Name: temperature effects on gibbs free energy
Uploaded: 2023-04-30T18:28:32.000Z
Duration: 2 min 23 s
Channel: JoVE

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gibbs-free-energy-and-thermodynamic-favorability

Gibbs Free Energy and Thermodynamic Favorability

Die Änderung der freien Gibbs-Energie oder Delta G ist die Energie, die absorbiert oder freigesetzt wird, wenn energiespeichernde Bindungen gebildet bzw. gebrochen werden.
Das Vorzeichen von Delta G hängt von den Vorzeichen und den relativen Werten von Enthalpie, Entropie und Temperatur ab.
Wenn Delta H negativ und Delta S positiv ist, ist Delta G bei allen Temperaturen negativ. Exotherme Reaktionen, bei denen die Entropie des Systems zunimmt, sind also immer spontan.
Wenn sowohl Delta H als auch Delta S negativ sind, hängt Delta G von der Temperatur ab. Reaktionen mit negativen Enthalpie- und Entropieänderungen sind nur bei niedrigen Temperaturen spontan.
Delta G ist auch von der Temperatur abhängig, wenn sowohl Delta H als auch Delta S positiv sind. Reaktionen mit positiven Enthalpie- und Entropieänderungen sind nur bei höheren Temperaturen spontan.
Wenn Delta H positiv und Delta S negativ ist, ist Delta G immer positiv, und die Reaktion ist bei allen Temperaturen nicht spontan.
Für jede Zusammensetzung des Reaktionsgemischs ist das Delta G für die Reaktion die Summe aus der freien Standardenergie und RT multipliziert mit dem natürlichen Logarithmus des Reaktionsquotienten.
Wenn sich die Reaktanten und Produkte im Gleichgewicht befinden, ist die Änderung der freien Energie Null, und der Reaktionsquotient entspricht der Gleichgewichtskonstante. Die standardmäßige Änderung der freien Energie entspricht also der negativen RT ln(K).
Wenn Delta G null kleiner als Null ist, ist ln(K) positiv, was bedeutet, dass K größer als 1 ist. In diesem Fall wird die Produktbildung im Gleichgewicht begünstigt. Je größer die Gleichgewichtskonstante ist, desto größer ist die Abnahme der freien Gibbs-Energie. 
Reaktionen mit negativen Delta-G-Werten beinhalten die Abgabe von freier Energie an die Umgebung und werden als "exergonische" Reaktionen bezeichnet.
Umgekehrt, wenn delta G null größer als Null ist, ist ln(K) negativ, was bedeutet, dass K kleiner als 1 ist, und die umgekehrte Richtung der Reaktion wird bevorzugt.
Reaktionen mit einem positiven delta G absorbieren freie Energie aus der Umgebung und werden als "endergonische" Reaktionen bezeichnet.

gibbs freie energie und thermodynamische günstigkeit

Die Spontaneität eines Prozesses hängt von der Temperatur des Systems ab. Phasenübergänge verlaufen beispielsweise spontan in die eine oder andere Richtung, abhängig von der Temperatur der betreffenden Substanz. Ebenso können einige chemische Reaktionen auch temperaturabhängige Spontaneitäten aufweisen. Um dieses Konzept zu veranschaulichen, wird die Gleichung betrachtet, die die Änderung der freien Energie mit den Enthalpie- und Entropieänderungen für den Prozess in Beziehung setzt:

<img alt="Eq1" src="/files/ftp_upload/11694/11694%20PT%20eq%201.svg" style="text-align: center; height: 15px;">

Die Spontaneität eines Prozesses, die sich im arithmetischen Vorzeichen seiner freien Energieänderung widerspiegelt, wird dann durch die Vorzeichen der Enthalpie- und Entropieänderungen und in einigen Fällen durch die absolute Temperatur bestimmt. Da T die absolute Temperatur (Kelvin) ist, kann sie nur positive Werte annehmen. Bezüglich der Vorzeichen der Enthalpie- und Entropieänderungen gibt es also vier Möglichkeiten:

