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

Il cambiamento di energia libera, o ΔG, di una particolare reazione è la somma dell'energia libera standard e RT moltiplicato per il logaritmo naturale del rapporto tra le concentrazioni dei prodotti e i reagenti.
Se il ΔG di una reazione è negativo, allora la reazione progredisce in avanti. Il ΔG diventa meno negativo man mano che le concentrazioni dei reagenti diminuiscono e quella dei prodotti aumenta.
Alla fine, il ΔG è uguale a zero quando la reazione raggiunge l'equilibrio, dove i tassi di reazione in avanti e indietro sono uguali.
All'equilibrio, il rapporto tra la concentrazione di prodotti e reagenti è uguale alla costante di equilibrio, K.
L'equazione può essere riorganizzata per mostrare la relazione matematica tra ΔG0 e K.

energia libera ed equilibrio

Free Energy and Equilibrium

La variazione di energia libera di un processo può essere vista come una misura della sua forza trainante. Un valore negativo per ΔG rappresenta una forza trainante per il processo nella direzione in avanti, mentre un valore positivo rappresenta una forza trainante per il processo nella direzione opposta. Quando ΔG è zero, le forze motrici avanti e indietro sono uguali e il processo avviene in entrambe le direzioni alla stessa velocità (il sistema è all'equilibrio).

Il quoziente di reazione, Q, è una misura conveniente dello stato di un sistema in equilibrio. Q è il valore numerico dell'espressione dell'azione di massa per il sistema e può essere utilizzato per identificare la direzione in cui procederà una reazione per raggiungere l'equilibrio. Quando Q è inferiore alla costante di equilibrio, K, la reazione procederà nella direzione in avanti fino al raggiungimento dell'equilibrio e Q = K. Al contrario, se Q > K, il processo procederà nella direzione opposta fino al raggiungimento dell'equilibrio.

La variazione di energia libera per un processo che avviene con reagenti e prodotti presenti in condizioni non standard (pressioni diverse da 1 bar; concentrazioni diverse da 1 M) è correlata alla variazione di energia libera standard secondo questa equazione:

ΔG = ΔG° + RT lnQ

R è la costante dei gas (8,314 J/K mol), T è il Kelvin o temperatura assoluta e Q è il quoziente di reazione. Per gli equilibri in fase gassosa viene utilizzato il quoziente di reazione basato sulla pressione, QP. Il quoziente di reazione basato sulla concentrazione, QC, viene utilizzato per gli equilibri di fase condensata.

Per un sistema all'equilibrio, Q = K e ΔG = 0, e l'equazione precedente può essere scritta come:

0 = ΔG° + RT lnK (all'equilibrio)

ΔG° = −RT lnK

Questa forma dell'equazione fornisce un utile collegamento tra queste due proprietà termodinamiche essenziali e può essere utilizzata per ricavare costanti di equilibrio da variazioni standard di energia libera e viceversa.

Questo testo è adattato da Openstax, Chemistry 2e, <a href="https://openstax.org/books/chemistry-2e/pages/16-4-free-energy">Section 16.4: Free Energy.</a>

Energia libera ed equilibrio

Il cambiamento di energia libera per un processo può essere visto come una misura della sua forza motrice. Un valore negativo per ΔG rappresenta una forza trainante per il ...

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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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