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

Le changement d’énergie libre, ou ΔG, d’une réaction particulière est la somme de l’énergie libre standard et de RT multipliée par le logarithme naturel du rapport entre les concentrations de produits et les réactifs.
Si le ΔG d’une réaction est négative, alors la réaction progresse vers l’avant. Le ΔG devient moins négatif à mesure que les concentrations des réactifs diminuent et que celles des produits augmentent.
Finalement, le ΔG est égal à zéro lorsque la réaction atteint l’équilibre, où les vitesses de réaction avant et arrière sont égales.
À l’équilibre, le rapport de la concentration des produits et des réactifs est égal à la constante d’équilibre, K.
L’équation peut être réarrangée pour montrer la relation mathématique entre ΔG0 et K.

Énergie libre et équilibre

Free Energy and Equilibrium

Le changement d'énergie libre pour un processus donné peut être considéré comme une mesure de sa force motrice. Une valeur ΔG négative signifie une force motrice au sens direct, tandis qu'une valeur positive représente une force motrice dans le sens inverse. Lorsque ΔG est nulle, les forces motrices de traction et de propulsion sont égales et le processus se produit dans les deux sens au même rythme (le système est dit dans un état d’équilibre).

Le quotient de réaction Q, quant à lui, est une mesure pratique de l’état d’un système en équilibre. Q est la valeur numérique d’expression d’action de masse du système. Elle peut être utilisée pour identifier la direction dans laquelle une réaction se déroulera en atteignant son état d’équilibre. Lorsque Q est inférieur à la constante d'équilibre K, la réaction se déroulera dans le sens direct jusqu'à ce que l'équilibre soit atteint et Q = K. Inversement, si Q > K, le processus se déroulera dans le sens inverse jusqu'à atteindre l’équilibre.

Le changement d'énergie libre pour un processus se déroulant avec des réactifs et des produits présents dans des conditions non standards (pressions autres que 1 bar ; concentrations autres que 1 M) est lié au changement d'énergie libre standard selon l’équation:

ΔG = ΔG° + RT lnQ

R is the gas constant (8.314 J/K mol), T is the kelvin or absolute temperature, and Q is the reaction quotient. For gas-phase equilibria, the pressure-based reaction quotient, QP, is used. The concentration-based reaction quotient, QC, is used for condensed phase equilibria.

Pour un système à l’état d’équilibre, Q = K et ΔG = 0, et l'équation précédente peut s'écrire de la manière suivante:

0 = ΔG° + RT lnK (à l'équilibre)

ΔG° = −RT lnK

Cette forme d'équation fournit un lien utile entre ces deux propriétés thermodynamiques essentielles et peut être utilisée pour dériver des constantes d'équilibre à partir des changements d'énergie libre standard et vice versa.

Ce texte est adapté d'Openstax, Biologie 2e <a href="https://openstax.org/books/chemistry-2e/pages/16-4-free-energy">Section 16.4: Free Energy.</a>

Le changement d’énergie libre d’un processus peut être considéré comme une mesure de sa force motrice. Une valeur négative pour ΔG représente une force motrice pour le ...

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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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