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

La variation de l’énergie libre de Gibbs, ou delta G, est l’énergie absorbée ou libérée lorsque des liaisons de stockage d’énergie sont formées ou rompues, respectivement.
Le signe du delta G dépend des signes et des valeurs relatives de l’enthalpie, de l’entropie et de la température.
Si delta H est négatif et delta S est positif, delta G est négatif à toutes les températures. Ainsi, les réactions exothermiques où l’entropie du système augmente sont toujours spontanées.
Si delta H et delta S sont négatifs, delta G dépend de la température. Les réactions avec des variations négatives d’enthalpie et d’entropie ne sont spontanées qu’à basse température.
Le delta G dépend également de la température si le delta H et le delta S sont positifs. Les réactions avec des changements positifs d’enthalpie et d’entropie ne sont spontanées qu’à des températures plus élevées.
Lorsque delta H est positif et delta S est négatif, delta G est toujours positif et la réaction n’est pas spontanée à toutes les températures.
Pour toute composition de mélange réactionnel, le delta G de la réaction est la somme de l’énergie libre standard et de RT multipliée par le logarithme naturel du quotient réactionnel.
Lorsque les réactifs et les produits sont à l’équilibre, la variation d’énergie libre est nulle et le quotient de réaction est égal à la constante d’équilibre. Ainsi, la variation d’énergie libre standard est égale à moins RT ln(K).
Si delta G zéro est inférieur à zéro, ln(K) est positif, ce qui signifie que K est supérieur à 1. Dans ce cas, la formation du produit est favorisée à l’équilibre. Plus la constante d’équilibre est grande, plus la diminution de l’énergie libre de Gibbs est importante. 
Les réactions avec des valeurs delta G négatives impliquent la libération d’énergie gratuite dans l’environnement et sont appelées réactions « exergoniques ».
Inversement, si delta G zéro est supérieur à zéro, ln(K) est négatif, ce qui signifie que K est inférieur à 1, et le sens inverse de la réaction est favorisé.
Les réactions avec un delta G positif absorbent l’énergie gratuite de l’environnement et sont appelées réactions « endergoniques ».

Énergie libre de gibbs et favorabilité thermodynamique

La spontanéité d'un processus dépend de la température du système. Les transitions de phase, par exemple, se dérouleront spontanément dans un sens ou dans l’autre en fonction de la température de la substance en question. De même, certaines réactions chimiques peuvent également présenter des spontanéités dépendantes de la température. Pour illustrer ce concept, l'équation reliant le changement d'énergie libre aux changements d'enthalpie et d'entropie pour le processus est considérée :

<img alt="Eq1" src="/files/ftp_upload/11694/11694%20PT%20eq%201.svg" style="text-align: center; height: 15px;">

La spontanéité d'un processus, reflétée dans le signe arithmétique de son changement d'énergie libre, est alors déterminée par les signes des changements d'enthalpie et d'entropie et, dans certains cas, par la température absolue. Puisque T est la température absolue (kelvin), elle ne peut avoir que des valeurs positives. Quatre possibilités existent donc quant aux signes des changements d'enthalpie et d'entropie :

<ol>
	<li>Both ΔH et ΔS sont tous deux positifs. Cette condition décrit un processus endothermique qui implique une augmentation de l'entropie du système. Dans ce cas, ΔG sera négatif si la grandeur du termeTΔS est supérieure à ΔHH. Si le terme TΔS est inférieur à ΔHH, le changement d'énergie libre sera positif. Un tel processus est spontané à haute température et non spontané à basse température.</li>
	<li>Both ΔH et ΔS sont négatifs. Cette condition décrit un processus exothermique qui implique une diminution de l'entropie du système. Dans ce cas, ΔG sera négatif si la grandeur du terme TΔS est inférieure à ΔH. Si l’amplitude du terme TΔS est supérieure à ΔH, le changement d’énergie libre sera positif. Un tel processus est spontané à basse température et non spontané à haute température.</li>
	<li>ΔH est positif et ΔS est négatif. Cette condition décrit un processus endothermique qui implique une diminution de l'entropie du système. Dans ce cas, ΔG sera positif quelle que soit la température. Un tel processus n’est pas spontané à toutes les températures.</li>
	<li>ΔH est négatif et ΔS est positif. Cette condition décrit un processus exothermique qui implique une augmentation de l'entropie du système. Dans ce cas, ΔG sera négatif quelle que soit la température. Un tel processus est spontané à toutes les températures.</li>
</ol>

Le changement d'énergie libre pour un processus peut être considéré comme une mesure de sa force motrice. Une valeur négative pour ΔG représente une force motrice pour le processus dans le sens direct, tandis qu'une valeur positive représente une force motrice pour le processus dans le sens inverse. Lorsque ΔGrxn est nul, les forces motrices avant et arrière sont égales et le processus se produit dans les deux sens au même rythme (le système est à l'équilibre).

Rappelez-vous que Q est la valeur numérique de l’expression de l’action de masse du système et que sa valeur peut être utilisée pour identifier la direction dans laquelle une réaction se déroulera afin d’atteindre l’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 &gt; K, le processus se déroulera dans le sens inverse jusqu'à ce que l'équilibre soit atteint.

Le changement d'énergie libre pour un processus se déroulant avec des réactifs et des produits présents dans des conditions non standard (pressions autres que 100 kPa ; concentrations autres que 1 M) est lié au changement d'énergie libre standard selon cette équation :

<img alt="Eq2" src="/files/ftp_upload/11694/11694%20PT%20eq%202.svg" style="text-align: center; height: 18px;">

R est la constante du gaz (8,314 J/K mol), T est le kelvin ou la température absolue et Q est le quotient de réaction. Pour un système à l'équilibre, Q= K et ΔG = 0, et l'équation précédente peut s'écrire

<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;">

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 de changements d'énergie libre standard et vice versa. Les relations entre les changements d'énergie libre standard et les constantes d'équilibre sont résumées ci-dessous.

If K &gt; 1, ΔG° &lt; 0 et les produits sont plus abondants dans le mélange réactionnel.

If K &lt; 1, ΔG° &gt; 0 et les réactifs sont plus abondants dans le mélange réactionnel.

If K = 1, ΔG° = 0 et les réactifs et produits sont comparativement abondants dans le mélange réactionnel.

Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/16-4-free-energy">Openstax, Chemistry 2e, Section 16.4: Free Energy.</a>

Énergie libre de Gibbs et favorabilité thermodynamique

La spontanéité d’un processus dépend de la température du système. Les transitions de phase, par exemple, se dérouleront spontanément dans une direction ou ...

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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.

Temperature Effects on Gibbs Free Energy

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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...

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Temperature Effects on Gibbs Free Energy

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