Name: chemical equilibrium and kc rules
Uploaded: 2025-08-11T09:37:07.000Z
Duration: 3 min 10 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

NC K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.2

The Equilibrium Constant

質量作用の法則は、反応物と生成物の平衡濃度を関連付けるために平衡定数を導入します。
AとBが反応物、CとDが生成物、小文字のa、b、c、およびdがそれぞれの化学量論的係数である平衡化学方程式の場合、平衡定数の式は、生成物のモル濃度を反応物のモル濃度で割った値によって与えられ、すべて化学量論的係数に引き上げられます。
平衡定数は、多くの場合、記号KまたはKCで表され、添え字cはモル濃度に対する平衡定数を示します。モル濃度の比であるため、平衡定数は単位がありません。
1よりはるかに大きい大きな平衡定数は、分子、つまり製品の濃度が分母、つまり反応物の濃度よりも大きいことを意味します。これは、平衡が生成物に向かっており、順反応に有利であることを示しています。
逆に、平衡定数が 1 よりはるかに小さい場合は、分母が分子よりも大きいことを意味します。これは、平衡が反応物に向かっており、逆反応に有利であることを意味します。
1 以上の平衡定数の場合、反応物と生成物の相対濃度はほぼ同じです。これは、順方向も逆方向の反応も好まれないことを示しています。
化学式を変更すると、平衡定数も変化します。
平衡定数xの化学反応が逆になると、逆反応の平衡定数の表現は順反応の逆数になります。したがって、新しい平衡定数は x に対して 1 になります。
係数に別の係数nを掛けると、平衡定数は同じ係数に上がります。新しい平衡定数は xn になります。
2つ以上の個々の反応を加算すると、反応全体の平衡定数は個々の平衡定数の積になります。したがって、新しい平衡定数は y × z になります。
不均一平衡の場合、平衡定数の発現は純粋な固体または純粋な液体のエンティティを除外します。
たとえば、固体の硝酸アンモニウムを気体の亜酸化窒素と水蒸気に分解するための平衡定数の式は、純粋な固体である硝酸アンモニウムを除外します。
これは、純粋な固体または液体の相対的なモル濃度が反応中に一定のままであるためです。

平衡定数

二酸化硫黄の酸化について考えてみましょう。
 <img alt="Eq1" src="/files/ftp_upload/11386/11386_Equation_1.png" style="text-align: center; height: 50px;" />
 反応物のみの混合物から始まる反応では、最初の生成物濃度はゼロに等しいです。反応が平衡に向かって順方向に進むにつれて、反応物の濃度は減少し、生成物の濃度は増加します。平衡状態になると、反応物と生成物の濃度は一定になります。
 生成物のみが存在する状態で反応が開始された場合、反応は逆方向の平衡に向かって進む。生成物の濃度は時間とともに減少し、反応物の濃度は平衡状態で濃度が一定になるまで増加します。
 質量作用の法則とは、平衡状態における反応物の濃度に対する生成物の濃度の比を、それぞれの化学量論係数でべき乗したものが、平衡定数K（またはKc）に等しいというものです。
 したがって、上記の反応の平衡定数式は次のように書かれます。
 <img alt="Eq2" src="/files/ftp_upload/11386/11386_Equation_2.png" style="text-align: center; height: 100px;" />
 ここで、添字 ‘c’は、平衡定数が反応物と生成物のモル濃度を考慮していることを示しています。
 平衡定数の大きさは、平衡状態にある反応混合物の組成を反映します。大きなKを示す反応は、反応物のほとんどが生成物に変換されたときに平衡に達しますが、小さなK は、反応物がほとんど変換されないで平衡に達したことを示します。ここで注意すべき点は、Kの大きさは、平衡に達するまでの速さや遅さを示すものではないということです。平衡は、ほぼ瞬間的に成立するものもあれば、数日、数年、あるいはそれ以上の時間をかけても知覚できる変化が観察されないほどゆっくりと成立するものもあります。ある反応の平衡定数は、その反応物や生成物を含む混合物の挙動を予測するのに用いることができます。前述の二酸化硫黄の酸化プロセスで示されたように、化学反応は平衡を達成するために必要な方向に進行します。
 連動した平衡状態
 多くの平衡系では、2つ以上の 連結した 平衡反応、つまり1つ以上の反応種または生成物種を共通に持つ反応が関与しています。結合平衡を含む系の K 値は、個々の反応の K 値と関連付けることができます。この方法には、以下のような3つの基本的な手順からなります。 
 
 • 化学式の向きを変えると、「反応物」と「生成物」が入れ替わるため、反応式を逆転させた式の平衡定数は、もとの式の平衡定数の逆数になります。 
 
 • 式中の化学量論的係数をある係数 x だけ変化させると、平衡定数も同じ係数だけ指数関数的に変化します。 
 
 • 2つ以上の化学平衡式を組み合わせると、個々の反応’K 値の積を平衡定数とする全体の化学平衡式が得られます。
 不均一系平衡における化学平衡式
 2つ以上の異なる相の反応物と生成物を含む不均一系平衡の場合、以下の例で示すように、純粋な固体または純粋な液体の濃度は平衡定数の式には含まれません。
 <img alt="Eq3" src="/files/ftp_upload/11386/11386_Equation_3.png" style="text-align: center; height: 150px;" />
 これは、純粋な液体と純粋な固体の相対的な濃度が、反応中に一定であるためです。
 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/13-2-equilibrium-constants">Openstax, Chemistry 2e, Section 13.2 Equilibrium Constants</a>.

