Exothermic compounds

Nitration Combination with oxidation possible, both highly exothermic, compounds... 

The monochloride is soluble in various inert organic liquids, more particularly in benzene, chloroform, carbon tetrachloride and carbon disulphide, without undergoing chemical change. It is an exothermic compound, its heat of formation from gaseous chlorine and the amorphous modification of selenium being 22-1 Cals.1 Water causes a gradual decomposition of the chloride, selenium dioxide and selenium being formed 2... 

Selenium dioxide is an exothermic compound, the heat of formation being 57-2 Calories per gram-molecule, referred to vitreous selenium, whilst the value is somewhat less for the monoclinic element and less still for metallic selenium.1... 

As might be expected, xenon does not form any strong bonds, bur it does form exothermic compounds with fluorine. Some typical bond strengths are listed in Table 17.1. Bartlett22 has shown that such values might have been expected by extrapolation of known bond energy in related nonmeta] compounds. 

An exothermic compound is a compound whose formation is accompanied by a negative change in heat content, i.e.. with the liberation of heat. 

Like nitrous oxide, carbon disulphide is an endothermic compound, and can consequently be decomposed by shock when a fulminate is exploded in it, it is resolved into carbon and sulphur. On the other hand, carbon dioxide and oxysulphide are exothermic compounds, heat being evolved during their formation. 

In view of the instability of the trioxide it is interesting to speculate on the stability of the oxyfluorides. Xenon oxide tetrafluoride should be an exothermic compound since the atomic heat of formation using average bond energies of 32 kcal. and 17 kcal. respectively for Xe-F and Xe-O is —160 kcal. mole-. The combined heat of atomization of four fluorine atoms (4 x 18.3 kcal.) and one oxygen atom (59.2 kcals.), 132.4 kcal., is less than this. Xenon dioxide difluoride, by such considerations, should be endothermic. It is of interest that xenon oxide tetrafluoride is easily made by controlled hydrolysis of xenon hexafluoride and is a thermally stable entity (m.p. —28) whereas xenon dioxide difluoride has been observed only mass spectrometrically < >. 

When, in certain circumstances, the formation of a compound from its elements liberates heat, or when its decomposition absorbs heat, the compound is called an exothermic compound in the circumstances considered. 

Water, hydrochloric acid, carbonous oxide, carbonic anhydride, are, in all circumstances, exothermic compounds. 

Dissociation of exothermic compounds and formation of endothermic compounds by rise in temperature.—Some purely chemical applications will indicate more clearly the importance of the law stated by Van Hoff. 

As the temperature is raised higher and higher, we see increase the number of reactions, decompositions of exothermic compounds, or syntheses of endothermic compounds which are exceptions to the principle of maximum work. According to the happy ex ... 

Analogous phenomena shown by endothermic combinations.—A, compound formed from its elements with absorption of heat may very well present phenomena analogous to those we have just described for an exothermic compound for Fig. 121 we should then substitute a representation such as Fig. 122. Here also x con-... 

Explosive combinations.— All we have just said on the subject of a system in which an exothermic compound is formed, according to the laws of which the reaction... 

X74. Dissociation of exothermic compounds and formation of endothermic compounds by rise in temperature, 205.—175. Actions produced by a series of electric sparks interpretation given by H. Saint-Claire I ville. Apparatus with cold and hot tubes, 206.—X76. Dissociation of carbonous oxide, of sulphurous and hydrochloric add gases. Synthesis of ozone, 208.—177 Synthesis of acetylene,... 

This interesting substance, which is an exothermic compound, was obtained by Rosolovskii, Krivtsov and Titova [97]. Us enthalpy of formation is — A//f = kcal/mol. 

We can now use this lattice energy to construct the Bom-Haber cycle for CaCl and show that it is indeed an exothermic compound. The cycle is shown below with all values taken from Table 1.5 in your textbook except the Ca sublimation energy given in the exercise text ... 

One reason why enthalpies of formation are important is that they give us some idea of the stabilities of compounds. Compounds for which AH is negative are apt to be stable, since they can only be converted into their elements by the addition of a positive amount of heat. Such compounds are known as exothermic compounds. 

The heat of formation of the paraffin hydrocarbons is positive, that is, heat is given off in their formation. They are exothermic compounds. The heats of formation of some hydrocarbons are negative. This means that the compounds when burned produce more heat than is produced when the equivalent weights of carbon and hydrogen are burned. Such compounds are called endothermic. 

The value of AH is an approximate measure of the stability of a substance relative to the elements from which it is made. The standard enthalpies of formation of graphite, diamond, water, ethyne (acetylene, C2H2), ammonia and sodium chloride are shown in Fig. 13.6. The reference states of elements define an energy baseline or sea level . Compounds such as ethyne, for which AHf is positive, and which therefore possess a greater enthalpy than their constituent elements, appear above sea level and are called endothermic compounds. Compounds such as water, ammonia and sodium chloride, for which AHf is negative and which therefore possess a lower enthalpy than their constituent elements, appear below sea level and are called exothermic compounds. 

Notice that the eflect of producinga highly exothermic compound (AIzOsCs)), is to make the overall reaction highly exothermic.)... 

Still another way to increase energy output is the use of endothermic compounds as oxidizers. The oxides of nitrogen (NgO, NO, NgO,) are such endothermic compounds. Ozone (Os) has been considered and tried, though it is hazardous to handle in undiluted state. On the other hand, even a stable exothermic compound such as nitrogen trifluoride (NFj) can furnish with hydrogen an extremely hot flame (fCirk-Othtner, Volume, 9, p. 629). 

Taking by convention the heat of formation of all elements in the standard state (explained below) as zero, the heat of formation of a compound can be calculated from heat of combustion data. Again, the pyrotechnician will use established values from previously quoted sources but must be careful in the use of such figures to observe their positive or negative character An exothermic compound has, for the scientisty a negative heat of formation because the system loses energy, and the same applies to an exothermic reaction. 

Fig. 12d shows a particular case of the type 2b. Here the mixture shows ideal behaviour at great dilution of the non-volatile component, whereas at low concentrations of the volatile component the behaviour of an exothermic mixture is clearly demonstrated. Such an S-shaped curve is e. g., observed in the system sulphuric acid-water, where the formation of strongly exothermic compounds (hydrates) of the acid is known to exist. The occurrence of such specific chemical binding forces will always tend to result in a vapour tension curve which is concave to the pressure axis in the region where compound formation dominates the process of mixing. It would seem that in this case the mixing of the hydrates with further amounts of water is a less thermally positive or even a thermally neutral process. 

A myriad of flame types are met in and out of the laboratory so it is perhaps worthwhile listing the principal types and indicating their relationships. Most flames involve two reactants, usually designated fuel and oxidizer. This is not an absolute requirement, however, since one can have a decomposition flame with a single exothermic compound such as ozone or, conversely, flames can be burned with mixed fuels or oxidizers. Even the presence of oxidizing and reducing species or groups is not completely necessary since flames have been obtained with such unlikely pairs as hydrazine and diborane (Berl and Wilson, 1961). 

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