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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Hess's law ： ウィキペディア英語版 Hess's law Hess's law of constant heat summation, also known as Hess's law (or Hess' law), is a relationship in physical chemistry named after Germain Hess, a Swiss-born Russian chemist and physician who published it in 1840. The law states that the total enthalpy change during the complete course of a chemical reaction is the same whether the reaction is made in one step or in several steps.〔(【引用サイトリンク】url=http://www.science.uwaterloo.ca/~cchieh/cact/c120/hess.html )〕 Hess's law is now understood as an expression of the principle of conservation of energy, also expressed in the first law of thermodynamics, and the fact that the enthalpy of a chemical process is independent of the path taken from the initial to the final state (i.e. enthalpy is a state function). Reaction enthalpy changes can be determined by calorimetry for many reactions. The values are usually stated for processes with the same initial and final temperatures and pressures, although the conditions can vary during the reaction. Hess's law can be used to determine the overall energy required for a chemical reaction, when it can be divided into synthetic steps that are individually easier to characterize. This affords the compilation of standard enthalpies of formation, that may be used as a basis to design complex syntheses. ==Definition == Hess's law states that the change of enthalpy in a chemical reaction (i.e. the heat of reaction at constant pressure) is independent of the pathway between the initial and final states. In other words, if a chemical change takes place by several different routes, the overall enthalpy change is the same, regardless of the route by which the chemical change occurs (provided the initial and final condition are the same). Hess's law allows the enthalpy change (ΔH) for a reaction to be calculated even when it cannot be measured directly. This is accomplished by performing basic algebraic operations based on the chemical equations of reactions using previously determined values for the enthalpies of formation. Addition of chemical equations leads to a net or overall equation. If enthalpy change is known for each equation, the result will be the enthalpy change for the net equation. If the net enthalpy change is negative (ΔHnet < 0), the reaction is exothermic and is more likely to be spontaneous; positive ΔH values correspond to endothermic reactions. Entropy also plays an important role in determining spontaneity, as some reactions with a positive enthalpy change are nevertheless spontaneous. Hess's law states that enthalpy changes are additive. Thus the ΔH for a single reaction : $\backslash Delta\; H\_^\backslash ominus\; =\; \backslash sum\; \backslash Delta\; H\_^\; -\; \backslash sum\; \backslash Delta\; H\_^$ where $\backslash Delta\; H\_f$ is an enthalpy of formation, and the o superscript indicates standard state values. This may be considered as the sum of two (real or fictitious) reactions: :Reactants → Elements :$\backslash Delta\; H^\backslash ominus\; =\; -\; \backslash sum\; \backslash Delta\; H\_^$ and Elements → Products :$\backslash Delta\; H^\backslash ominus\; =\; \backslash sum\; \backslash Delta\; H\_^$ 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hess's law」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.