Name: enthalpy and heat at constant pressure
Uploaded: 2020-09-03T13:38:27.000Z
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PS.Chm.7.2

Enthalpy

Некоторые химические реакции выделяют огромное количество тепла и воздействуют на окружающую среду, например, при сгорании ракетного топлива космический шаттл отрывается от земли. Сумма тепла q и работы w представляет собой изменение внутренней энергии ΔE, как это определено первым законом термодинамики. Для химических реакций с участием газов, происходящих при атмосферном давлении, выполняемая работа это механическая работа, связанная с изменением объема расширением или сжатием.Следовательно, работа равна отрицательному значению давления, умноженному на изменение объема. Замена w в первом законе термодинамики и перестановка членов в уравнении показывает, что q равно ΔE плюс P, умноженное на ΔV, что дает выражение для теплового потока при постоянном давлении. В других химических реакциях, таких как сжигание дров чтобы приготовить пищу, более уместно количественное определение выделяемого тепла для ее приготовления, чем измерение объема работы по расширению, производимой в окружающей среде.Поскольку внутренняя энергия учитывает как тепло, так и работу, ΔE не используется для условий постоянного давления. Чтобы рассматривать исключительно поток энергии в форме тепла, определена новая термодинамическая функция энтальпия. Энтальпия H равна сумме внутренней энергии E и произведения давления на объем PV.Поскольку энергия, давление и объем являются функциями состояния, энтальпия также является функцией состояния.Абсолютные значения энтальпии для определенных веществ не могут быть измерены. Можно определить только изменение энтальпии. Изменение энтальпии, ΔH, равно изменению внутренней энергии, ΔE, плюс P, умноженное на ΔV.Вспоминая, что изменение энергии является суммой тепла и произведения давления и объема, уравнения можно объединить, чтобы показать, что в условиях постоянного давления ΔH равно теплоте q, полученной или потерянной системой.Если система теряет энергию в окружающую среду в виде тепла, как при сжигании дерева, температура окружающей среды повышается. Это описывается знаком минус для q. Следовательно, ΔH становится отрицательным, и процесс описывается как экзотермический.Напротив, если система получает энергию из окружающей среды в виде тепла например, в реакции, происходящей в химической холодильной упаковке температура окружающей среды падает. Теплота в этом случае описывается положительным знаком. Это делает ΔH положительным, и процесс называется эндотермическим.

<ol>
	<li>Канагаратна, Себастьян Г. "Наглядное пособие по вычислениям энтальпии." Журнал химического образования 77, No9 (2000): 1178.</li>
	<li>Говард, Ирмгард К. "H означает энтальпию, благодаря Хайке Камерлингу Оннесу и Альфреду В. Портеру." Журнал химического образования 79, No6 (2002): 697.</li>
	<li>Ван Несс, Хендрик К. Классическая термодинамика неэлектролитных растворов. Elsevier, 2015. 
	 </li>
</ol>

Энтальпия

Химики обычно используют свойство, известное как энтальпия (H), для описания термодинамики химических и физических процессов. Энтальпия определяется как сумма внутренней энергии системы (E) и математического продукта ее давления (P) и объема (V): 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/796/796_Equations-01.png"" style=""height: 40px;"" />
 
 Энтальпия - это функция состояния. Значения энтальпии для конкретных веществ нельзя измерить напрямую; могут быть определены только изменения энтальпии для химических или физических процессов. Для процессов, которые происходят при постоянном давлении (обычное условие для многих химических и физических изменений), изменение энтальпии (& Delta; H ) составляет: 
 
 <img alt=""Eq2"" src=""https://www.jove.com/files/core/CDNSource/lessons/796/796_Equations-02.png"" style=""height: 40px;"" />
 
 Математическое произведение PΔV представляет работу (w), а именно, работу по расширению или по давлению-объему. По своим определениям, арифметические признаки ΔV и w всегда будут противоположными: 
 
 <img alt=""Eq3"" src=""https://www.jove.com/files/core/CDNSource/lessons/796/796_Equations-03.png"" style=""height: 40px;"" />
 
 Подстановка этого уравнения и определение внутренней энергии при постоянном давлении (ΔE = qp + w) в уравнение энтальпии-изменения дает: 
 
 <img alt=""Eq4"" src=""https://www.jove.com/files/core/CDNSource/lessons/796/796_Equations-04.png"" style=""height: 40px;"" />
 
 где qp — тепло реакции в условиях постоянного давления.  
 
