Name: using standard enthalpy values in chemistry
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PS.Chm.7.2

Standard Enthalpy of Formation

El cambio de entalpía de una reacción. se puede medir con un calorímetro, o se puede encontrar calculando la diferencia de entalpía entre los reactivos y los productos. Sin embargo, las entalpías absolutas de los reactivos y productos no se puede medir directamente;por lo tanto, los químicos generalmente usan el cambio de entalpía, o delta H, relativo a una sustancia de referencia en estado estándar acordado.El estado estándar está definido por un conjunto específico de condiciones. Esto incluye una temperatura establecida, generalmente de 25 grados Celsius o 298 Kelvin, y una presión constante de 1 bar para los gases. Para las soluciones, el estado estándar es una concentración de 1 molar de un soluto puro en un solvente.El estado estándar de una sustancia también incluye el estado físico de la materia en el que la sustancia existe en estas condiciones. Por ejemplo, el cloruro de sodio como sólido, el mercurio como líquido, o el helio como gas. Si un elemento existe en más de una forma en estas condiciones, la forma más estable del elemento se define como el estado estándar.Por ejemplo, el carbono puede existir como cristales de grafito o como un diamante, pero el grafito es la forma más estable y, por tanto, el estado estándar del carbono. Cuando los elementos en sus estados estándar se combinan para formar 1 mol de un compuesto puro, la entalpía de la reacción se llama entalpía estándar, o calor estándar, de formación. Esto lo indica delta H cero, subíndice f, donde cero indica los estados estándar de los elementos constitutivos, mientras que f indica la formación.La entalpía estándar de formación para elementos puros en condiciones estándar es siempre cero porque no hay reacción, y por lo tanto sin cambios en la entalpía, cuando el elemento ya está en su estado estándar. Los valores de la entalpía estándar de formación en kilojulios por mol para un compuesto se puede encontrar en las tablas de referencia. Estas sustancias incluyen elementos en estados no estándar, como sodio gaseoso, y compuestos como cloruro de sodio.El cambio de entalpía estándar de una reacción se puede calcular a partir de la diferencia de productos y reactivos, que a su vez, pueden calcularse utilizando los valores de la entalpía de la tabla de referencia.

<ol>
	<li>Halcón, Eric Leigh. "El cálculo de entalpías estándar de formación de alcanos: ilustración de la mecánica molecular y los programas de hojas de cálculo". Revista de educación química 76, no. 2 (1999): 278.</li>
	<li>Mazzuca, James W., Alexis R. Downing y Christopher Potter. "Cálculos de estructura electrónica corregidos empíricamente aplicados al laboratorio de química física de la entalpía de la combustión". Revista de Educación Química 96, no. 6 (2019): 1165-1170.</li>
	<li>Jansen, Michael P. "El costo de convertir un vehículo de gasolina a propano. Un problema práctico de repaso para la escuela secundaria superior o introducción a la química". Revista de Educación Química 77, no. 12 (2000): 1578. 
	 </li>
</ol>

entalpía estándar de formación

Los cambios en la entalpía suelen tabularse para las reacciones en las que tanto los reactivos como los productos se encuentran en las mismas condiciones. Un estado estándar es un conjunto de condiciones comúnmente aceptadas que se utiliza como punto de referencia para la determinación de propiedades bajo otras condiciones diferentes. Para los químicos, el estado estándar de la IUPAC se refiere a materiales bajo una presión de 1 bar y soluciones a 1 M y no especifica una temperatura. Muchas tablas termoquímicas enumeran valores con un estado estándar de 1 atm. Debido a que la ΔH de una reacción cambia muy poco con cambios tan pequeños en la presión (1 bar = 0,987 atm), los valores de ΔH (excepto los valores medidos con mayor precisión) son esencialmente los mismos en ambos conjuntos de condiciones estándar. Un superíndice “o” en el símbolo de cambio de entalpía designa el estado estándar. Puesto que la temperatura habitual (pero no técnicamente estándar) es de 298,15 K, se asumirá esta temperatura a menos que se especifique otra temperatura. Por lo tanto, el símbolo (ΔH°) se utiliza para indicar un cambio de entalpía en un proceso que se produce en estas condiciones. (El símbolo ΔH se utiliza para indicar un cambio de entalpía en una reacción que se produce en condiciones no estándar.)
 
 Los cambios en la entalpía para muchos tipos de procesos químicos y físicos están disponibles en la literatura de referencia, incluyendo aquellos para reacciones de combustión, transiciones de fase y reacciones de formación. Dado que el cambio de entalpía para una reacción dada es proporcional a las cantidades de las sustancias involucradas, puede ser reportado sobre esa base (es decir, como el ΔH para cantidades específicas de reactantes). Sin embargo, a menudo nos resulta más útil dividir una propiedad extensiva (ΔH) por otra (cantidad de sustancia), y reportar un valor intensivo por cantidad de ΔH, a menudo “normalizado” a una base por mol. 
 
 <h4>Entalpía estándar de formación</h4>
 
 La entalpía estándar de formación ΔHf° es un cambio de entalpía para una reacción en la que se forma exactamente 1 mol de una sustancia pura a partir de elementos libres en sus estados más estables bajo condiciones de estado estándar. Estos valores son especialmente útiles para calcular o predecir cambios de entalpía para reacciones químicas que son poco prácticas o peligrosas de realizar, o para procesos para los que es difícil realizar mediciones. Utilizando valores conocidos de entalpías estándar de formación, se puede determinar el cambio de entalpía para cualquier reacción.
 
 La entalpía estándar de formación del CO2 (g) es −393,5 kJ/mol. Este es el cambio de entalpía para la reacción exotérmica: 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/801/801_Equations-01.png" style="height: 40px;"" />
 
 comenzando con los reactantes a una presión de 1 atm y 25 °C (con el carbono presente como grafito, la forma más estable de carbono en estas condiciones) y terminando con un mol de CO2, también a 1 atm y 25 °C. Para el dióxido de nitrógeno, NO2 (g), la ΔHf° es 33,2 kJ/mol. Este es el cambio de entalpía para la reacción endotérmica:
 
 <img alt="Eq2" src="https://www.jovecore.com/CDNSource/lessons/801/801_Equations-02.png" style="height: 40px;" />
 
 Una ecuación de reacción con 1/2 mol de N2 y 1 mol de O2 es correcta en este caso porque la entalpía estándar de formación siempre se refiere a 1 mol de producto: NO2 (g).
 
 Por definición, la entalpía estándar de formación de un elemento en su forma más estable es igual a cero en condiciones estándar. Por ejemplo, las entalpías estándar de formación de carbono (grafeno), gas de oxígeno diatómico, gas de nitrógeno diatómico, metal de sodio y mercurio líquido son cero en condiciones estándar.
 
 Este texto ha sido adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">Openstax, Química 2e, Sección 5.3: Entalpía.</a>

Entalpía Estándar de Formación

Los cambios de entalpía se suelen tabular para reacciones en las que tanto los reactivos como los productos están en las mismas condiciones. Un estado estándar es un estado comúnmente ...

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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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