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Enthalpy is defined as a thermodynamic state function, designated by the letter "H", that consists of the internal energy of the system (U) plus the product of pressure (p) and volume (V) of the system:〔Mark W. Zemansky (1968), ''Heat and Thermodynamics'', Chapter 11 (5th edition) page 275, McGraw Hill, New York.〕 :: Since U, p and V are all functions of the state of the thermodynamic system, enthalpy is a state function. The unit of measurement for enthalpy in the International System of Units (SI) is the joule, but other historical, conventional units are still in use, such as the British thermal unit and the calorie. The enthalpy is the preferred expression of system energy changes in many chemical, biological, and physical measurements at constant pressure, because it simplifies the description of energy transfer. At constant pressure, the enthalpy change equals the energy transferred from the environment through heating or work other than expansion work. The total enthalpy, ''H'', of a system cannot be measured directly. The same situation exists in classical mechanics: only a change or difference in energy carries physical meaning. Enthalpy itself is a thermodynamic potential, so in order to measure the enthalpy of a system, we must refer to a defined reference point; therefore what we measure is the change in enthalpy, Δ''H''. The Δ''H'' is a positive change in endothermic reactions, and negative in heat-releasing exothermic processes. For processes under constant pressure, Δ''H'' is equal to the change in the internal energy of the system, plus the pressure-volume work that the system has done on its surroundings.〔G.J. Van Wylen and R.E. Sonntag (1985), ''Fundamentals of Classical Thermodynamics'', Section 5.5 (3rd edition), John Wiley & Sons Inc. New York. ISBN 0-471-82933-1〕 This means that the change in enthalpy under such conditions is the heat absorbed (or released) by the material through a chemical reaction or by external heat transfer. Enthalpies for chemical substances at constant pressure assume standard state: most commonly 1 bar pressure. Standard state does not, strictly speaking, specify a temperature (see standard state), but expressions for enthalpy generally reference the standard heat of formation at 25 °C. Enthalpy of ideal gases and incompressible solids and liquids does not depend on pressure, unlike entropy and Gibbs energy. Real materials at common temperatures and pressures usually closely approximate this behavior, which greatly simplifies enthalpy calculation and use in practical designs and analyses. ==Origins== The word ''enthalpy'' is based on the Greek ''enthalpein'' (ἐνθάλπειν), which means "to warm in".〔("ἐνθάλπω" ). ''A Greek–English Lexicon''.〕 It comes from the Classical Greek prefix ἐν- ''en-'', meaning "to put into", and the verb θάλπειν ''thalpein'', meaning "to heat". The word ''enthalpy'' is often incorrectly attributed to Benoît Paul Émile Clapeyron and Rudolf Clausius through the 1850 publication of their Clausius–Clapeyron relation. This misconception was popularized by the 1927 publication of ''The Mollier Steam Tables and Diagrams''. However, neither the concept, the word, nor the symbol for enthalpy existed until well after Clapeyron's death. The earliest writings to contain the concept of enthalpy did not appear until 1875, when Josiah Willard Gibbs introduced "a heat function for constant pressure". However, Gibbs did not use the word "enthalpy" in his writings.〔''The Collected Works of J. Willard Gibbs, Vol. I'' do not contain reference to the word enthalpy, but rather reference the heat function for constant pressure.〕 The actual word first appears in the scientific literature in a 1909 publication by J. P. Dalton. According to that publication, Heike Kamerlingh Onnes (1853-1926) actually coined the word. Over the years, many different symbols were used to denote enthalpy. It was not until 1922 that Alfred W. Porter proposed the symbol "H" as the accepted standard, thus finalizing the terminology still in use today. 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「enthalpy」の詳細全文を読む スポンサード リンク
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