Name: le chatelier's principle in heating and cooling
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 19 s
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Le Chatelier's Principle: Changing Temperature

温度は化学反応の速度に影響を与えるため、平衡状態での反応の温度変化は、システムにストレスとして作用します。ル・シャトリエの原理は、このような撹乱を最小限に抑えるためにシステムがどのように反応するかを予測します。
温度の変化は、Kの値を変化させずに平衡をシフトする濃度や体積の変化とは異なり、平衡定数の値を変化させます。
ガス状五塩化リンの三塩化リンと塩素ガスへの分解を考えてみましょう。この吸熱反応では、吸収された熱を反応物と考えることができます。
温度が上昇すると、反応物をさらに追加するのと同様に、システムに熱が追加されます。したがって、平衡位置は製品にシフトし、平衡定数Kの値が増加したため、余分な熱を消費するために三塩化リンと塩素をより多く生成します。
一方、温度が下がると、反応物が除去されるのと同様に、システムから熱が除去されます。平衡位置は反応物に向かってシフトし、Kの値が減少したため、熱を放出するためにより多くの五塩化リンを生成します。
二酸化硫黄と酸素の間の気体反応で三酸化硫黄を生成するような発熱反応の場合、放出される熱は生成物と考えることができます。
温度の上昇は、製品の追加と似ています。これにより、平衡位置が反応物に向かってシフトし、Kの値が減少したため、追加された熱の一部を吸収するために二酸化硫黄と酸素がより多くの生成されます。
逆に、この発熱反応の温度を下げると、生成物を取り除くように熱が取り除かれます。平衡位置は製品にシフトし、Kが増加するにつれて、より多くの三酸化硫黄を生成して熱を放出します。
したがって、温度の上昇は吸熱反応の生成物に有利に働き、温度の低下は発熱反応の生成物に有利になります。

ルシャトリエの原理：温度の変更

質量作用の法則に従い、濃度の変化によって外乱を受けた平衡は、平衡定数Kの値が変化することなく、平衡を回復するために移動します。しかし、温度変化に応じて平衡が移動すると、異なる相対的な組成で平衡定数の値が変化するため、平衡が再構築されます。
 この現象を理解するために、次の反応を考えます。
  <img alt="Eq1" src="/files/ftp_upload/11393/11393_Equation_1.png" style="height: 50px;" />
 この反応は素過程にあたるので、平衡式の化学量論から直接、順方向と逆方向の速度則を導くことができます。’
  <img alt="Eq2" src="/files/ftp_upload/11393/11393_Equation_2.png" style="height: 65px;" />
 系が平衡にある時、次が成立します。
  <img alt="Eq3" src="/files/ftp_upload/11393/11393_Equation_3.png" style="height: 50px;" />
 この等式に反応速度則を代入して再整理すると、
  <img alt="Eq4" src="/files/ftp_upload/11393/11393_Equation_4.png" style="height: 100px;" />
 平衡定数は、順反応と逆反応の速度定数の数学的な関数として表すことができます。速度定数はアレニウスの式で表されるように温度によって変化するため、平衡定数も同様に温度によって変化する（速度定数が温度変化によって異なる影響を受けると仮定した場合）。多段階の反応機構を含むより複雑な反応の場合、平衡定数と機構内の各段階の速度定数の間には、同様の、より複雑な数学的関係が存在します。どんなに複雑な反応であっても、平衡定数の温度依存性は変わりません。
 温度変化に対して平衡がどのように変化するかを予測するには、反応のエンタルピー変化を考慮するのが最も簡便です。例えば、ハーバー・ボッシュ法によるアンモニアの生成は、発熱を伴うプロセスです。
  <img alt="Eq5" src="/files/ftp_upload/11393/11393_Equation_5.png" style="height: 50px;" />
 ルシャトリエの原理を適用するために、ここでの熱qは生成物と見なすことができます。
  <img alt="Eq6" src="/files/ftp_upload/11393/11393_Equation_6.png" style="height: 50px;" />
 反応系の温度を上げることは、生成物の量を増やすことと同じなので、平衡は左にシフトします。系の温度を下げると、同様に平衡が右にシフトします。吸熱過程では、熱は反応の反応物とみなすことができ、逆の温度依存性が観察されます。
 上記の文章は以下から引用しました。 <a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">Openstax, Chemistry 2e, Section 13.3 Shifting Equilibria: Le </a><a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">Ch</a>â<a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">t</a><a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle">elier’s Principle</a>.

