Name: enthalpy and heat at constant pressure
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 2 min 59 s
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Enthalpy

Niektóre reakcje chemiczne wydzielają ogromne ciepło i działają na otoczenie, takie jak spalanie paliwa rakietowego, powodujące oderwanie się promu kosmicznego od ziemi.

Suma ciepła "q" i pracy "w" jest zmianą energii wewnętrznej, "ΔE", określoną przez pierwszą zasadę termodynamiki.

W przypadku reakcji chemicznych z udziałem gazów, które zachodzą pod ciśnieniem atmosferycznym, wykonywana praca jest pracą mechaniczną związaną ze zmianami objętości - rozszerzaniem się lub kurczeniem. Praca jest zatem równa ujemnej wartości ciśnienia pomnożonej przez zmianę objętości.

Podstawienie w w pierwszej zasadzie termodynamiki i przegrupowanie wyrazów w równaniu pokazuje, że q = ΔE + PΔV, co daje wyrażenie na przepływ ciepła pod stałym ciśnieniem.

W innych reakcjach chemicznych, takich jak spalanie drewna w celu ugotowania żywności, bardziej istotne jest ilościowe określenie ciepła wydzielanego w celu ułatwienia gotowania niż zmierzenie ilości pracy związanej z rozszerzalnością wykonanej w otoczeniu.

Ponieważ energia wewnętrzna odpowiada zarówno za ciepło, jak i za pracę, ΔE nie jest używane w warunkach stałego ciśnienia. Aby omówić wyłącznie przepływ energii w postaci ciepła, zdefiniowano nową funkcję termodynamiczną – entalpię.

Entalpia, H, równa się sumie energii wewnętrznej, E, i pracy ciśnienie-objętość, P-V. Ponieważ energia, ciśnienie i objętość są funkcjami stanu, entalpia jest również funkcją stanu. 

Nie można zmierzyć bezwzględnych wartości entalpii dla określonych substancji. Można określić tylko zmianę entalpii.  Zmiana entalpii, ΔH, równa się zmianie energii wewnętrznej, ΔE + PΔV.

Przypominając, że zmiana energii jest sumą pracy cieplnej i ciśnieniowo-objętościowej, równania można połączyć, aby pokazać, że w warunkach stałego ciśnienia ΔH równa się ciepłu, q, uzyskanemu lub utraconemu przez układ.

Jeśli system traci energię do otoczenia w postaci ciepła – jak w przypadku spalania drewna – temperatura otoczenia wzrasta. Jest to opisane konwencją znaku ujemnego dla q. W konsekwencji ΔH staje się ujemny, a proces określa się jako egzotermiczny.

Wręcz przeciwnie, jeśli system pozyskuje energię z otoczenia w postaci ciepła – takiej jak reakcja zachodząca w chemicznym zimnym opakowaniu – temperatura otoczenia spada. Upał w tym przypadku jest opisany konwencją znaku dodatniego. To sprawia, że ΔH dodatnie, a proces ten nazywa się endotermicznym.

<ol>
	<li>Canagaratna, Sebastian G. "Wizualna pomoc w obliczeniach entalpii." Journal of Chemical Education 77, nr 9 (2000): 1178.</li>
	<li>Howard, Irmgard K. "H jak entalpia, dzięki Heike Kamerlingh Onnes i Alfredowi W. Porterowi." Dziennik Edukacji Chemicznej 79, nr 6 (2002): 697.</li>
	<li>Van Ness, Hendrick C. Klasyczna termodynamika roztworów nieelektrolitowych. Elsevier, 2015. 
	 </li>
</ol>

entalpia

Chemicy zwykle używają właściwości zwanej entalpią (H) do opisu termodynamiki procesów chemicznych i fizycznych. Entalpię definiuje się jako sumę energii wewnętrznej układu (E) i iloczynu matematycznego jego ciśnienia (P) i objętości (V):

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/796/796_Equations-01.png" style="height: 40px;">

Entalpia jest funkcją stanu. Wartości entalpii dla określonych substancji nie można zmierzyć bezpośrednio; można określić jedynie zmiany entalpii procesów chemicznych lub fizycznych. W przypadku procesów zachodzących pod stałym ciśnieniem (powszechny warunek wielu zmian chemicznych i fizycznych) zmiana entalpii (ΔH) wynosi:

<img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/796/796_Equations-02.png" style="height: 40px;">

Iloczyn matematyczny PΔV reprezentuje pracę (w) polegającą na rozszerzaniu lub ścieśnianiu objętości. Zgodnie z ich definicjami znaki arytmetyczne ΔV i w będą zawsze przeciwne:

<img alt="Eq3" src="https://www.jove.com/files/core/CDNSource/lessons/796/796_Equations-03.png" style="height: 40px;">

Podstawiając to równanie i definicję energii wewnętrznej przy stałym ciśnieniu (ΔE = qp + w) do równania zmiany entalpii otrzymuje się:

<img alt="Eq4" src="https://www.jove.com/files/core/CDNSource/lessons/796/796_Equations-04.png" style="height: 40px;">

gdzie qp jest ciepłem reakcji w warunkach stałego ciśnienia.

