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

Изменение свободной энергии Гиббса, или дельта G, — это энергия, поглощаемая или высвобождаемая при образовании или разрыве связей, накапливающих энергию, соответственно.
Знак дельты G зависит от знаков и относительных значений энтальпии, энтропии и температуры.
Если дельта H отрицательна, а дельта S положительна, то дельта G отрицательна при всех температурах. Таким образом, экзотермические реакции, в которых энтропия системы возрастает, всегда спонтанны.
Если и дельта H, и дельта S отрицательны, дельта G зависит от температуры. Реакции с отрицательными изменениями энтальпии и энтропии спонтанны только при низких температурах.
Дельта G также зависит от температуры, если и дельта H, и дельта S положительны. Реакции с положительными изменениями энтальпии и энтропии спонтанны только при более высоких температурах.
Когда дельта H положительна, а дельта S отрицательна, дельта G всегда положительна, и реакция не происходит спонтанно при любых температурах.
Для любого состава реакционной смеси дельта G для реакции является суммой стандартной свободной энергии и RT, умноженной на естественный логарифм коэффициента реакции.
Когда реагенты и продукты находятся в равновесии, изменение свободной энергии равно нулю, а коэффициент реакции равен константе равновесия. Таким образом, стандартное изменение свободной энергии равно отрицательному RT ln(K).
Если дельта G nought меньше нуля, ln(K) положительна, что означает, что K больше 1. В этом случае формирование продукта благоприятствует равновесию. Чем больше константа равновесия, тем больше уменьшается свободная энергия Гиббса. 
Реакции с отрицательными значениями дельта G включают высвобождение свободной энергии в окружающую среду и называются «эксергоническими» реакциями.
И наоборот, если дельта G nought больше нуля, ln(K) отрицательна, то есть K меньше 1, и предпочтение отдается обратному направлению реакции.
Реакции с положительной дельтой G поглощают свободную энергию из окружающей среды и называются «эндергоническими» реакциями.

Свободная энергия Гиббса и термодинамическая благоприятность

Спонтанность процесса зависит от температуры системы. Фазовые переходы, например, будут происходить спонтанно в том или ином направлении в зависимости от температуры рассматриваемого вещества. Аналогичным образом, некоторые химические реакции могут проявлять спонтанность, зависящую от температуры. Чтобы проиллюстрировать эту концепцию, рассматривается уравнение, связывающее изменение свободной энергии с изменениями энтальпии и энтропии процесса:

<img alt="Eq1" src="/files/ftp_upload/11694/11694%20PT%20eq%201.svg" style="text-align: center; height: 15px;">

Спонтанность, или самопроизвольность, процесса, отражающаяся в арифметическом знаке изменения его свободной энергии, в данном случае определяется знаками изменения энтальпии и энтропии, а в некоторых случаях и абсолютной температурой. Поскольку T — абсолютная температура (по шкале Кельвина), она может иметь только положительные значения. Таким образом, существуют четыре возможности относительно знаков изменения энтальпии и энтропии:


<ol>
 <li>И ΔH, и ΔS положительны. Это условие описывает эндотермический процесс, который включает увеличение энтропии системы. В этом случае ΔG будет отрицательным, если величина члена TΔS больше, чем ΔH. Если член TΔS меньше ΔH, изменение свободной энергии будет положительным. Такой процесс является спонтанным при высоких температурах и неспонтанным при низких.</li>
 
 <li> И ΔH, и ΔS отрицательны. Это условие описывает экзотермический процесс, сопровождающийся уменьшением энтропии системы. В этом случае ΔG будет отрицательным, если величина члена TΔS меньше ΔH. Если величина члена TΔS больше, чем ΔH, изменение свободной энергии будет положительным. Такой процесс является спонтанным при низких температурах и неспонтанным при высоких.</li>
 
 <li> ΔH положителен, а ΔS отрицателен. Это условие описывает эндотермический процесс, сопровождающийся уменьшением энтропии системы. В этом случае ΔG будет положительным независимо от температуры. Такой процесс является неспонтанным при любых температурах.</li>
 
 <li> ΔH отрицателен, а ΔS положителен. Это условие описывает экзотермический процесс, который включает увеличение энтропии системы. В этом случае ΔG будет отрицательным независимо от температуры. Такой процесс является спонтанным при любых температурах.</li>
</ol>

Изменение свободной энергии процесса можно рассматривать как меру его движущей силы. Отрицательное значение ΔG представляет собой движущую силу процесса в прямом направлении, а положительное значение представляет собой движущую силу процесса в обратном направлении. Когда ΔGrxn равен нулю, прямая и обратная движущие силы равны, и процесс происходит в обоих направлениях с одинаковой скоростью (система находится в равновесии).

