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

El cambio en la energía libre de Gibbs, o delta G, es la energía absorbida o liberada cuando se forman o rompen enlaces que almacenan energía, respectivamente.
El signo de delta G depende de los signos y los valores relativos de entalpía, entropía y temperatura.
Si delta H es negativo y delta S es positivo, delta G es negativo a todas las temperaturas. Por lo tanto, las reacciones exotérmicas en las que aumenta la entropía del sistema son siempre espontáneas.
Si tanto delta H como delta S son negativos, delta G depende de la temperatura. Las reacciones con entalpía negativa y cambios de entropía son espontáneas solo a bajas temperaturas.
Delta G también depende de la temperatura si tanto delta H como delta S son positivos. Las reacciones con entalpía positiva y cambios de entropía son espontáneas solo a temperaturas más altas.
Cuando delta H es positivo y delta S es negativo, delta G es siempre positivo y la reacción no es espontánea a todas las temperaturas.
Para cualquier composición de mezcla de reacción, el delta G de la reacción es la suma de la energía libre estándar y el RT por el logaritmo natural del cociente de reacción.
Cuando los reactivos y productos están en equilibrio, el cambio de energía libre es cero y el cociente de reacción es igual a la constante de equilibrio. Por lo tanto, el cambio de energía libre estándar es igual a RT ln(K) negativo.
Si delta G naught es menor que cero, ln(K) es positivo, lo que significa que K es mayor que 1. En este caso, la formación de productos se ve favorecida en el equilibrio. Cuanto mayor sea la constante de equilibrio, mayor será la disminución de la energía libre de Gibbs. 
Las reacciones con valores negativos de delta G implican la liberación de energía libre al entorno y se denominan reacciones "exergónicas".
Por el contrario, si delta G naught es mayor que cero, ln(K) es negativo, lo que significa que K es menor que 1, y se favorece la dirección inversa de la reacción.
Las reacciones con un delta G positivo absorben energía libre del entorno y se denominan reacciones "endergónicas".

energía libre de gibbs y favorabilidad termodinámica

La espontaneidad de un proceso depende de la temperatura del sistema. Las transiciones de fase, por ejemplo, se producirán espontáneamente en una dirección u otra dependiendo de la temperatura de la sustancia en cuestión. Asimismo, algunas reacciones químicas también pueden presentar espontaneidades que dependen de la temperatura. Para ilustrar este concepto, se considera la ecuación que relaciona el cambio de energía libre con la entalpía y los cambios de entropía del proceso:


 <img alt="Eq1" src="/files/ftp_upload/11694/11694%20PT%20eq%201.svg" style="text-align: center; height: 15px;">


 La espontaneidad de un proceso, tal como se refleja en el signo aritmético de su cambio de energía libre, está entonces determinada por los signos de los cambios de entalpía y entropía y, en algunos casos, por la temperatura absoluta. Como T es la temperatura absoluta (kelvin), solo puede tener valores positivos. Por tanto, existen cuatro posibilidades con respecto a los signos de los cambios de entalpía y de entropía:


 <ol>
 <li>Tanto ΔH como ΔS son positivos. Esta condición describe un proceso endotérmico que implica un aumento en la entropía del sistema. En este caso, ΔG será negativo si la magnitud del término TΔS es mayor que ΔH. Si el término TΔS es menor que ΔH, el cambio de energía libre será positivo. Este proceso es espontáneo a altas temperaturas y no espontáneo a bajas temperaturas.</li>
 <li>Tanto ΔH como ΔS son negativos. Esta condición describe un proceso exotérmico que implica una disminución de la entropía del sistema. En este caso, ΔG será negativo si la magnitud del término TΔS es menor que ΔH. Si la magnitud del término TΔS es mayor que ΔH, el cambio de energía libre será positivo. Este proceso es espontáneo a bajas temperaturas y no espontáneo a altas temperaturas.</li>
 <li>ΔH es positivo y ΔS es negativo. Esta condición describe un proceso endotérmico que implica una disminución de la entropía del sistema. En este caso, ΔG será positivo independientemente de la temperatura. Este proceso no es espontáneo a todas las temperaturas.</li>
 <li>ΔH es negativo y ΔS es positivo. Esta condición describe un proceso exotérmico que implica un aumento en la entropía del sistema. En este caso, ΔG será negativo independientemente de la temperatura. Este proceso es espontáneo a todas las temperaturas.</li>
 </ol>


 El cambio de energía libre de un proceso puede verse como una medida de su fuerza impulsora. Un valor negativo para ΔG representa una fuerza impulsora para el proceso en la dirección de avance, mientras que un valor positivo representa una fuerza impulsora para el proceso en la dirección inversa. Cuando ΔGrxn es cero, las fuerzas impulsoras hacia adelante y hacia atrás son iguales y el proceso ocurre en ambas direcciones a la misma velocidad (el sistema está en equilibrio).



 Recuerde que Q es el valor numérico de la expresión de la acción de masas para el sistema, y ​​su valor puede usarse para identificar la dirección en la que procederá una reacción para lograr el equilibrio. Cuando Q es menor que la constante de equilibrio, K, la reacción procederá en dirección directa hasta que se alcance el equilibrio y Q = K. Por el contrario, si Q &gt; K, el proceso procederá en dirección inversa hasta que se alcance el equilibrio.



 El cambio de energía libre para un proceso que tiene lugar con reactivos y productos presentes en condiciones no estándar (presiones distintas de 100 kPa; concentraciones distintas de 1 M) está relacionado con el cambio de energía libre estándar según esta ecuación:


 <img alt="Eq2" src="/files/ftp_upload/11694/11694%20PT%20eq%202.svg" style="text-align: center; height: 18px;">


 R es la constante de los gases (8,314 J/K mol), T es el kelvin o temperatura absoluta y Q es el cociente de reacción. Para un sistema en equilibrio, Q = K y ΔG = 0, y la ecuación anterior se puede escribir como


 <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;">


 Esta forma de ecuación proporciona un vínculo útil entre estas dos propiedades termodinámicas esenciales y puede usarse para derivar constantes de equilibrio a partir de cambios estándar de energía libre y viceversa. Las relaciones entre los cambios de energía libre estándar y las constantes de equilibrio se resumen a continuación.


 Si K &gt; 1, ΔG° &lt; 0 y los productos son más abundantes en la mezcla de reacción.


 Si K &lt; 1, ΔG° &gt; 0 y los reactivos son más abundantes en la mezcla de reacción.


 Si K = 1, ΔG° = 0 y los reactivos y productos son comparablemente abundantes en la mezcla de reacción.

Energía libre de Gibbs y favorabilidad termodinámica

La espontaneidad de un proceso depende de la temperatura del sistema. Las transiciones de fase, por ejemplo, procederán espontáneamente en una dirección o ...

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Thermodynamics and Chemical Kinetics

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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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JoVE Core Química Orgánica explica los conceptos básicos de la estereoquímica, las reacciones y sus mecanismos mediante lecciones de vídeo animadas, concisas y fáciles de entender. Además, los vídeos de científicos en acción muestran la aplicación de esos conceptos en experimentos de investigación clásicos y originales, desarrollados actualmente en laboratorios de todo el mundo.

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HS-PS1-3

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Temperature Effects on Gibbs Free Energy

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