Name: temperature effects on gibbs free energy
Uploaded: 2023-04-30T18:28:32.000Z
Duration: 2 min 23 s
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Gibbs Free Energy and Thermodynamic Favorability

Zmiana energii swobodnej Gibbsa, czyli delta G, to energia pochłonięta lub uwolniona, gdy wiązania magazynujące energię są odpowiednio tworzone lub zrywane.
Znak delta G zależy od znaków i względnych wartości entalpii, entropii i temperatury.
Jeśli delta H jest ujemna, a delta S jest dodatnia, delta G jest ujemna we wszystkich temperaturach. Tak więc reakcje egzotermiczne, w których wzrasta entropia układu, są zawsze spontaniczne.
Jeśli zarówno delta H, jak i delta S są ujemne, delta G zależy od temperatury. Reakcje z ujemnymi zmianami entalpii i entropii są spontaniczne tylko w niskich temperaturach.
Delta G jest również zależna od temperatury, jeśli zarówno delta H, jak i delta S są dodatnie. Reakcje z dodatnimi zmianami entalpii i entropii są spontaniczne tylko w wyższych temperaturach.
Gdy delta H jest dodatnia, a delta S jest ujemna, delta G jest zawsze dodatnia, a reakcja jest niespontaniczna we wszystkich temperaturach.
Dla dowolnego składu mieszaniny reakcyjnej delta G dla reakcji jest sumą standardowej energii swobodnej i RT pomnożonej przez logarytm naturalny ilorazu reakcji.
Gdy reagenty i produkty są w równowadze, zmiana energii swobodnej wynosi zero, a iloraz reakcji jest równy stałej równowagi. Zatem standardowa zmiana energii swobodnej jest równa ujemnemu RT ln(K).
Jeśli delta G naught jest mniejsza od zera, ln(K) jest dodatnia, co oznacza, że K jest większe niż 1. W takim przypadku tworzenie produktu sprzyja tworzeniu się w równowadze. Im większa stała równowagi, tym większy spadek energii swobodnej Gibbsa. 
Reakcje z ujemnymi wartościami delta G polegają na uwalnianiu swobodnej energii do otoczenia i nazywane są reakcjami "egzergonicznymi".
I odwrotnie, jeśli delta G zero jest większa od zera, ln(K) jest ujemna, co oznacza, że K jest mniejsze niż 1, a odwrotny kierunek reakcji jest faworyzowany.
Reakcje z dodatnią deltą G pochłaniają wolną energię z otoczenia i nazywane są reakcjami "endergonicznymi".

energia swobodna gibbsa i korzystne warunki termodynamiczne

Spontaniczność procesu zależy od temperatury układu. Na przykład przejścia fazowe będą zachodzić samoistnie w jednym lub drugim kierunku, w zależności od temperatury danej substancji. Podobnie niektóre reakcje chemiczne mogą również wykazywać spontaniczność zależną od temperatury. Aby zilustrować tę koncepcję, rozważono równanie odnoszące zmianę energii swobodnej do zmian entalpii i entropii procesu:

<img alt="Eq1" src="/files/ftp_upload/11694/11694%20PT%20eq%201.svg" style="text-align: center; height: 15px;">

Spontaniczność procesu, odzwierciedlona w arytmetycznym znaku zmiany jego energii swobodnej, jest następnie określana przez znaki zmian entalpii i entropii oraz, w niektórych przypadkach, temperaturę bezwzględną. Ponieważ T jest temperaturą bezwzględną (kelwin), może ona przyjmować tylko wartości dodatnie. Istnieją zatem cztery możliwości w odniesieniu do znaków zmian entalpii i entropii:

<ol style="display: flex; flex-direction: column; gap: 10px;">
<li>Zarówno ΔH, jak i ΔS są dodatnie. Warunek ten opisuje proces endotermiczny, który wiąże się ze wzrostem entropii układu. W tym przypadku ΔG będzie ujemne, jeśli wielkość składnika TΔS będzie większa niż ΔH. Jeśli składnik TΔS jest mniejszy niż ΔH, zmiana energii swobodnej będzie dodatnia. Proces taki jest samoistny w wysokich temperaturach i niespontaniczny w niskich temperaturach.</li>

<li>Zarówno ΔH, jak i ΔS są ujemne. Warunek ten opisuje proces egzotermiczny, który polega na spadku entropii układu. W tym przypadku ΔG będzie ujemne, jeśli wielkość składnika TΔS będzie mniejsza niż ΔH. Jeśli wielkość składnika TΔS jest większa niż ΔH, zmiana energii swobodnej będzie dodatnia. Proces taki jest samoistny w niskich temperaturach i niespontaniczny w wysokich temperaturach.</li>

<li>ΔH jest dodatnie, a ΔS jest ujemne. Warunek ten opisuje proces endotermiczny, który polega na spadku entropii układu. W tym przypadku ΔG będzie dodatnie niezależnie od temperatury. Proces taki nie jest spontaniczny w każdej temperaturze.</li>

<li>ΔH jest ujemne, a ΔS jest dodatnie. Warunek ten opisuje proces egzotermiczny, który wiąże się ze wzrostem entropii układu. W tym przypadku ΔG będzie ujemne niezależnie od temperatury. Proces taki zachodzi samoistnie w każdej temperaturze.</li>
</ol>

Zmiana energii swobodnej procesu może być postrzegana jako miara jego siły napędowej. Ujemna wartość ΔG reprezentuje siłę napędową procesu w kierunku do przodu, natomiast wartość dodatnia reprezentuje siłę napędową procesu w kierunku odwrotnym. Gdy ΔGrxn wynosi zero, siły napędowe do przodu i do tyłu są równe, a proces zachodzi w obu kierunkach z tą samą szybkością (układ jest w równowadze).