 Таким образом, если химический или физический процесс выполняется при постоянном давлении с единственной работой, выполняемой вследствие расширения или сжатия (P-V работа), то тепловой поток (qp) и изменение энтальпии (ΔH) для процесса равны. 
 
 Тепло, выделяемое при работе горелки Бунзена, равно энтальпирующей смене реакции сгорания метана, которая происходит, так как происходит при существенно постоянном давлении в атмосфере. Химики обычно проводят эксперименты в нормальных атмосферных условиях при постоянном внешнем давлении с qp = ΔH, что делает энтальпию наиболее удобным выбором для определения изменений температуры химических реакций. 
 
 Отрицательное значение изменения энтальпии, ΔH < 0, указывает на экзотермическую реакцию (тепло, выделенное окружающей среде); положительное значение, ΔH > 0, указывает на эндотермическую реакцию (тепло, поглощенное окружающей средой). Если направление химического уравнения противоположно, то изменяется арифметический знак его ΔH (процесс, который является эндотермическим в одном направлении, является экзотермическим в противоположном направлении). 
 
 Концептуально, ΔE (мера тепла и работы) и ΔH (мера тепла при постоянном давлении) представляют изменения в функции состояния системы. В процессах, где изменение объема, ΔV, невелико (плавление льда), и ΔE и ΔH идентичны. Однако, если изменение объема значительно (испарение воды), то количество энергии, передаваемой в качестве работы, будет значительным; таким образом, ΔE и ΔH имеют значительно разные значения. 
 
 Этот текст адаптирован из <a href=""https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy"">Openstax, Химия 2е изд., раздел 5.3: Энтальпия</a>. "

Химики обычно используют свойство, известное как энтальпия (H), для описания термодинамики химических и физических процессов. Энтальпия определяется как сумма ...

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enthalpy and heat at constant pressure

Enthalpy describes heat changes in chemical and physical processes. Chemists use the symbol H for enthalpy. It is defined as a system’s internal energy, E, plus the product of pressure, P, and volume, V. Enthalpy is a state function, so only changes in enthalpy can be measured, not the enthalpy of a substance by itself.

For many reactions, the pressure stays constant. Under those conditions, the enthalpy change, ΔH, can be written in a way that includes pressure-volume work. The term PΔV represents expansion work, also called pressure-volume work. The signs of ΔV and w are always opposite, because expansion and work use opposite sign conventions.

At constant pressure, the internal energy change is ΔE = qp + w, where qp is the heat of reaction. When the only work done is due to expansion or contraction, qp and ΔH are equal. This is why enthalpy is a useful choice for measuring heat changes in many lab reactions and physical changes. A Bunsen burner is one example, since methane combustion occurs at about atmospheric pressure.

A negative ΔH means the process is exothermic, which means heat is given off to the surroundings. A positive ΔH means the process is endothermic, which means heat is absorbed from the surroundings. If a chemical equation is reversed, the sign of ΔH also changes. A process that is endothermic in one direction becomes exothermic in the other.

Enthalpy change and internal energy change both describe changes in a state function for the system. When the volume change, ΔV, is small, such as during melting of ice, ΔE and ΔH are essentially the same. When the volume change is larger, such as during evaporation of water, more energy is transferred as work. In that case, ΔE and ΔH can differ by a significant amount.

Enthalpy and Heat at Constant Pressure

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Enthalpy and Heat at Constant Pressure

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