Name: temperature effects on gibbs free energy
Uploaded: 2023-04-30T18:28:32.000Z
Duration: 2 min 23 s
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Gibbs Free Energy and Thermodynamic Favorability

La variazione dell'energia libera di Gibbs, o delta G, è l'energia assorbita o rilasciata quando si formano o si rompono legami che immagazzinano energia, rispettivamente.
Il segno del delta G dipende dai segni e dai valori relativi di entalpia, entropia e temperatura.
Se il delta H è negativo e il delta S è positivo, il delta G è negativo a tutte le temperature. Pertanto, le reazioni esotermiche in cui l'entropia del sistema aumenta sono sempre spontanee.
Se sia il delta H che il delta S sono negativi, il delta G dipende dalla temperatura. Le reazioni con variazioni negative di entalpia ed entropia sono spontanee solo a basse temperature.
Il delta G dipende anche dalla temperatura se sia il delta H che il delta S sono positivi. Le reazioni con variazioni positive di entalpia ed entropia sono spontanee solo a temperature più elevate.
Quando il delta H è positivo e il delta S è negativo, il delta G è sempre positivo e la reazione non è spontanea a tutte le temperature.
Per qualsiasi composizione di miscela di reazione, il delta G per la reazione è la somma dell'energia libera standard e RT moltiplicato per il log naturale del quoziente di reazione.
Quando i reagenti e i prodotti sono all'equilibrio, la variazione di energia libera è zero e il quoziente di reazione è uguale alla costante di equilibrio. Quindi, la variazione standard di energia libera è uguale a RT negativo ln(K).
Se delta G nulla è minore di zero, ln(K) è positivo, ovvero K è maggiore di 1. In questo caso, la formazione del prodotto è favorita all'equilibrio. Maggiore è la costante di equilibrio, maggiore è la diminuzione dell'energia libera di Gibbs. 
Le reazioni con valori di delta G negativi comportano il rilascio di energia libera nell'ambiente circostante e sono chiamate reazioni "esoergoniche".
Al contrario, se delta G nulla è maggiore di zero, ln(K) è negativo, il che significa che K è minore di 1, e la direzione inversa della reazione è favorita.
Le reazioni con un delta G positivo assorbono energia libera dall'ambiente circostante e sono chiamate reazioni "endergoniche".

energia libera di gibbs e vantaggio termodinamico

La spontaneità di un processo dipende dalla temperatura del sistema. Le transizioni di fase, ad esempio, procederanno spontaneamente in una direzione o nell'altra a seconda della temperatura della sostanza in questione. Allo stesso modo, alcune reazioni chimiche possono anche mostrare spontaneità dipendenti dalla temperatura. Per illustrare questo concetto, viene considerata l'equazione che mette in relazione la variazione di energia libera con le variazioni di entalpia ed entropia del processo:

<img alt="Eq1" src="/files/ftp_upload/11694/11694%20PT%20eq%201.svg" style="text-align: center; height: 15px;">

La spontaneità di un processo, come si riflette nel segno aritmetico della sua variazione di energia libera, è quindi determinata dai segni delle variazioni di entalpia ed entropia e, in alcuni casi, dalla temperatura assoluta. Poiché T è la temperatura assoluta (kelvin), può avere solo valori positivi. Esistono quindi quattro possibilità per quanto riguarda i segni delle variazioni di entalpia ed entropia:

<ol>
	<li>Sia ΔH sia ΔS sono positivi. Questa condizione descrive un processo endotermico che comporta un aumento dell'entropia del sistema. In questo caso, ΔG sarà negativo se el magnitudo del termine TΔS è maggiore di ΔH. Se il termine TΔS è inferiore a ΔH, la variazione di energia libera sarà positiva. Tale processo è spontaneo alle alte temperature e non spontaneo alle basse temperature.</li>
	<li>Sia ΔH sia ΔS sono negativi. Questa condizione descrive un processo esotermico che comporta una diminuzione dell'entropia del sistema. In questo caso, ΔG sarà negativo se magnitudo del termine TΔS è inferiore a ΔH. Se magnitudo del termine TΔS è maggiore di ΔH, la variazione di energia libera sarà positiva. Tale processo è spontaneo alle basse temperature e non spontaneo alle alte temperature.</li>
	<li>ΔH è positivo e ΔS è negativo. Questa condizione descrive un processo endotermico che comporta una diminuzione dell'entropia del sistema. In questo caso, ΔG sarà positivo indipendentemente dalla temperatura. Tale processo non è spontaneo a tutte le temperature.</li>
	<li>ΔH è negativo e ΔS è positivo. Questa condizione descrive un processo esotermico che comporta un aumento dell'entropia del sistema. In questo caso, ΔG sarà negativo indipendentemente dalla temperatura. Tale processo è spontaneo a tutte le temperature.</li>
</ol>

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 ΔGrxn 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).

Ricordiamo che Q è il valore numerico dell'espressione dell'azione di massa per il sistema, e il suo valore 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 &gt; 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 100 kPa; concentrazioni diverse da 1 M) è correlata alla variazione di energia libera standard secondo questa equazione:

<img alt="Eq2" src="/files/ftp_upload/11694/11694%20PT%20eq%202.svg" style="text-align: center; height: 18px;">

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 un sistema all'equilibrio, Q = K e ΔG = 0, e l'equazione precedente può essere scritta come

<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;">

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. Le relazioni tra le variazioni standard di energia libera e le costanti di equilibrio sono riassunte di seguito.

Se K &gt; 1, ΔG° &lt; 0 e i prodotti sono più abbondanti nella miscela di reazione.

Se K &lt; 1, ΔG° &gt; 0 e i reagenti sono più abbondanti nella miscela di reazione.

Se K = 1, ΔG° = 0 e sia reagenti sia prodotti sono relativamente abbondanti nella miscela di reazione.

Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/16-4-free-energy">Openstax, Chemistry 2e, Section 16.4: Free Energy.</a>

Energia libera di Gibbs e vantaggio termodinamico

La spontaneità di un processo dipende dalla temperatura del sistema. Le transizioni di fase, ad esempio, procederanno spontaneamente in una direzione o ...

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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 Effects on Gibbs Free Energy

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