Name: using standard enthalpy values in chemistry
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Standard Enthalpy of Formation

Изменение энтальпии реакции можно измерить с помощью калориметра или вычислить разность энтальпии между исходными веществами и продуктами. Однако абсолютные энтальпии исходных веществ и продуктов нельзя измерить напрямую;поэтому химики обычно используют изменение энтальпии или дельта H относительно эталонного вещества в согласованном стандартном состоянии. Стандартное состояние определяется определенным набором условий.Это включает установленную температуру, обычно 25 градусов Цельсия или 298 Кельвинов, и постоянное давление 1 бар для газов. Для растворов стандартным состоянием является 1-молярная концентрация чистого растворенного вещества в растворителе. Стандартное состояние вещества также включает физическое состояние вещества, в котором оно существует в этих условиях.Например, хлорид натрия в твердом состоянии, ртуть в виде жидкости или гелий в виде газа. Если элемент существует более чем в одной форме при этих условиях, наиболее стабильная форма элемента определяется как стандартное состояние. Например, углерод может существовать в виде кристаллов графита или алмаза, но графит является наиболее стабильной формой и, следовательно, стандартным состоянием углерода.Когда элементы в их стандартных состояниях соединяются с образованием 1 моля чистого соединения, энтальпия реакции называется стандартной энтальпией или стандартной теплотой образования. Она обозначается как дельта H ноль, индекс f, где ноль указывает на стандартные состояния составляющих элементов, а f указывает на образование. Стандартная энтальпия образования чистых элементов в стандартных условиях всегда равна нулю, поскольку реакция не протекает и, следовательно, изменения энтальпии не происходит, когда элемент уже находится в своем стандартном состоянии.Значения стандартной энтальпии образования в килоджоулях на моль для соединения можно найти в справочных таблицах. Эти вещества включают элементы в нестандартных состояниях, такие как газообразный натрий, и такие соединения, как хлорид натрия. Стандартное изменение энтальпии реакции может быть вычислено из разницы продуктов и исходных веществ, которые, в свою очередь, могут быть рассчитаны с использованием значений энтальпии из справочной таблицы.

<ol>
	<li>Хоук, Эрик Ли. "Расчет стандартных энтальпий образования алканов: иллюстрирование молекулярной механики и программ электронных таблиц." Журнал химического образования 76, No2 (1999): 278.</li>
	<li>Маццука, Джеймс В., Алексис Р. Даунинг и Кристофер Поттер. «Эмпирически скорректированные расчеты электронной структуры, примененные к лаборатории физической химии горения». Журнал химического образования 96, No 6 (2019): 1165-1170.</li>
	<li>Янсен, Майкл. «Стоимость перевода автомобиля с бензиновым двигателем на пропан. Практическая задача для старших классов средней школы или введение в химию." Журнал химического образования 77, No12 (2000): 1578. 
	 </li>
</ol>

Стандартная энтальпия образования

Изменения энтальпии обычно табулируются для реакций, в которых реагенты и продукты находятся в одинаковых условиях. Стандартное состояние — это общепринятый набор условий, используемых в качестве опорной точки для определения свойств при других различных условиях. Для химиков стандартное состояние, согласно международному Союзу Чистой и Прикладной Химии (IUPAC)относится к материалам под давлением 1 бар и растворами при 1 м и не указывает температуру. Многие термохимические таблицы содержат значения со стандартным состоянием 1 атм. Поскольку ΔH реакции изменяется очень мало при таких небольших изменениях давления (1 бар = 0.987 атм), значения ΔH (за исключением наиболее точно измеренных значений) по существу одинаковы при обоих наборах стандартных условий. В символе смены энтальпии “o” обозначает стандартное состояние. Поскольку обычная (но нетехнически стандартная ) температура составляет 298.15 К, эта температура будет учитываться, если не указана другая температура. Таким образом, символ (ΔH°) используется для обозначения изменения энтальпии процесса, происходящего в этих условиях. (Символ ΔH используется для обозначения изменения энтальпии для реакции, возникающей в нестандартных условиях.) 
 
