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

질량 작용의 법칙은 반응물과 생성물의 평형 농도를 연관시키기 위해 평형 상수를 도입합니다. A와 B가 반응물이고, C와 D가 생성물이며, 소문자 a, b, c, d가 각각의 화학양론계수인 균형 화학 방정식의 경우 평형 상수 표현식은 화학양론계수로 제곱한 생성물의 몰 농도를 화학양론계수로 제곱한 반응물들의 몰 농도로 나누어 얻어집니다. 평형 상수는 종종 K 또는 Kc 기호로 표현됩니다.

여기서 첨자 c는 몰 농도와 관련된 평형 상수를 나타냅니다. 몰 농도의 비율인 평형 상수는 단위가 없습니다. 평형 상수가 1보다 훨씬 크다는 것은 생성물의 농도인 분자가 반응물의 농도인 분모보다 크다는 것을 의미합니다.

이것은 또한 균형은 생성물에 있고 정반응을 선호한다는 것을 나타냅니다. 반대로, 평형 상수가 1보다 훨씬 작다는 것은 분모가 분자보다 크다는 것을 의미합니다. 이것은 평형이 반응물에서 이루어지며 역반응에 유리하다는 것을 의미합니다.

크기가 1과 같거나 가까운 평형 상수의 경우 반응물 및 생성물의 상대 농도는 거의 같습니다. 이는 정방향과 역방향 모두 반응이 선호되지 않음을 나타냅니다. 화학 방정식을 수정하면 평형 상수도 변경됩니다.

평형 상수 x가 있는 화학 반응이 반전되는 경우 역반응에 대한 평형 상수 방정식은 정반응의 역수입니다. 따라서, 새로운 평형 상수는 x분의 1이 됩니다. 계수에 다른 인자 n을 곱하면 평형 상수도 동일한 요인만큼 곱해집니다.

이제 새로운 평형 상수는 n을 제곱한 x로 됩니다. 두 개 이상의 개별 반응을 서로 합하면 전체 반응에 대한 평형 상수는 개별 평형 상수의 곱으로 됩니다. 따라서, 새로운 평형 상수는 y 곱하기 z로 됩니다.

불균일 평형의 경우 평형 상수 식에는 순수한 고체 또는 순수한 액체를 제외합니다. 예를 들어, 고체 질산암모늄을 기체 아산화질소와 수증기로 분해하기 위한 평형 상수식은 순수한 고체인 질산암모늄을 제외합니다. 이것은 순수한 고체나 액체의 상대적인 몰 농도가 반응하는 동안 일정하게 유지되기 때문입니다.

평형 상수

이산화황의 산화에 대해 생각해 보세요.

<img alt="Eq1" src="/files/ftp_upload/11386/11386_Equation_1.png" style="text-align: center; height: 50px;" />

반응물의 혼합물만으로 시작되는 반응의 경우, 생성물 농도는 초기에 0과 같습니다. 반응이 순방향으로 평형을 향해 진행됨에 따라 반응물 농도는 감소하고 생성물 농도는 증가합니다. 평형이 달성되면 반응물과 생성물의 농도는 일정하게 유지됩니다.

생성물만 존재하는 상태에서 반응이 시작되면 반응은 반대 방향으로 평형을 향해 진행됩니다. 생성물 농도는 시간이 지남에 따라 감소하고 반응물 농도는 농도가 평형 상태에서 일정해질 때까지 증가합니다.

질량 작용의 법칙은 생성물의 농도와 평형 상태의 반응물의 농도 비율을 각각의 화학량론적 계수로 올리면 평형 상수인 K 또는 K_c라고 불리는 상수와 같다고 명시합니다.

따라서 위 반응의 평형 상수 표현식은 다음과 같이 작성됩니다.

<img alt="Eq2" src="/files/ftp_upload/11386/11386_Equation_2.png" style="text-align: center; height: 100px;" />

여기서 아래 첨자 'c'는 평형 상수가 반응물과 생성물의 몰 농도를 고려함을 나타냅니다.

평형 상수의 크기는 평형 상태의 반응 혼합물의 조성을 명시적으로 반영합니다. 큰 K를 나타내는 반응은 대부분의 반응물이 생성물로 변환될 때 평형에 도달하는 반면, 작은 K는 반응물이 거의 변환되지 않은 후에 반응이 평형에 도달함을 나타냅니다. K의 크기는 평형에 얼마나 빨리 또는 느리게 도달하는지를 나타내지 않는다는 점을 명심하는 것이 중요합니다. 일부 평형은 거의 즉각적으로 이루어질 정도로 빠르게 확립되고, 다른 평형은 너무 느리게 형성되어 며칠, 몇 년 또는 그 이상 동안 눈에 띄는 변화가 관찰되지 않습니다. 반응의 평형 상수는 반응물 및/또는 생성물을 포함하는 혼합물의 거동을 예측하는 데 사용될 수 있습니다. 위에서 설명한 이산화황 산화 과정에서 입증된 것처럼 화학 반응은 평형을 달성하는 데 필요한 모든 방향으로 진행됩니다.

