Name: using standard enthalpy values in chemistry
Uploaded: 2020-09-03T13:38:27.000Z
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North Carolina K-12 Science Standard Course of Study

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PS.Chm.7.2

Standard Enthalpy of Formation

La variazione di entalpia di una reazione può essere misurata utilizzando un calorimetro, oppure può essere trovata calcolando la differenza di entalpia fra i reagenti e i prodotti. Tuttavia, le entalpie assolute dei reagenti e dei prodotti non possono essere misurate direttamente;pertanto, i chimici generalmente utilizzano la variazione di entalpia, o delta H, relativa ad una sostanza di riferimento in uno stato standard concordato. Lo stato standard è definito da un insieme specifico di condizioni.

Ciò include una temperatura impostata, di solito a 25 gradi Celsius o 298 Kelvin, e una pressione costante di 1 bar per i gas. Per quanto riguarda le soluzioni, lo stato standard è una concentrazione molare di un soluto puro in un solvente. Lo stato standard per una sostanza include anche lo stato fisico della materia in cui la sostanza esiste, a determinate condizioni.

Per esempio, cloruro di sodio come solido, mercurio come liquido o elio come gas. Se un elemento esiste in più di una forma in queste condizioni, la forma più stabile dell'elemento è definita come stato standard. Per esempio, il carbonio può esistere come cristalli di grafite o come diamante, ma la grafite è la forma più stabile e dunque lo stato standard del carbonio.

Quando gli elementi nei loro stati standard si combinano per formare 1 mole di un composto puro, l'entalpia della reazione è detta entalpia standard, o calore standard di formazione. Questo è denotato da delta H zero, pedice f, dove zero indica gli stati standard degli elementi costitutivi, mentre f indica la formazione. L'entalpia standard di formazione per elementi puri in condizioni di stato standard è sempre zero perché non c'è reazione, e dunque nessuna variazione di entalpia, quando l'elemento è già nel suo stato standard.

I valori dell'entalpia standard di formazione in kilojoule per mole per un composto possono essere trovati nelle tabelle di riferimento. Queste sostanze includono elementi in stati non standard, come il sodio gassoso, e composti, come il cloruro di sodio. La variazione di entalpia standard di una reazione può essere calcolata dalla differenza di prodotti e reagenti, che a sua volta può essere calcolata utilizzando i valori di entalpia dalla tabella di riferimento.

<ol>
	<li>Falco, Eric Leigh. "Il calcolo delle entalpie standard di formazione degli alcani: illustrazione della meccanica molecolare e dei programmi per fogli di calcolo." Journal of chemical education 76, n. 2 (1999): 278.</li>
	<li>Mazzuca, James W., Alexis R. Downing e Christopher Potter. "Calcoli di struttura elettronica corretti empiricamente applicati all'entalpia del laboratorio di chimica fisica della combustione." Journal of Chemical Education 96, n. 6 (2019): 1165-1170.</li>
	<li>Jansen, Michael P. "Il costo della conversione di un veicolo a benzina in propano. Un problema di revisione pratica per la scuola superiore o la chimica introduttiva. Journal of Chemical Education 77, n. 12 (2000): 1578. 
	 </li>
</ol>

entalpia standard di formazione

Le variazioni di entalpia vengono tipicamente tabulate per reazioni in cui sia i reagenti che i prodotti si trovano nelle stesse condizioni. Uno stato standard un insieme di condizioni comunemente accettate utilizzate come punto di riferimento per la determinazione delle propriet in condizioni diverse. Per i chimici, lo stato standard IUPAC si riferisce a materiali sotto una pressione di 1 bar e soluzioni a 1 M e non specifica una temperatura. Molte tabelle termochimiche elencano valori con uno stato standard di 1 atm. Poich lΔH di una reazione cambia molto poco con variazioni di pressione cos piccole (1 bar = 0,987 atm), i valori ΔH (ad eccezione dei valori misurati pi precisamente) sono essenzialmente gli stessi in entrambi i gruppi di condizioni standard. Una “o? in apice nel simbolo di variazione di entalpia indica lo stato standard. Poich la temperatura abituale (ma non tecnicamente standard) 298,15 K, verr presupposta questa temperatura a meno che non ne venga specificata un'altra. Pertanto, il simbolo (ΔH°) viene utilizzato per indicare una variazione di entalpia per un processo che avviene in queste condizioni. (Il simbolo ΔH viene utilizzato per indicare una variazione di entalpia per una reazione che avviene in condizioni non standard.)

Le variazioni di entalpia per molti tipi di processi chimici e fisici sono disponibili nella letteratura di riferimento, comprese quelle per reazioni di combustione, transizioni di fase e reazioni di formazione. Poich la variazione di entalpia per una data reazione proporzionale alla quantit di sostanze coinvolte, pu essere riportata su quella base (cio come ΔH per quantit specifiche di reagenti). Tuttavia, spesso troviamo pi utile dividere una propriet estensiva (ΔH) per un'altra (quantit di sostanza) e riportare un valore intensivo per quantit di ΔH, spesso “normalizzato? su base per mole.

Entalpia standard di formazione

L'entalpia standard di formazione ΔHf° una variazione di entalpia per una reazione in cui esattamente 1 mole di una sostanza pura formata da elementi liberi nei loro stati pi stabili in condizioni di stato standard. Questi valori sono particolarmente utili per calcolare o prevedere variazioni di entalpia per reazioni chimiche poco pratiche o pericolose da eseguire o per processi per i quali difficile effettuare misurazioni. Utilizzando valori noti delle entalpie standard di formazione, possibile determinare la variazione di entalpia per qualsiasi reazione.

L'entalpia standard di formazione di CO2 (g) −393,5 kJ/mol. Questa la variazione di entalpia per la reazione esotermica:

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/801/801_Equations-01.png" style="height: 40px;" />

iniziando con i reagenti alla pressione di 1 atm e 25C (con il carbonio presente come grafite, la forma di carbonio pi stabile in queste condizioni) e terminando con una mole di CO_2, sempre a 1 atm e 25C. Per il biossido di azoto, N_O2 (g), ΔHf° 33,2 kJ/mol. Questa la variazione di entalpia per la reazione endotermica:

<img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/801/801_Equations-02.png" style="height: 40px;" />

Un'equazione di reazione con 1/2 mole di N_2 e 1 mole di O_2 in questo caso corretta perch l'entalpia standard di formazione si riferisce sempre a 1 mole di prodotto: N_O2 (g).

Per definizione, l'entalpia standard di formazione di un elemento nella sua forma pi stabile pari a zero in condizioni standard. Ad esempio, le entalpie standard di formazione di carbonio (grafene), ossigeno biatomico, azoto biatomico, sodio metallico e mercurio liquido sono pari a zero in condizioni standard.

Questo testo adattato da <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">Openstax, Chemistry 2e, Section 5.3: Enthalpy.</a>

Entalpia standard di formazione

Le variazioni di entalpia sono tipicamente tabulate per le reazioni in cui sia i reagenti che i prodotti si trovano nelle stesse condizioni. Uno stato standard è 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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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...

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Using Standard Enthalpy Values in Chemistry

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