Name: temperature effects on gibbs free energy
Uploaded: 2023-04-30T18:28:32.000Z
Duration: 2 min 23 s
Channel: JoVE

K12 Standards

Louisiana Student Standards for Science

High School

Chemistry

Matter And Its Interactions

HS-PS1-6

gibbs-free-energy-and-thermodynamic-favorability

Gibbs Free Energy and Thermodynamic Favorability

ギブス自由エネルギーの変化、またはデルタGは、エネルギー貯蔵結合が形成されるか切断されるときにそれぞれ吸収または放出されるエネルギーです。
デルタGの符号は、エンタルピー、エントロピー、および温度の符号と相対値によって異なります。
デルタ H が負でデルタ S が正の場合、デルタ G はすべての温度で負です。したがって、システムのエントロピーが増加する発熱反応は、常に自発的です。
デルタ H とデルタ S の両方が負の場合、デルタ G は温度によって異なります。負のエンタルピーおよびエントロピー変化を伴う反応は、低温でのみ自発的です。
デルタGは、デルタHとデルタSの両方が正の場合、温度にも依存します。正のエンタルピーおよびエントロピー変化を伴う反応は、より高い温度でのみ自発的である。
デルタHが正でデルタSが負の場合、デルタGは常に正であり、反応はすべての温度で非自発的です。
どの反応混合物組成についても、反応のデルタGは、標準自由エネルギーとRTに反応商の自然対数を掛けた合計です。
反応物と生成物が平衡状態にあるとき、自由エネルギー変化はゼロであり、反応商は平衡定数に等しくなります。したがって、標準の自由エネルギー変化は負のRTLN(K)に等しくなります。
デルタ G naught が 0 より小さい場合、ln(K) は正であり、K は 1 より大きいことを意味します。この場合、生成物の形成は平衡状態で有利に働きます。平衡定数が大きいほど、ギブスの自由エネルギーの減少は大きくなります。 
負のデルタG値を持つ反応は、周囲への自由エネルギーの放出を伴い、「エクセルゴニック」反応と呼ばれます。
逆に、デルタGがゼロより大きい場合、ln(K)は負、つまりKは1未満であり、反応の逆方向が有利になります。
正のデルタGを持つ反応は、周囲から自由エネルギーを吸収し、「エンダーゴニック」反応と呼ばれます。

ギブズの自由エネルギーと熱力学的好感度

プロセスの自発性はシステムの温度に依存します。 たとえば、相転移は、問題の物質の温度に応じて一方向または他の方向に自発的に進行します。 同様に、一部の化学反応も温度に依存して自発性を示すことがあります。 この概念を説明するために、自由エネルギーの変化をプロセスのエンタルピーおよびエントロピーの変化に関連付ける方程式を考慮します。

<img alt="Eq1" src="/files/ftp_upload/11694/11694%20PT%20eq%201.svg" style="text-align: center; height: 15px;">

プロセスの自由エネルギー変化の算術符号に反映されるプロセスの自発性は、エンタルピーとエントロピー変化の符号、場合によっては絶対温度によって決まります。 T は絶対 (ケルビン) 温度であるため、正の値のみを取ることができます。 したがって、エンタルピーとエントロピーの変化の兆候に関しては、次の 4 つの可能性が存在します。

1 ΔH とΔS は両方とも正です。 この状態は、システムのエントロピーの増加を伴う吸熱プロセスを表します。 この場合、TΔS 項の大きさが ΔH より大きい場合、ΔG は負になります。 TΔS 項が ΔH より小さい場合、自由エネルギー変化は正になります。 このようなプロセスは、高温では自発的ですが、低温では非自発的です。

2 ΔH とΔS は両方とも負です。 この状態は、システムのエントロピーの減少を伴う発熱プロセスを表します。 この場合、TΔS 項の大きさが ΔH より小さい場合、ΔG は負になります。 TΔS 項の大きさが ΔH より大きい場合、自由エネルギー変化は正になります。 このようなプロセスは、低温では自発的ですが、高温では非自発的です。