결합 평형

많은 평형 시스템에는 둘 이상의 결합 평형 반응이 포함되며, 이 반응은 공통적으로 하나 이상의 반응물 또는 생성물 종을 갖습니다. 결합평형을 포함하는 시스템의 K 값은 개별 반응의 K 값과 관련될 수 있습니다. 이 접근 방식에는 아래에 설명된 대로 세 가지 기본 조작이 포함됩니다.

<ol style="list-style-type: disc;">
	<li>화학 방정식의 방향을 바꾸면 본질적으로 "반응물"과 "생성물"의 동일성이 바뀌므로 역방정식의 평형 상수는 단순히 순방정식의 평형 상수와 같습니다.</li>
	<li>방정식의 화학량론적 계수를 일부 x 요소로 변경하면 동일한 요소로 평형 상수가 기하급수적으로 변경됩니다.</li>
	<li>두 개 이상의 평형 방정식을 함께 추가하면 평형 상수가 개별 반응 K 값의 수학적 곱인 전체 방정식이 생성됩니다.</li>
</ol>

이종 평형에 대한 평형 상수 표현

두 개 이상의 서로 다른 상의 반응물과 생성물을 포함하는 이종 평형의 경우, 다음 예에서 설명한 것처럼 순수한 고체 또는 순수한 액체의 농도는 평형 상수 표현식에 포함되지 않습니다.

<img alt="Eq3" src="/files/ftp_upload/11386/11386_Equation_3.png" style="text-align: center; height: 150px;" />

이는 순수한 액체와 순수한 고체의 상대 농도가 반응 중에 일정하게 유지되기 때문입니다.

이 문서는 에서 발췌되었습니다 <a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">Openstax, Chemistry 2e, Section 13.2 Equilibrium Constants</a>.

이산화황의 산화를 고려하십시오.

반응물의 혼합물로만 시작하는 반응의 경우, 생성물 농도는 초기에 다음과 같습니다 ...

Education

교육

JoVE Core

화학

하나

Chemical Equilibrium

화학 평형

KEY TERMS AND CONCEPTS

주요 용어 및 개념

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 화학은 간결하고 이해하기 쉬운 애니메이션을 통해 일반 화학 개념을 설명합니다. 또한, 과학자가 직접 고전실험과 연구실험을 진행하는 실사 영상을 제공합니다.

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.F

ohios-learning-standards-and-model-curriculum

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

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

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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The Lewis structure of a nitrite anion (NO2&minus;) may actually be drawn in two different ways, distinguished by the locations of the N-O and N=O ...

Formal Charges

In some cases, there are seemingly more than one valid Lewis structures for molecules and polyatomic ions. The concept of formal charges can be used to ...

Exceptions to the Octet Rule

Many covalent molecules have central atoms that do not have eight electrons in their Lewis structures. These molecules fall into three categories:


	...

Bond Energies and Bond Lengths

Stable molecules exist because covalent bonds hold the atoms together. The strength of a covalent bond is measured by the energy required to break it, ...

Analyze and interpret data to explain the mechanisms and properties of the two main types of intramolecular (ionic and covalent) bonds.

PS.Chm.3.1

molecular-comparison-of-gases-liquids-and-solids

Molecular Comparison of Gases, Liquids, and Solids

Particles in a solid are tightly packed together (fixed shape) and often arranged in a regular pattern; in a liquid, they are close together with no ...

Intermolecular vs Intramolecular Forces

Intermolecular forces (IMF) are electrostatic attractions arising from charge-charge interactions between molecules. The strength of the intermolecular ...

Intermolecular Forces

Atoms and molecules interact through bonds (or forces): intramolecular and intermolecular. The forces are electrostatic as they arise from interactions ...

comparing-intermolecular-forces-melting-point-boiling-point

Comparing Intermolecular Forces: Melting Point, Boiling Point, and Miscibility

Intermolecular forces are attractive forces that exist between molecules. They dictate several bulk properties, such as melting points, boiling points, ...

surface-tension-capillary-action-and-viscosity

Surface Tension, Capillary Action, and Viscosity

Surface Tension
The various IMFs between identical molecules of a substance are examples of cohesive forces. The molecules within a liquid are surrounded ...

Construct an explanation to summarize the influences intermolecular forces have on the properties of chemical compounds.

PS.Chm.3.2

Analyze the structure of atoms and isotopes.

Chemical Equilibrium and Kc Rules

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