I tak, jeśli proces chemiczny lub fizyczny przebiega pod stałym ciśnieniem, a jedyna praca jest spowodowana rozszerzaniem się lub kurczeniem (praca P-V), to przepływ ciepła (qp) i zmiana entalpii (ΔH) w procesie są równe.

Ciepło wydzielane podczas pracy palnika Bunsena jest równe zmianie entalpii reakcji spalania metanu, która zachodzi przy zasadniczo stałym ciśnieniu atmosfery. Chemicy zazwyczaj przeprowadzają eksperymenty w normalnych warunkach atmosferycznych, przy stałym ciśnieniu zewnętrznym przy qp = ΔH, co sprawia, że entalpia jest najwygodniejszym wyborem do określania zmian ciepła w reakcjach chemicznych.

Ujemna wartość zmiany entalpii, ΔH < 0, wskazuje na reakcję egzotermiczną (oddawanie ciepła do otoczenia); wartość dodatnia ΔH > 0 wskazuje na reakcję endotermiczną (ciepło pochłaniane z otoczenia). Jeśli kierunek równania chemicznego zostanie odwrócony, zmienia się znak arytmetyczny jego ΔH (proces endotermiczny w jednym kierunku jest egzotermiczny w przeciwnym).

Koncepcyjnie ΔE (miara ciepła i pracy) i ΔH (miara ciepła przy stałym ciśnieniu) reprezentują zmiany funkcji stanu układu. W procesach, w których zmiana objętości ΔV jest niewielka (topnienie lodu), a ΔE i ΔH są identyczne. Jeśli jednak zmiana objętości jest znaczna (odparowanie wody), ilość energii przekazanej w postaci pracy będzie znacząca; zatem ΔE i ΔH mają znacząco różne wartości.

Entalpia

Chemicy zwykle używają właściwości znanej jako entalpia (H) do opisania termodynamiki procesów chemicznych i fizycznych. Entalpia jest definiowana jako suma ...

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enthalpy and heat at constant pressure

Enthalpy describes heat changes in chemical and physical processes. Chemists use the symbol H for enthalpy. It is defined as a system’s internal energy, E, plus the product of pressure, P, and volume, V. Enthalpy is a state function, so only changes in enthalpy can be measured, not the enthalpy of a substance by itself.

For many reactions, the pressure stays constant. Under those conditions, the enthalpy change, ΔH, can be written in a way that includes pressure-volume work. The term PΔV represents expansion work, also called pressure-volume work. The signs of ΔV and w are always opposite, because expansion and work use opposite sign conventions.

At constant pressure, the internal energy change is ΔE = qp + w, where qp is the heat of reaction. When the only work done is due to expansion or contraction, qp and ΔH are equal. This is why enthalpy is a useful choice for measuring heat changes in many lab reactions and physical changes. A Bunsen burner is one example, since methane combustion occurs at about atmospheric pressure.

A negative ΔH means the process is exothermic, which means heat is given off to the surroundings. A positive ΔH means the process is endothermic, which means heat is absorbed from the surroundings. If a chemical equation is reversed, the sign of ΔH also changes. A process that is endothermic in one direction becomes exothermic in the other.

Enthalpy change and internal energy change both describe changes in a state function for the system. When the volume change, ΔV, is small, such as during melting of ice, ΔE and ΔH are essentially the same. When the volume change is larger, such as during evaporation of water, more energy is transferred as work. In that case, ΔE and ΔH can differ by a significant amount.

Enthalpy and Heat at Constant Pressure

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Enthalpy describes heat changes in chemical and physical processes. Chemists use the symbol H for enthalpy. It is defined as a system’s internal energy, E, plus the product of pressure, P, and volume, V. Enthalpy is a state function, so only changes in enthalpy can be measured, not the enthalpy of a substance by itself. For many reactions, the pressure stays constant. Under those conditions, the enthalpy change, ΔH, can be written in a way that includes pressure-volume work. The term PΔV...

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Enthalpy and Heat at Constant Pressure

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