Name: free energy and reaction direction
Uploaded: 2023-04-30T10:09:12.000Z
Duration: 55 s
Channel: JoVE

Die Änderung der freien Energie oder ΔG, einer bestimmten Reaktion ist die Summe der standardmäßigen freien Energie und RT multipliziert mit dem natürlichen Logarithmus des Verhältnisses der Konzentrationen von Produkten zu Reaktanten.
Wenn die ΔG einer Reaktion negativ ist, dann schreitet die Reaktion fort. Die ΔG wird weniger negativ, wenn die Konzentrationen der Reaktanten abnehmen und die der Produkte steigen.
Schließlich wird die ΔG ist gleich Null, wenn die Reaktion das Gleichgewicht erreicht, wobei die Vorwärts- und Rückwärtsreaktionsraten gleich sind.
Im Gleichgewicht ist das Verhältnis der Konzentration von Produkten und Reaktanten gleich der Gleichgewichtskonstante K.
Die Gleichung kann umgestellt werden, um die mathematische Beziehung zwischen ΔG0 und K anzuzeigen.

freie energie und gleichgewicht

Free Energy and Equilibrium

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 ΔG 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).

Der Reaktionsquotient Q ist ein geeignetes Maß für den Zustand eines Gleichgewichtssystems. Q ist der numerische Wert des Massenwirkungsausdrucks für das System und kann verwendet werden, 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 > K, läuft der Prozess in der Rückwärtsrichtung ab, bis das Gleichgewicht erreicht ist.

Die Änderung der freien Energie für einen Prozess, der mit Reaktanten und Produkten unter nicht standardmäßigen Bedingungen (andere Drücke als 1 bar, andere Konzentrationen als 1 M) abläuft, hängt mit der standardmäßigen Änderung der freien Energie gemäß dieser Gleichung zusammen:

ΔG = ΔG° + RT lnQ

R ist die Gaskonstante (8,314 J/K mol), T ist das Kelvin oder die absolute Temperatur und Q ist der Reaktionsquotient. Für Gasphasengleichgewichte wird der druckbasierte Reaktionsquotient QP verwendet. Der konzentrationsbasierte Reaktionsquotient QC wird für kondensierte Phasengleichgewichte verwendet.

Für ein System im Gleichgewicht gilt Q = K und ΔG = 0, und die vorherige Gleichung kann geschrieben werden als

0 = ΔG° + RT lnK (im Gleichgewicht)

ΔG° = −RT lnK

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.

Dieser Text ist adaptiert von Openstax, Chemie 2e, <a href="https://openstax.org/books/chemistry-2e/pages/16-4-free-energy">Section 16.4: Free Energy.</a>

Freie Energie und Gleichgewicht

Die Änderung der freien Energie für einen Prozess kann als Maß für seine treibende Kraft angesehen werden. Ein negativer Wert für ΔG stellt eine treibende Kraft für die ...

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free energy and reaction direction

Free energy and reaction direction are linked in chemical equilibrium. A free energy change, ΔG, shows the driving force for a process. When ΔG is negative, the process is driven forward. When ΔG is positive, the reverse direction is favored. When ΔG is zero, the forward and reverse driving forces are equal, and the system is at equilibrium.

The reaction quotient, Q, helps show how close a system is to equilibrium. Q is the numerical value of the mass action expression for the system. It can be used to predict which way a reaction will shift to reach equilibrium. If Q is less than the equilibrium constant, K, the reaction proceeds forward until Q = K. If Q is greater than K, the process shifts in reverse until equilibrium is reached.

For reactions under nonstandard conditions, ΔG depends on both the standard free energy change and the reaction quotient. The relationship is ΔG = ΔG° + RT ln Q, where R is the gas constant, T is temperature in kelvin, and Q is the reaction quotient. For gas-phase equilibria, the pressure-based quotient QP is used. For condensed-phase equilibria, the concentration-based quotient QC is used.

At equilibrium, Q = K and ΔG = 0. Substituting that condition into the equation gives 0 = ΔG° + RT ln K. This can be rearranged to ΔG° = −RT ln K. That link lets chemists find an equilibrium constant from a standard free energy change, or find a standard free energy change from K.

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Free Energy and Reaction Direction

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