Przypomnijmy, że Q jest wartością liczbową wyrażenia na działanie masy układu, a jego wartość można wykorzystać do określenia kierunku, w jakim będzie przebiegać reakcja, aby osiągnąć równowagę. Gdy Q jest mniejsze niż stała równowagi K, reakcja będzie przebiegać w kierunku do przodu, aż do osiągnięcia równowagi i Q = K. I odwrotnie, jeśli Q &gt; K, proces będzie przebiegał w odwrotnym kierunku, aż do osiągnięcia równowagi.

Zmiana energii swobodnej podczas procesu zachodzącego z reagentami i produktami obecnymi w niestandardowych warunkach (ciśnienia inne niż 100 kPa; stężenia inne niż 1 M) jest powiązana ze standardową zmianą energii swobodnej zgodnie z poniższym równaniem:

<img alt="Eq2" src="/files/ftp_upload/11694/11694%20PT%20eq%202.svg" style="text-align: center; height: 18px;">

R to stała gazowa (8,314 J/K mol), T to temperatura Kelvina lub temperatura bezwzględna, a Q to iloraz reakcji. Dla układu w równowadze Q = K i ΔG = 0, a poprzednie równanie można zapisać jako

<img alt="Eq3" src="/files/ftp_upload/11694/11694%20PT%20eq%203.svg" style="text-align: center; height: 18px;">

<img alt="Eq4" src="/files/ftp_upload/11694/11694%20PT%20eq%204.svg" style="text-align: center; height: 18px;">

Ta postać równania zapewnia użyteczne powiązanie między tymi dwoma podstawowymi właściwościami termodynamicznymi i może być wykorzystana do wyprowadzenia stałych równowagi ze standardowych zmian energii swobodnej i odwrotnie. Poniżej podsumowano zależności pomiędzy standardowymi zmianami energii swobodnej a stałymi równowagi.

<ul style="list-style-type: disc; display: flex; flex-direction: column; padding: 0 0 0 40px;">
 <li>Jeżeli K &gt; 1, ΔG° &lt; 0 i produkty występują w większej ilości w mieszaninie reakcyjnej.
</li>
 <li>Jeżeli K &lt; 1, ΔG° &gt; 0 i reagentów jest więcej w mieszaninie reakcyjnej.
</li>
 <li>Jeżeli K = 1, ΔG° = 0, a reagenty i produkty są porównywalnie liczne w mieszaninie reakcyjnej.
</li>
</ul>


Ten tekst jest adaptacją <a href="https://openstax.org/books/chemistry-2e/pages/16-4-free-energy">Openstax, Chemistry 2e, Section 16.4: Free Energy.</a>

Energia swobodna Gibbsa i korzystne warunki termodynamiczne

Spontaniczność procesu zależy od temperatury systemu. Na przykład przejścia fazowe będą przebiegać spontanicznie w jednym kierunku lub ...

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temperature effects on gibbs free energy

Temperature changes can decide whether a process is spontaneous. This idea applies to phase changes and to some chemical reactions. The key link is Gibbs free energy, which connects free energy change with enthalpy and entropy.

The sign of the Gibbs free energy change, or ΔG, depends on the signs of enthalpy change (ΔH) and entropy change (ΔS), and sometimes on the absolute temperature. Because temperature is measured in kelvin, it is always a positive value. That means there are four main sign combinations to consider.

If both ΔH and ΔS are positive, the process is endothermic and the system gains entropy. In this case, ΔG is negative only when the TΔS term is larger than ΔH. So the process is spontaneous at high temperatures and nonspontaneous at low temperatures. If both ΔH and ΔS are negative, the process is exothermic and the system loses entropy. Then ΔG is negative when the TΔS term is smaller than ΔH, which makes the process spontaneous at low temperatures and nonspontaneous at high temperatures.

If ΔH is positive and ΔS is negative, the process is endothermic and the system becomes less disordered. In that case, ΔG stays positive at any temperature, so the process is nonspontaneous at all temperatures. If ΔH is negative and ΔS is positive, the process is exothermic and the system becomes more disordered. Then ΔG stays negative at any temperature, so the process is spontaneous at all temperatures.

Gibbs free energy also shows the driving force of a process. A negative ΔG means the forward direction is favored. A positive ΔG means the reverse direction is favored. When ΔGrxn is zero, the forward and reverse driving forces are equal, and the system is at equilibrium.

The reaction quotient, Q, helps predict which way a reaction will shift to reach equilibrium. When Q is less than the equilibrium constant, K, the reaction moves forward until Q = K. When Q is greater than K, the process moves in reverse until equilibrium is reached. Under nonstandard conditions, the free energy change is related to the standard free energy change by ΔG = ΔG° + RT ln Q, where R is the gas constant and T is the kelvin temperature.

At equilibrium, Q equals K and ΔG is zero, so the equation can be written as a link between ΔG° and K. This relationship lets chemists find equilibrium constants from standard free energy changes, or the reverse. If K is greater than 1, ΔG° is negative and products are more abundant. If K is less than 1, ΔG° is positive and reactants are more abundant. If K equals 1, ΔG° is zero and reactants and products are present in similar amounts.

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Temperature Effects on Gibbs Free Energy

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