 Изменения энтальпии для многих типов химических и физических процессов можно найти в справочной литературе, включая изменения в реакциях сгорания, фазовых переходах и реакциях на формирование. Поскольку изменение энтальпии для данной реакции пропорционально количеству вовлеченных веществ, оно может быть сообщено на этой основе (например, как ΔH для конкретных объемов реагенты). Однако мы часто находим более полезным разделить одно обширное свойство (ΔH) на другое (количество вещества) и сообщить о интенсивном значении в ΔH, часто “нормализованном” к базису в моле.  
 
 <h4>Стандартная энтальпия образования </h4>
 
 Стандартная энтальпия образования ΔHf° представляет собой энтальпию изменения для реакции, при которой ровно 1 моль чистого вещества образуется из свободных элементов в их наиболее стабильных состояниях в стандартных условиях. Эти значения особенно полезны для вычисления или прогнозирования изменений энтальпии химических реакций, которые непрактичны или опасны для выполнения, или для процессов, для которых трудно проводить измерения. Используя известные значения стандартных энтальпий образования, можно определить изменение энтальпии для любой реакции. 
 
 Стандартная энтальпия образования CO2 (г) составляет − 393.5 кДж/моль. Это изменение энтальпии экзотермической реакции:  
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/801/801_Equations-01.png"" style=""height: 40px;"" />
 
 Начиная с реагенты при давлении 1 атм и 25 °C (с углеродом, присутствующим в графите, наиболее стабильной формой углерода в этих условиях) и заканчивая одним моль CO2, также при 1 атм и 25 °C. Для диоксида азота, NО2 (г), ΔHf° составляет 33.2 кДж/моль. Это изменение энтальпии эндотермической реакции: 
 
 <img alt=""Eq2"" src=""https://www.jove.com/files/core/CDNSource/lessons/801/801_Equations-02.png"" style=""height: 40px;"" />
 
 Уравнение реакции с 1/2 моль N2 и 1 моль O2 в данном случае верно, так как стандартная энтальпия образования всегда относится к 1 моль продукта: NО2 (г). 
 
 По определению, стандартная энтальпия образования элемента в его наиболее устойчивой форме равна нулю при стандартных условиях. Например, стандартные энтальпии образования углерода (графена), диатомного кислорода, диатомного азота, натриевого металла и жидкой ртути в стандартных условиях равны нулю. 
 
 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>

Изменения энтальпии обычно сводятся в таблицу для реакций, в которых реагенты и продукты находятся в одинаковых условиях. Стандартное состояние — это обычное ...

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using standard enthalpy values in chemistry

Enthalpy changes are often listed for reactions that start and end under the same conditions. Chemists use a standard state as a reference set of conditions for comparing properties. In IUPAC terms, the standard state means 1 bar of pressure and 1 M solutions, but it does not set a temperature.

Many thermochemical tables still use 1 atm as the reference pressure. Because the difference between 1 bar and 1 atm is very small, enthalpy change usually stays almost the same. A superscripted o in the enthalpy symbol shows that the value is for standard state. The usual temperature is 298.15 K, so ΔH° is often used for a process under these conditions. ΔH is used for nonstandard conditions.

Standard enthalpy values are available for many chemical and physical processes. These include combustion reactions, phase changes, and formation reactions. Enthalpy change depends on the amount of substance involved, so it can be reported for a specific reaction amount. It is often more useful to divide by moles and give a per-mole value.

The standard enthalpy of formation, ΔHf°, is the enthalpy change when 1 mole of a pure substance forms from its elements in their most stable states under standard conditions. This value is useful when a reaction is hard to measure directly or would be unsafe to run. Known formation enthalpies can also be used to calculate the enthalpy change of other reactions.

For example, the standard enthalpy of formation of CO2(g) is −393.5 kJ/mol. That value matches the exothermic formation of one mole of carbon dioxide from graphite, the most stable form of carbon under these conditions, with all substances at 1 atm and 25 °C. For NO2(g), ΔHf° is 33.2 kJ/mol. Here the balanced formation reaction uses 1/2 mole of N2 and 1 mole of O2 because the value must refer to 1 mole of NO2 product.

By definition, an element in its most stable form has a standard enthalpy of formation of zero. Examples include carbon as graphene, diatomic oxygen gas, diatomic nitrogen gas, sodium metal, and liquid mercury. These zero values help make thermochemical calculations consistent.

Using Standard Enthalpy Values in Chemistry

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Using Standard Enthalpy Values in Chemistry

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