Compounds, chemical, formation endothermic

We next consider the synthesis and chemical reactions of the oxides of chlorine. Because the compounds are strongly endothermic and have large positive free energies of formation it is not possible to prepare them by direct reaction of CI2 and O2. Dichlorine monoxide, CI2O, is best obtained by treating freshly prepared yellow HgO and CI2 gas (diluted with dry air or by dissolution in CCI4) ... 

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. 

THERMOCHEMISTRY. That aspect of chemistry which deals with die heat changes which accompany chemical reactions and processes, the heal produced by them, and die influence of temperature and odier thermal quantities upon them. Tt is closely related to chemical thermodynamics. The heat of formation of a compound is the heat absorbed when it is formed from its elements in their standard states. An exothermic reaction evolves heat and endothermic reaction requires heat for initiation. 

When the only work performed is of the pAV type, Qp is termed the enthalpy change AH). It can be measured in a constant-pressure calorimeter. If heat is given off in such a calorimeter experiment, the reaction is exothermic and AH is negative. If heat is absorbed, the reaction is endothermic and AH is positive. In chemical compounds, enthalpies of formation are related to the sum of their bond energies. 

The quantity of heat set free by a system which undergoes a transformation does not depend solely upon the initial and final states, page 86.-32. Example from the study of perfect gases, 37.—33. Case in which the quantity of heat set free by a system depends solely upon the initial and final states, 88.—34. Utility, in chemical calorimetry, of the preceding law, 89.-35. Exothermic and endothermic reactions, 41.—36. Heats of formation under constant pressure and at constant volume, 44.—37. Case in which the two heats of formation are equal to each other, 45.— 38. General relation between the two heats of formation, 45.—39. Case in which the compound is a perfect gas, 46.—40. The distinction between the two heats of formation has small importance in practice, 46.—41. Infiuence of temperature on the heats of formation, 47.-42. Heat of formation referred to a temperature at which the reaction considered is impossible, 48.—... 

The reference states of the elements define a thermochemical sea level , and enthalpies of formation can be regarded as thermo chemical altitudes above or below sea level (Fig. 1.25). Compounds that have negative standard enthalpies of formation (such as water) are classified as exothermic compounds, for they lie at a lower enthalpy than their component elements (they lie below thermo chemical sea level). Compounds that have positive standard enthalpies of formation (such as carbon disulfide) are classified as endothermic compounds and possess a higher enthalpy than their component elements (they lie above sea level). 

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