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

A mudança na energia livre de Gibbs, ou delta G, é a energia absorvida ou liberada quando as ligações de armazenamento de energia são formadas ou quebradas, respectivamente.
O sinal de delta G depende dos sinais e dos valores relativos de entalpia, entropia e temperatura.
Se delta H for negativo e delta S for positivo, delta G é negativo em todas as temperaturas. Assim, as reações exotérmicas em que a entropia do sistema aumenta são sempre espontâneas.
Se delta H e delta S forem negativos, delta G depende da temperatura. Reações com mudanças negativas de entalpia e entropia são espontâneas apenas em baixas temperaturas.
O delta G também depende da temperatura se o delta H e o delta S forem positivos. Reações com mudanças positivas de entalpia e entropia são espontâneas apenas em temperaturas mais altas.
Quando delta H é positivo e delta S é negativo, delta G é sempre positivo e a reação é não espontânea em todas as temperaturas.
Para qualquer composição de mistura de reação, o delta G para a reação é a soma da energia livre padrão e RT vezes o log natural do quociente de reação.
Quando os reagentes e produtos estão em equilíbrio, a mudança de energia livre é zero e o quociente de reação é igual à constante de equilíbrio. Portanto, a mudança de energia livre padrão é igual a RT negativo ln(K).
Se delta G zero é menor que zero, ln(K) é positivo, o que significa que K é maior que 1. Nesse caso, a formação do produto é favorecida no equilíbrio. Quanto maior a constante de equilíbrio, maior a diminuição da energia livre de Gibbs. 
Reações com valores delta G negativos envolvem a liberação de energia livre para o ambiente e são chamadas de reações 'exergônicas'.
Por outro lado, se delta G zero é maior que zero, ln(K) é negativo, o que significa que K é menor que 1, e a direção inversa da reação é favorecida.
As reações com um delta G positivo absorvem energia livre do ambiente e são chamadas de reações 'endergônicas'.

energia livre de gibbs e favorabilidade termodinâmica

A espontaneidade de um processo depende da temperatura do sistema. As transições de fase, por exemplo, ocorrerão espontaneamente em uma direção ou outra dependendo da temperatura da substância em questão. Da mesma forma, algumas reações químicas também podem apresentar espontaneidade dependente da temperatura. Para ilustrar este conceito, é considerada a equação que relaciona a mudança da energia livre com as variações de entalpia e entropia do processo:


<img alt="Eq1" src="/files/ftp_upload/11694/11694%20PT%20eq%201.svg" style="text-align: center; height: 15px;">

A espontaneidade de um processo, refletida no sinal aritmético de sua mudança de energia livre, é então determinada pelos sinais das mudanças de entalpia e entropia e, em alguns casos, pela temperatura absoluta. Como T é a temperatura absoluta (kelvin), ela só pode ter valores positivos. Existem, portanto, quatro possibilidades em relação aos sinais das mudanças de entalpia e entropia:


<ol style="display: flex; flex-direction: column; gap: 10px;">
 <li>Tanto ΔH quanto ΔS são positivos. Esta condição descreve um processo endotérmico que envolve um aumento na entropia do sistema. Neste caso, ΔG será negativo se a magnitude do termo TΔS for maior que ΔH. Se o termo TΔS for menor que ΔH, a variação da energia livre será positiva. Tal processo é espontâneo em altas temperaturas e não espontâneo em baixas temperaturas.</li>
 <li>Tanto ΔH quanto ΔS são negativos. Esta condição descreve um processo exotérmico que envolve uma diminuição na entropia do sistema. Neste caso, ΔG será negativo se a magnitude do termo TΔS for menor que ΔH. Se a magnitude do termo TΔS for maior que ΔH, a variação da energia livre será positiva. Tal processo é espontâneo em baixas temperaturas e não espontâneo em altas temperaturas.</li>
 <li>ΔH é positivo e ΔS é negativo. Esta condição descreve um processo endotérmico que envolve uma diminuição na entropia do sistema. Neste caso, ΔG será positivo independentemente da temperatura. Tal processo não é espontâneo em todas as temperaturas.</li>
 <li>ΔH é negativo e ΔS é positivo. Esta condição descreve um processo exotérmico que envolve um aumento na entropia do sistema. Neste caso, ΔG será negativo independentemente da temperatura. Tal processo é espontâneo em todas as temperaturas.</li>
</ol>

A mudança de energia livre para um processo pode ser vista como uma medida de sua força motriz. Um valor negativo para ΔG representa uma força motriz para o processo na direção direta, enquanto um valor positivo representa uma força motriz para o processo na direção inversa. Quando ΔGrxn é zero, as forças motrizes direta e inversa são iguais, e o processo ocorre em ambas as direções na mesma proporção (o sistema está em equilíbrio).

Lembre-se de que Q é o valor numérico da expressão da ação de massa para o sistema, e seu valor pode ser usado para identificar a direção na qual uma reação prosseguirá para atingir o equilíbrio. Quando Q é menor que a constante de equilíbrio, K, a reação prosseguirá na direção direta até que o equilíbrio seja alcançado e Q = K. Por outro lado, se Q &gt; K, o processo prosseguirá na direção inversa até que o equilíbrio seja alcançado.

A mudança de energia livre para um processo que ocorre com reagentes e produtos presentes sob condições não padronizadas (pressões diferentes de 100 kPa; concentrações diferentes de 1 M) está relacionada à mudança de energia livre padrão de acordo com esta equação:


<img alt="Eq2" src="/files/ftp_upload/11694/11694%20PT%20eq%202.svg" style="text-align: center; height: 18px;">

R é a constante dos gases (8,314 J/K mol), T é a temperatura em Kelvin (absoluta), e Q é o quociente da reação. Para um sistema em equilíbrio, Q = K e ΔG = 0, e a equação anterior pode ser escrita 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 da equação fornece uma ligação útil entre estas duas propriedades termodinâmicas essenciais e pode ser usada para derivar constantes de equilíbrio a partir de mudanças padrão de energia livre e vice-versa. As relações entre mudanças padrão de energia livre e constantes de equilíbrio estão resumidas abaixo.



Se K &gt; 1, ΔG° &lt; 0 e os produtos são mais abundantes na mistura reacional. 
Se K &lt; 1, ΔG° &gt; 0 e os reagentes são mais abundantes na mistura reacional. 
Se K = 1, ΔG° = 0 e os reagentes e produtos são comparativamente abundantes na mistura de reação.


Este texto foi adaptado do <a href="https://openstax.org/books/chemistry-2e/pages/16-4-free-energy">Openstax, Chemistry 2e, Section 16.4: Free Energy.</a>

Energia Livre de Gibbs e Favorabilidade Termodinâmica

A espontaneidade de um processo depende da temperatura do sistema. As transições de fase, por exemplo, prosseguirão espontaneamente em uma direção 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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