Напомним, что Q — это числовое значение выражения действия масс для системы, и его значение можно использовать для определения направления, в котором будет протекать реакция для достижения равновесия. Когда Q меньше константы равновесия K, реакция будет идти в прямом направлении до тех пор, пока не будет достигнуто равновесие и Q = K. И наоборот, если Q &gt; K, процесс будет идти в обратном направлении до тех пор, пока не будет достигнуто равновесие.

Изменение свободной энергии процесса, происходящего с реагентами и продуктами, присутствующими в нестандартных условиях (давление, отличное от 100 кПа; концентрации, отличные от 1 М), связано со стандартным изменением свободной энергии согласно этому уравнению:

<img alt="Eq2" src="/files/ftp_upload/11694/11694%20PT%20eq%202.svg" style="text-align: center; height: 18px;">
R — газовая постоянная (8,314 Дж/К моль), T — кельвин или абсолютная температура, а Q — коэффициент реакции. Для системы, находящейся в равновесии, Q = K и ΔG = 0, и предыдущее уравнение можно записать как

 
<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;">

Эта форма уравнения обеспечивает полезную связь между этими двумя важными термодинамическими свойствами и может использоваться для вывода констант равновесия из стандартных изменений свободной энергии и наоборот. Взаимосвязь между стандартными изменениями свободной энергии и константами равновесия кратко изложена ниже.



Если К &gt; 1, ΔG° &lt; 0 и продуктов в реакционной смеси больше. 
Если K &lt; 1, ΔG° &gt; 0 и реагентов в реакционной смеси больше. 
Если К = 1, ΔG° = 0 и реагентов и продуктов сравнительно много в реакционной смеси.


Адаптированная версия текста <a href="https://openstax.org/books/chemistry-2e/pages/16-4-free-energy">Openstax, Chemistry 2e, Section 16.4: Free Energy.</a>

Спонтанность процесса зависит от температуры системы. Фазовые переходы, например, будут протекать самопроизвольно в одном направлении или...

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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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Chemical reactions require sufficient energy to cause the matter to collide with enough precision and force that old chemical bonds can be broken and new ...

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Factors Influencing the Rate of Chemical Reactions

A variety of factors influence the rate of chemical reactions. For a chemical reaction to happen, atoms must collide with enough energy to overcome the ...

Predicting Reaction Outcomes

Kinetics describes the rate and path by which a reaction occurs. In contrast, thermodynamics deals with state functions and describes the properties, ...

States of Matter and Phase Changes

The internal energy of a substance&#8212;the total kinetic energy of all its molecules and the potential energy of their associated forces&#8212;depends ...

Reaction Mechanisms

Chemical reactions often occur in a stepwise fashion, involving two or more distinct reactions taking place in a sequence. A balanced equation indicates ...

Temperature Dependence

Chemical Kinetics 
The reaction rate is the speed at which a chemical reaction occurs. The reaction rate is defined as the change in concentration of a ...

Molecular Kinetic Energy

The word &#34;gas&#34;&#160;comes from the Flemish word meaning &#34;chaos,&#34; first used to describe vapors by the chemist J. B. van Helmont. Consider ...

Energy Basics

Chemical reactions, such as those that occur when you light a match, involve changes in energy as well as matter.

Chemical changes and their accompanying ...

Reaction Rate

The rate of reaction is the change in the amount of a reactant or product per unit time. Reaction rates are therefore determined by measuring the time ...

Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.  Clarification Statement: Emphasis is on the idea that a chemical reaction is a system that affects the energy change.  Examples of models could include molecular-level drawings and diagrams of reactions, graphs showing the relative energies of reactants and products, and representations showing energy is conserved.

HS-PS1-4

Temperature Effects on Gibbs Free Energy

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