Thermochemistry Henry reaction

Germain Henri Hess is noted today for two fundamental principles of thermochemistry the law of constant summation of heat (known simply as Hess s law) and the law of thermoneutrality. These discoveries were remarkable in that they were postulated without any supporting theoretical framework and took place in a field of study almost totally neglected by his contemporaries. Hess s law is of immense practical importance and is used to this day to determine heats of reaction when their direct measurements are difficult or impossible. 

This laN was first formulated by the Swiss-Russian chemist Germain Henri Hess (180 "" 1 50), who is generally regarded as the founder of the field of thermochemistry. The law follows from the principle of conservation of energy. Thus, if reactions (a) arid (bf occur there is a net evolution of 30 fccal when 1 mol of Y is produced. We can then reconvert Y into 2A + B by the reverse of reaction (c). If the heat required to do this differed from 30 kcal, we would have obtained the starting materials with a net gain or loss of heat, which would violate the principle of conservation of energy. 

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