<ol>
	<li>Sowohl ΔH als auch ΔS sind positiv. Dieser Zustand beschreibt einen endothermen Prozess, der eine Erhöhung der    Systementropie mit sich bringt. In diesem Fall ist ΔG negativ, wenn der Betrag des TΔS-Terms größer als ΔH ist. Wenn der TΔS-Term kleiner als ΔH ist, ist die Änderung der freien Energie positiv. Ein solcher Prozess verläuft bei hohen Temperaturen spontan und bei niedrigen Temperaturen nicht spontan.</li>
	<li>Sowohl ΔH als auch ΔS sind negativ. Dieser Zustand beschreibt einen exothermen Prozess, der eine Abnahme der Systementropie mit sich bringt. In diesem Fall ist ΔG negativ, wenn der Betrag des TΔS-Terms kleiner als ΔH ist. Wenn der Betrag des TΔS-Terms größer als ΔH ist, ist die Änderung der freien Energie positiv. Ein solcher Prozess verläuft bei niedrigen Temperaturen spontan und bei hohen Temperaturen nicht spontan.</li>
	<li>ΔH ist positiv und ΔS ist negativ. Dieser Zustand beschreibt einen endothermen Prozess, der eine Abnahme der Systementropie mit sich bringt. In diesem Fall ist ΔG unabhängig von der Temperatur positiv. Ein solcher Prozess findet nicht spontan bei allen Temperaturen statt.</li>
	<li>ΔH ist negativ und ΔS ist positiv. Dieser Zustand beschreibt einen exothermen Prozess, der eine Erhöhung der Systementropie mit sich bringt. In diesem Fall ist ΔG unabhängig von der Temperatur negativ. Ein solcher Prozess läuft bei allen Temperaturen spontan ab.</li>
</ol>

Die Änderung der freien Energie eines Prozesses kann als Maß für seine treibende Kraft angesehen werden. Ein negativer Wert für ΔG stellt eine treibende Kraft für den Prozess in Vorwärtsrichtung dar, während ein positiver Wert eine treibende Kraft für den Prozess in Rückwärtsrichtung darstellt. Wenn ΔGrxn Null ist, sind die Vorwärts- und Rückwärtsantriebskräfte gleich und der Prozess läuft in beide Richtungen mit der gleichen Geschwindigkeit ab (das System befindet sich im Gleichgewicht).

Es sei daran erinnert, dass Q der numerische Wert des Ausdrucks für die Massenwirkung des Systems ist und sein Wert verwendet werden kann, um die Richtung zu identifizieren, in der eine Reaktion abläuft, um ein Gleichgewicht zu erreichen. Wenn Q kleiner als die Gleichgewichtskonstante K ist, läuft die Reaktion in Vorwärtsrichtung ab, bis das Gleichgewicht erreicht ist und Q = K. Wenn umgekehrt Q &gt; K, läuft der Prozess in der Rückwärtsrichtung ab, bis das Gleichgewicht erreicht ist.

<img alt="Eq2" src="/files/ftp_upload/11694/11694%20PT%20eq%202.svg" style="text-align: center; height: 18px;">

Die Änderung der freien Energie für einen Prozess, der mit Reaktanten und Produkten unter nicht standardmäßigen Bedingungen (andere Drücke als 100 kPa; andere Konzentrationen als 1 M) abläuft, hängt mit der standardmäßigen Änderung der freien Energie gemäß dieser Gleichung zusammen:

<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;">

R ist die Gaskonstante (8,314 J/K mol), T ist das Kelvin oder die absolute Temperatur und Q ist der Reaktionsquotient. Für ein System im Gleichgewicht gilt Q = K und ΔG = 0, und die vorherige Gleichung kann geschrieben werden als

Diese Form der Gleichung stellt eine nützliche Verbindung zwischen diesen beiden wesentlichen thermodynamischen Eigenschaften her und kann verwendet werden, um Gleichgewichtskonstanten aus Standardänderungen der freien Energie abzuleiten und umgekehrt. Die Beziehungen zwischen Standardänderungen der freien Energie und Gleichgewichtskonstanten sind unten zusammengefasst.

Wenn K &gt; 1, ΔG° &lt; 0 und die Produkte in der Reaktionsmischung häufiger vorkommen.

Wenn K &lt; 1, ΔG° &gt; 0 und die Reaktanten in der Reaktionsmischung häufiger vorkommen.

Wenn K = 1, ΔG° = 0 und die Reaktanten und Produkte sind in der Reaktionsmischung vergleichsweise reichlich vorhanden.

Dieser Text wurde angepasst von <a href="https://openstax.org/books/chemistry-2e/pages/16-4-free-energy">Openstax, Chemistry 2e, Abschnitt 16.4: Free Energy.</a>

Gibbs freie Energie und thermodynamische Günstigkeit

Die Spontaneität eines Prozesses hängt von der Temperatur des Systems ab. Phasenübergänge verlaufen zum Beispiel spontan in eine Richtung oder ...

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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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