union of the constituent elements in their standard states, but by some process in which the necessary energy is provided indirectly. Many known covalent hydrides (Chapter 5) are made by indirect methods (for example from other hydrides) or by supplying energy (in the form of heat or an electric discharge) to the direct reaction to dissociate the hydrogen molecules and also possibly vaporise the other element. Other known endothermic compounds include nitrogen oxide and ethyne (acetylene) all these compounds have considerable kinetic stability. 

Most chemical reactions are exothermic. In the few endothermic reactions that are known, heat is absorbed into the reaction product or products, which are known as endothermic or energy-rich compounds. Such compounds are thermodynamically unstable because heat woiild be released on decomposition of their elements. The majority of endothermic compounds possess a tendency toward insta-bihty and possibly explosive decomposition under various circumstances of initiation. 

Besides the weak bonds listed in the previous table, there are other multiple bonds that endow the molecules in which they are situated with a positive enthalpy of formation. Such compounds are termed endothermic compounds. The danger they represent does not necessarily come from the fact that they are unstable, but is related to the exothermicity of their decomposition reaction. The most convincing examples are the acetylenic compounds, and in particular, acetylene. It is also the case for ethylene, aromatic compounds, imines and nitriles. 

The tetroxide, which forms an equilibrium with nitrogen dioxide, is a very strong oxidant and an endothermic compound. To analyse all the dangerous reactions of... 

An endothermic compound, which is hardiy stable thermodynamically. However, it only decomposes under extreme conditions (very violent shock, high temperature). When it is involved in a reaction, this compound can combust or detonate easily because of the exothermicity of the transformation. Since it is hardly ever handled in the pure state (it forms when compounds containing arsenic are handled) it gives rise to a limited number of accidents. 

Endothermic compounds, formed as the energy-rich products of endothermic reactions, are thermodynamically unstable and may be liable to energetic decomposition with low energy of activation. 

Of the factors associated with the high reactivity of cyanuric chloride (high exother-micity, rapid hydrolysis in presence of water-containing solvents, acid catalysed reactions, liberation of up to 3 mol hydrogen chloride/mol of chloride, formation of methyl chloride gas with methanol, formation of carbon dioxide from bicarbonates), several were involved in many of the incidents recorded [1] (and given below). The acid catalysed self acceleration and high exothermicity are rated highest [2]. It is also a mildly endothermic compound (AH°f (s) +91.6 kJ/mol, 0.49 kJ/g). 

The autocatalytic exothermic polymerisation reaction exhibited by styrene was involved in a plant-scale incident where accidental heating caused violent ejection of liquid and vapour from a storage tank [1], Polymerisation becomes self-sustaining above 95°C [2], The monomer has been involved in several plant-scale explosions, and must be stored at below 32°C, and for less than 3 months [3], Styrene is a moderately endothermic compound (AH°f (1) +147.7 kJ/mol,... 

All reactive hazards involve the release of energy in quantities or at rates too high to be absorbed by the immediate environment of the reacting system, and material damage results. The source of the energy may be an exothermic multi-component reaction, or the exothermic decomposition of a single unstable (often endothermic) compound. 

Many, but not all, endothermic compounds have been involved in violent decompositions, reactions or explosions, and in general, compounds with significantly positive values of standard heat of formation may be considered suspect on stability grounds. Notable exceptions are benzene and toluene (AH°f +82.2, 50.0 kJ/mol 1.04, 0.54 kJ/g, respectively), where there is the resonance stabilising effect of aromaticity. Values of thermodynamic constants for elements and compounds are tabulated conveniently [1], but it should be noted that endothermicity may change to exothermicity with increase in temperature [2], There is a more extended account of the implications of endothermic compounds and energy release in the context of fire and explosion hazards [3], Many examples of endothermic compounds will be found in the groups ... 

High temperature (often exceeding 1000 K) drives the endothermic chemical reactions. Multistep cycles for water splitting are used because very high temperatures are required before an appreciable amount of water decomposes in single-step cycles. Thus, in one or more subsequent chemical reactions, the intermediary compounds can be recovered to the original substance, which is used repeatedly. The thermochemical water decomposition steps involve the following five principal reactions ... 

At higher temperatures (>250°C), endothermic depolymerization reactions dominate and generate volatile anhydrosugars and related monomeric compounds, see pathway... 

The addition and insertion of one molecule of CO into the lithium-hydrogen bond of the (LiFI)4 model compound turned out to be a slightly endothermic partial reaction (23 kJmoU ) (Table 5) , which is hard to believe in light of the successful experiment in LXe. Using (LiMe)4 as substitute it was shown that the reaction is actually exothermic... 

Many, but not all, endothermic compounds have been involved in violent decompositions, reactions or explosions, and in general, compounds with significantly positive values of standard heat of formation may be considered suspect on stabihty groimds. Notable exceptions are benzene and toluene -1-82.2, 50.0 kJ/mol ... 

Acetylene is an endothermic compound, its heat of formation —AH ) being — 54.9 kcal/mole. Its heat of explosion is therefore very great, viz. 1870kcal/kg, although the explosion is not connected with an oxidation reaction ... 

The heat of formation (enthalpy of formation) of a compound is an important thermodynamic quantity, because a table of heats of formation of a limited number of compounds enables one to calculate the heats of reaction (reaction enthalpies) of a great many processes, that is, how exothermic or endothermic these reactions are. The heat of formation (enthalpy of formation) of a compound at a specified temperature T is defined [195] as the standard heat of reaction (standard reaction enthalpy) for formation of the compound at T from its elements in their standard states (their reference states). By the standard state of an element we mean the thermodynamically stablest state at 105 Pa (standard pressure, about normal atmospheric pressure), at the specified temperature (the exception is phosphorus, for... 

Both carbides, Fe3C and Fe2C, are thus endothermic compounds at temperatures above 700° C., and the diminished solubility of graphite in iron above 2220° C. is due to dissociation of the diferro carbide. Below 2220° C. the reaction... 

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