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

Le changement d'enthalpie d'une réaction peut être mesuré à l'aide d'un calorimètre, ou il peut être trouvé en calculant la différence d'enthalpie entre les réactifs et les produits. Cependant, les enthalpies absolues des réactifs et des produits ne peuvent pas être mesurées directement;par conséquent, les chimistes utilisent généralement le changement d'enthalpie, ou delta H, par rapport à une substance de référence dans un état standard convenu. L'état standard est défini par un ensemble spécifique de conditions.Cela comprend une température réglée, généralement 25 degrés Celsius ou 298 Kelvin et une pression constante de 1 bar pour les gaz. Pour les solutions, l'état standard est une concentration molaire 1 d'un soluté pur dans un solvant. L'état standard d'une substance comprend également l'état physique de la matière dans lequel la substance existe dans ces conditions.Par exemple, le chlorure de sodium sous forme solide, le mercure sous forme liquide ou l'hélium sous forme gazeuse. Si un élément existe dans plus d'une forme dans ces conditions, la forme la plus stable de l'élément est définie comme l'état standard. Par exemple, le carbone peut exister sous forme de cristaux de graphite ou de diamant, mais le graphite est la forme la plus stable et, par conséquent, l'état standard du carbone.Lorsque les éléments dans leurs états standard se combinent pour former 1 mole d'un composé pur, l'enthalpie de la réaction est appelée enthalpie standard, ou chaleur standard, de formation. Ceci est noté par delta H naught, indice f, où naught indique les états standard des éléments constitutifs, tandis que f indique la formation. L'enthalpie standard de formation pour les éléments purs dans des conditions d'état standard est toujours nulle car il n'y a pas de réaction, et donc pas de changement d'enthalpie, lorsque l'élément est déjà dans son état standard.Les valeurs de l'enthalpie standard de formation en kilojoules par mole pour un composé peuvent être trouvées dans les tableaux de référence. Ces substances comprennent des éléments dans des états non standard, tels que le sodium gazeux, et des composés tels que le chlorure de sodium. Le changement d'enthalpie standard d'une réaction peut être calculé à partir de la différence des produits et des réactifs, qui à son tour, peut être calculé en utilisant les valeurs d'enthalpie de la table de référence.

<ol>
	<li>Hawk, Eric Leigh. « Le calcul des enthalpies standard de formation d’alcanes : illustration de la mécanique moléculaire et des programmes de tableur. » Journal of chemical education 76, no. 2 (1999) : 278.</li>
	<li>Mazzuca, James W., Alexis R. Downing et Christopher Potter. « Calculs de structure électronique corrigés empiriquement appliqués à l’enthalpie de la combustion laboratoire de chimie physique. » Journal of Chemical Education 96, n° 6 (2019) : 1165-1170.</li>
	<li>Jansen, Michael P. "Le coût de la conversion d’un véhicule à essence au propane. Un problème d’examen pratique pour le lycée ou l’introduction à la chimie. Journal of Chemical Education 77, n° 12 (2000) : 1578. 
	 </li>
</ol>

enthalpie standard de formation

Les variations d'enthalpie sont généralement tabulées pour les réactions dans lesquelles les réactifs et les produits se trouvent dans les mêmes conditions. Un état standard est un ensemble de conditions communément admis, qui est utilisé comme point de référence pour la détermination des propriétés dans d'autres conditions différentes. Pour les chimistes, l'état standard UICPA se réfère aux matériaux sous une pression de 1 bar et aux solutions à 1 M et ne spécifie pas de température. De nombreuses tables thermochimiques répertorient des valeurs avec un état standard de 1 atm. Comme le ΔH d'une réaction change très peu avec de si petites variations de pression (1 bar = 0,987 atm), les valeurs de ΔH (à l'exception des valeurs mesurées avec la plus grande précision) sont essentiellement les mêmes dans les deux ensembles de conditions standard. Un “ o ” en exposant dans le symbole de la variation d'enthalpie désigne l’état standard. Étant donné que la température habituelle (mais pas techniquement standard) est de 298,15 K, cette température sera admise sauf si une autre température est précisée. Ainsi, le symbole (ΔH°) est utilisé afin d'indiquer une variation d'enthalpie pour un processus se produisant dans ces conditions (le symbole ΔH est utilisé afin d'indiquer une variation d'enthalpie pour une réaction se produisant dans des conditions non standard). 
 
 Les variations de l'enthalpie pour de nombreux types de processus chimiques et physiques sont disponibles dans la documentation de référence, y compris pour les réactions de combustion, les transitions de phase et les réactions de formation. Comme la variation d'enthalpie pour une réaction donnée est proportionnelle aux quantités de substances impliquées, elle peut être indiquée sur cette base (c'est-à-dire comme le ΔH pour des quantités précises de réactifs). Cependant, nous trouvons souvent qu'il est plus utile de diviser une propriété extensive (ΔH) par une autre (quantité de substance), et d'indiquer une valeur intensive de ΔH par quantité, souvent &ldquo normalisée ” à la base de par mole.   
 
 <h4>Enthalpie standard de formation </h4> 
 
 L'enthalpie standard de formation ΔHf° est une variation d'enthalpie pour une réaction dans laquelle exactement 1 mole d'une substance pure est formée à partir de corps simples dans leurs états les plus stables dans les conditions d'état standard. Ces valeurs sont particulièrement utiles pour calculer ou prédire les variations d'enthalpie pour les réactions chimiques qui sont peu pratiques ou dangereuses à effectuer, ou pour les processus pour lesquels il est difficile de réaliser des mesures. En utilisant les valeurs connues des enthalpies standard de formation, il est possible de déterminer la variation d'enthalpie pour n'importe quelle réaction. 
 
 L'enthalpie standard de formation de CO2 (g) est de −393,5 kJ/mol. Il s'agit de la variation d'enthalpie pour la réaction exothermique :   
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/801/801_Equations-01.png" style="height: 40px;" /> 
 
 En commençant par les réactifs à une pression de 1 atm et à 25 °C (avec le carbone présent sous forme de graphite, la forme de carbone la plus stable dans ces conditions) et en terminant par une mole de CO2, également à 1 atm et 25 °C. Pour le dioxyde d'azote, NO2 (g), ΔHf° est de 33,2 kJ/mol. Il s'agit de la variation d'enthalpie pour la réaction endothermique : 
 
 <img alt="Eq2" src="https://www.jovecore.com/CDNSource/lessons/801/801_Equations-02.png" style="height: 40px;" /> 
 
 Une équation de la réaction avec 1/2 mole de N2 et 1 mole d'O2 est correcte dans ce cas parce que l'enthalpie standard de formation se rapporte toujours à 1 mole de produit : NO2 (g). 
 Par définition, l'enthalpie standard de formation d'un élément dans sa forme la plus stable est égale à zéro dans des conditions standard. Par exemple, les enthalpies standard de formation du carbone (graphène), de l'oxygène diatomique gazeux, de l'azote diatomique gazeux, du sodium métallique et du mercure liquide sont nulles dans des conditions standard. 
 
 Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">Openstax, Chimie 2e, Section 5.3 : Enthalpie.</a>

Enthalpie standard de formation

Les variations d’enthalpie sont généralement présentées sous forme de tableau pour les réactions dans lesquelles les réactifs et les produits sont dans les mêmes conditions. Un état standard est un ...

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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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