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

Zmianę entalpii reakcji można zmierzyć za pomocą kalorymetru lub można ją znaleźć, obliczając różnicę entalpii między reagentami a produktami.

Jednak entalpii bezwzględne reagentów i produktów nie mogą być mierzone bezpośrednio, dlatego chemicy na ogół używają zmiany entalpii lub ΔH w stosunku do substancji odniesienia w uzgodnionym stanie standardowym.

Stan standardowy jest definiowany przez określony zestaw warunków. Obejmuje to ustawioną temperaturę, zwykle 25 °C lub 298 K, oraz stałe ciśnienie 1 bar dla gazów.

W przypadku roztworów stanem wzorcowym jest stężenie 1 molowe czystej substancji rozpuszczonej w rozpuszczalniku.

Stan standardowy substancji obejmuje również stan skupienia materii, w którym substancja występuje w tych warunkach. Na przykład chlorek sodu jako ciało stałe, rtęć jako ciecz lub hel jako gaz.  

Jeśli w tych warunkach element występuje w więcej niż jednej formie, najbardziej stabilna forma elementu jest definiowana jako stan standardowy. Na przykład węgiel może istnieć jako kryształy grafitu lub jako diament, ale grafit jest najbardziej stabilną formą, a zatem standardowym stanem węgla.

Kiedy pierwiastki w swoich stanach standardowych łączą się, tworząc 1 mol czystego związku, entalpia reakcji nazywana jest entalpią standardową lub standardowym ciepłem powstawania. Jest to oznaczone przez ΔHf°, gdzie nic nie wskazuje na standardowe stany elementów składowych, podczas gdy f oznacza powstawanie.

Standardowa entalpia tworzenia dla czystych pierwiastków w warunkach stanu standardowego wynosi zawsze zero, ponieważ nie ma reakcji, a zatem nie ma zmiany entalpii, gdy pierwiastek jest już w stanie standardowym.

Wartości standardowej entalpii tworzenia w kilodżulach na mol dla związku można znaleźć w tabelach referencyjnych. Substancje te obejmują pierwiastki w stanach niestandardowych, takie jak gazowy sód oraz związki, takie jak chlorek sodu.

Standardową zmianę entalpii reakcji można obliczyć na podstawie różnicy produktów i reagentów, które z kolei można obliczyć przy użyciu wartości entalpii z tabeli referencyjnej.

<ol>
	<li>Jastrząb, Eric Leigh. "Obliczanie standardowych entalpii powstawania alkanów: ilustrowanie mechaniki molekularnej i programów arkuszy kalkulacyjnych." Czasopismo Edukacji Chemicznej 76, nr 2 (1999): 278.</li>
	<li>Mazzuca, James W., Alexis R. Downing i Christopher Potter. "Empirycznie poprawione obliczenia struktury elektronowej zastosowane do entalpii spalania laboratorium fizykochemii." Czasopismo Edukacji Chemicznej 96, nr 6 (2019): 1165-1170.</li>
	<li>Jansen, Michael P. "Koszt konwersji pojazdu napędzanego benzyną na propan. Praktyczny problem przeglądowy dla liceum lub wstępu do chemii." Journal of Chemical Education 77, nr 12 (2000): 1578. 
	 </li>
</ol>

standardowa entalpia formowania

Zmiany entalpii są zazwyczaj zestawiane w tabelach dla reakcji, w których zarówno reagenty, jak i produkty znajdują się w tych samych warunkach. Stan standardowy to powszechnie przyjęty zestaw warunków używany jako punkt odniesienia przy określaniu właściwości w innych, odmiennych warunkach. Dla chemików stan standardowy IUPAC odnosi się do materiałów pod ciśnieniem 1 bara i roztworów pod ciśnieniem 1 M, i nie określa temperatury. Wiele tabel termochemicznych podaje wartości ze stanem standardowym 1 atm. Ponieważ ΔH reakcji zmienia się bardzo mało przy tak małych zmianach ciśnienia (1 bar = 0,987 atm), wartości ΔH (z wyjątkiem najdokładniej zmierzonych wartości) są zasadniczo takie same w obu zestawach warunków standardowych. Znak „o” z indeksem górnym w symbolu zmiany entalpii oznacza stan standardowy. Ponieważ typowa (ale nie jest to technicznie standardowa) temperatura wynosi 298,15 K, przyjmuje się ją, chyba że określono inną temperaturę. Zatem symbol (ΔH°) służy do wskazania zmiany entalpii dla procesu zachodzącego w tych warunkach. (Symbol ΔH służy do wskazania zmiany entalpii reakcji zachodzącej w niestandardowych warunkach.)

Zmiany entalpii dla wielu typów procesów chemicznych i fizycznych są dostępne w literaturze przedmiotu, w tym dla reakcji spalania, przejść fazowych i reakcji tworzenia. Ponieważ zmiana entalpii dla danej reakcji jest proporcjonalna do ilości zaangażowanych substancji, można ją na tej podstawie podać (tj. jako ΔH dla określonych ilości reagentów). Jednakże często okazuje się, że bardziej przydatne jest podzielenie jednej ekstensywnej właściwości (ΔH) przez inną (ilość substancji) i podanie intensywnej wartości ΔH w przeliczeniu na ilość, często „znormalizowanej” do przeliczeń na mol.

<h4>Standardowa entalpia formowania</h4>

Standardowa entalpia tworzenia ΔHf° jest zmianą entalpii reakcji, w której dokładnie 1 mol czystej substancji powstaje z wolnych pierwiastków w ich najbardziej stabilnych stanach w warunkach stanu standardowego. Wartości te są szczególnie przydatne do obliczania lub przewidywania zmian entalpii w reakcjach chemicznych, których przeprowadzenie jest niepraktyczne lub niebezpieczne, lub w procesach, dla których trudno jest dokonać pomiarów. Wykorzystując znane wartości standardowych entalpii tworzenia, można określić zmianę entalpii dowolnej reakcji.

Standardowa entalpia tworzenia CO2 (g) wynosi -393,5 kJ/mol. Oto zmiana entalpii reakcji egzotermicznej:

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/801/801_Equations-01.png" style="height: 40px;">

Zaczynając od reagentów pod ciśnieniem 1 atmosfery i 25 °C (z węglem obecnym w postaci grafitu, najbardziej stabilnej formy węgla w tych warunkach), a kończąc na jednym molu CO2, również pod ciśnieniem 1 atmosfery i 25 °C. Dla dwutlenku azotu, NO2 (g), ΔHf° wynosi 33,2 kJ/mol. To jest zmiana entalpii reakcji endotermicznej:

<img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/801/801_Equations-02.png" style="height: 40px;">

Równanie reakcji z 1/2 molem N2 i 1 molem O2 jest w tym przypadku poprawne, ponieważ standardowa entalpia tworzenia zawsze odnosi się do 1 mola produktu: NO2 (g).

Z definicji standardowa entalpia tworzenia pierwiastka w jego najbardziej stabilnej postaci jest równa zeru w standardowych warunkach. Na przykład standardowe entalpie tworzenia węgla (grafenu), dwuatomowego gazowego tlenu, dwuatomowego azotu, metalicznego sodu i ciekłej rtęci wynoszą zero w standardowych warunkach.

Standardowa entalpia formowania

Zmiany entalpii są zwykle zestawiane w tabelach dla reakcji, w których zarówno reagenty, jak i produkty znajdują się w tych samych warunkach. Stan standardowy to powszechnie ...

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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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