二酸化硫黄の酸化を考えてみましょう。

反応物の混合物のみで始まる反応の場合、生成物の濃度は最初は...

Education

JoVE Core

化学

Chemical Equilibrium

化学平衡

KEY TERMS AND CONCEPTS

主要用語と概念

chemical equilibrium and kc rules

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 in reverse toward equilibrium. The product concentration falls with time, and the reactant concentrations rise until both become constant. This shows that a chemical reaction will move in whatever direction is needed to reach equilibrium.

The law of mass action links equilibrium to a constant value. It states that the ratio of product concentrations to reactant concentrations, each raised to their stoichiometric coefficients, equals the equilibrium constant, K or Kc. The subscript c means the expression uses molar concentration. The size of K shows the mixture makeup at equilibrium. A large K means most reactant has changed to product before equilibrium is reached. A small K means only a little reactant changes to product. The value of K does not tell how fast equilibrium is reached.

Some equilibria form very quickly. Others take so long that no visible change may be seen for days, years, or longer. The equilibrium constant can still be used to predict how a mixture with reactants and/or products will behave. For coupled equilibria, two or more equilibrium reactions share one or more species. The K value for the overall system can be found from the K values of the separate reactions. Reversing a chemical equation changes K to its reciprocal. Multiplying the stoichiometric coefficients by a factor x changes K by that same factor as an exponent. Adding equilibrium equations gives an overall K equal to the product of the individual K values.

For heterogeneous equilibria, the reactants and products are in different phases. Pure solids and pure liquids are not included in the equilibrium constant expression. Their concentrations stay constant during the reaction, so they do not change the value of K. This rule is an important part of writing equilibrium expressions correctly.

Chemical Equilibrium and Kc Rules

the equilibrium constant

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...

homogeneous equilibria for gaseous reactions

homogeneous-equilibria-for-gaseous-reactions

Relating Kc and Kp in Gas Equilibria

Gas-phase equilibrium can be written using either concentration or pressure. The equilibrium constant may be shown as Kc, which uses molar concentration, or as Kp, which uses partial pressure of the reactants and products. The link between Kc and Kp comes from the ideal gas equation and the definition of molarity. Molarity is the number of moles divided by volume. In the ideal gas equation, P stands for partial pressure, V for volume, n for number of moles, R for the gas constant, and T for...

le chatelier's principle: changing concentration

le-chatelier-s-principle-changing-concentration

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...

calculating the equilibrium constant

Finding Kc from Equilibrium Data

The equilibrium constant, Kc, can be found from the equilibrium concentrations of reactants and products. When those concentrations are known, the calculation is straightforward. The values are placed into the Kc expression for the reaction. One example uses nitrogen dioxide and dinitrogen tetroxide. When 0.10 mol NO2 is added to a 1.0-L flask at 25 °C, the concentrations change until equilibrium is reached. At equilibrium, [NO2] is 0.016 M and [N2O4] is 0.042 M, and these values are used to...

reaction quotient

Using Q to Track Equilibrium Changes

Reaction quotient Q helps track how far a reversible reaction has moved toward equilibrium. For a balanced reaction such as m A + n B ⇌ x C + y D, Q comes directly from the stoichiometric coefficients in the equation. When concentrations are used, the expression is written as Qc. For gases, a similar form uses partial pressures. These expressions use relative concentration and pressure values, not absolute values. That means reaction quotients are dimensionless and have no units. As a reaction...

dynamic equilibrium

Chemical Equilibrium in Reversible Reactions

Chemical equilibrium in reversible reactions happens when a reaction moves in both directions at the same time. A reversible reaction uses a double arrow to show that products can turn back into reactants. At equilibrium, the concentrations of reactants and products stay constant over time because the forward and reverse reaction rates are equal. The video uses the reversible decomposition of colorless dinitrogen tetroxide to brown nitrogen dioxide as an example. At time zero, there is finite...

calculating equilibrium concentrations

Equilibrium Concentrations from an ICE Table

Equilibrium concentrations can be found from initial concentrations and an equilibrium constant. This skill matters in many areas of science and technology, including pharmaceutical product formulation and dosing. After a drug is ingested or injected, it may take part in several chemical equilibria that change its final concentration in the body system of interest. Scientists use these quantitative equilibrium relationships to help calculate a dosage that will produce the desired therapeutic...

le chatelier's principle: changing volume (pressure)

le-chatelier-s-principle-changing-volume-pressure

Gas Equilibrium Under Volume Changes

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-changing-temperature

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...

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

Explore chemical equilibrium, Kc, and the law of mass action. Learn how concentration changes, reaction direction, and mixed-phase equilibria shape equilibrium expressions.

Chemical Equilibrium and Kc Rules | JoVE K12

chemistry

JoVE Core 化学は、簡潔で理解しやすいアニメーションビデオレッスンを使用して、一般的な化学の概念を生き生きとしたものにします。さらに、活動する科学者のビデオでは、現在世界中の実験室で行われている古典的そして独創的な研究・実験をお見せします。

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.F

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Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Arizona Science Standards

High School

HS+C.P1

HS+C.P1U1.6

Next Generation Science Standards

Matter and Its Interactions

HS-PS1-6

Florida Department of Education

SC.912.P.12

SC.912.P.12.13

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South Carolina College and Career Ready Science Standards

C-PS1-6

new-york-state-science-learning-standards

New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-6.

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Chemical Equilibrium and Kc Rules

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