質量作用の法則と一致して、濃度の変化によってストレスを受けた平衡は、...

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le chatelier's principle in heating and cooling

Le Chatelier's principle helps explain how chemical equilibrium responds to heating and cooling. A system at equilibrium will shift to restore balance after a temperature change, but the equilibrium constant, K, will not stay the same. Instead, the mixture reaches a new equilibrium with a different composition.

This temperature effect is different from a concentration change, which shifts equilibrium without changing K. To understand why temperature matters, the reaction can be written with forward and reverse rate laws based on the balanced equation. At equilibrium, the forward and reverse reaction rates are equal. When those rate laws are combined and rearranged, the equilibrium constant can be expressed as a function of the forward and reverse rate constants.

Because rate constants change with temperature, as described by the Arrhenius equation, the equilibrium constant can also change with temperature. This idea applies to both elementary reactions and more complex multistep mechanisms. In multistep reactions, the math is more complicated, but the temperature dependence of the equilibrium constant still remains.

A practical way to predict the direction of the shift is to use the reaction’s enthalpy change. In the Haber process, ammonia formation is exothermic, which means it produces heat. For Le Chatelier's principle, heat can be treated like a product. Raising the temperature adds heat, so the equilibrium shifts left. Lowering the temperature removes heat, so the equilibrium shifts right. For endothermic processes, heat acts like a reactant, so the temperature effect is opposite.

Le Chatelier's Principle in Heating and Cooling

le-chateliers-principle-heating-and-cooling

the equilibrium constant

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Chemical equilibrium and the equilibrium constant Kc describe how reactants and products are balanced in a reaction. In the oxidation of sulfur dioxide, a mixture that starts with reactants only has no product at first. As the reaction moves forward toward equilibrium, reactant concentrations decrease and product concentration increases. When equilibrium is reached, the concentrations of both stay constant. A reaction can also begin with only products present. In that case, the reaction moves...

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le chatelier's principle: changing concentration

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Shifting Equilibrium by Changing Concentration

Chemical equilibrium is a dynamic balance. At equilibrium, the forward and reverse reactions happen at equal rates. If the system is stressed in a way that changes one rate more than the other, it is no longer at equilibrium and will shift until balance is restored. This behavior is described by Le Châtelier’s principle. Changes in concentration are a major stress on an equilibrium system. Reaction rates depend mainly on concentration and temperature. For that reason, changing the amount of a...

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le chatelier's principle: changing volume (pressure)

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Gas-phase equilibrium can shift when the volume of the system changes. In these reactions, a change in pressure changes concentration because the partial pressure of an ideal gas is proportional to its molar concentration. That means pressure changes affect equilibrium in the same way as concentration changes. When the volume of a gas mixture changes, the concentrations of reactants and products change together. Since every gas in the equilibrium occupies the same space, the same volume change...

le chatelier's principle: changing temperature

Le Chatelier's principle helps explain how chemical equilibrium responds to heating and cooling. A system at equilibrium will shift to restore balance after a temperature change, but the equilibrium constant, K, will not stay the same. Instead, the mixture reaches a new equilibrium with a different composition. This temperature effect is different from a concentration change, which shifts equilibrium without changing K. To understand why temperature matters, the reaction can be written with...

the small x assumption

Estimating Equilibrium with Small Changes

A small equilibrium constant can make equilibrium calculations easier. In these reactions, the equilibrium position favors the reactants. When the initial reactant concentration is high and the Kc value is small, the change in concentration may be negligible. In that case, the equilibrium concentration is close to the initial concentration. This idea helps simplify weak acid and weak base problems. Chemists can assume that some equilibrium concentrations are equal to the initial concentrations.

key terms and concepts

Learn how heating and cooling change chemical equilibrium, why K changes with temperature, and how exothermic and endothermic reactions shift.

Le Chatelier's Principle in Heating and Cooling | JoVE K12

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Le Chatelier's Principle in Heating and Cooling

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