3 ΔH は正、ΔS は負です。 この状態は、システムのエントロピーの減少を伴う吸熱プロセスを表します。 この場合、ΔG は温度に関係なく正になります。 このようなプロセスは、どの温度でも非自発的に行われます。

4 ΔH は負、ΔS は正です。 この状態は、システムのエントロピーの増加を伴う発熱プロセスを表します。 この場合、温度に関わらずΔGは負となります。 このようなプロセスは、すべての温度で自発的に発生します。

プロセスの自由エネルギー変化は、その推進力の尺度として見ることができます。 ΔＧの負の値は順方向のプロセスの駆動力を表し、正の値は逆方向のプロセスの駆動力を表す。 ΔGrxn がゼロの場合、順方向と逆方向の駆動力は等しく、プロセスは両方向に同じ速度で発生します (システムは平衡状態にあります)。

Q はシステムの質量作用式の数値であり、その値は平衡を達成するために反応が進行する方向を特定するために使用できることを思い出してください。 Q が平衡定数 K より小さい場合、反応は平衡に達し Q = K になるまで順方向に進みます。逆に、Q &gt; K の場合、プロセスは平衡に達するまで逆方向に進みます。

非標準条件（100 kPa 以外の圧力、1 M 以外の濃度）下で存在する反応物および生成物を使用して起こるプロセスの自由エネルギー変化は、次の方程式に従って標準自由エネルギー変化に関連します。

<img alt="Eq2" src="/files/ftp_upload/11694/11694%20PT%20eq%202.svg" style="text-align: center; height: 18px;">

R は気体定数 (8.314 J/K mol)、T はケルビンまたは絶対温度、Q は反応商です。 平衡状態にあるシステムの場合、Q = K および ΔG = 0 であり、前の式は次のように書くことができます。

<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;">

この形式の方程式は、これら 2 つの必須の熱力学特性間の有用なリンクを提供し、標準自由エネルギー変化から平衡定数を導出したり、その逆に平衡定数を導出したりするために使用できます。 標準自由エネルギー変化と平衡定数の関係を以下にまとめます。

K &gt; 1、ΔG° &lt; 0 の場合、生成物は反応混合物中でより豊富になります。

K &lt; 1、ΔG° &gt; 0 の場合、反応物は反応混合物中にさらに豊富に存在します。

K = 1、ΔG° = 0 の場合、反応物と生成物は反応混合物中に比較的豊富に存在します。

このテキストはから翻案されます <a href="https://openstax.org/books/chemistry-2e/pages/16-4-free-energy">Openstax, Chemistry 2e, Section 16.4: Free Energy.</a>

プロセスの自発性は、システムの温度に依存します。例えば、相転移は一方向に自発的に進行するか、または...

Education

JoVE Core

Organic Chemistry

有機化学

Unit 1

ユニット1

Thermodynamics and Chemical Kinetics

熱力学と化学反応速度論

KEY TERMS AND CONCEPTS

主要用語と概念

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

chemical reactions

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gibbs free energy and thermodynamic favorability

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...

chemical and solubility equilibria

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rate law and reaction order

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effect of temperature change on reaction rate

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Temperature Effects on Gibbs Free Energy | JoVE K12

organic chemistry

JoVE Core 有機化学では、立体化学、反応、メカニズムの基本概念を、簡潔でわかりやすいアニメーション ビデオ レッスンを使用して説明します。さらに、科学者の実際のビデオでは、世界中の今日の研究室で行われている古典的および独自の研究実験における関連概念の適用を示しています。

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Physical Science

Energy

HS-PS3-3

HS-PS3-4

south-carolina-college-and-career-ready-science-standards

South Carolina College and Career Ready Science Standards

C-PS3-4

Physics

P-PS3-3

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HS-PS1-3

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Temperature Effects on Gibbs Free Energy

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