Name: shifting equilibrium by changing concentration
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 27 s
Channel: JoVE

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.2

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Le Chatelier's Principle: Changing Concentration

当正向反应速率等于 逆向反应速率时，系统处于动态平衡。尽管反应仍在两个方向上进行，但反应物和产物的净浓度 保持不变。在这一点上，如果反应物或产物的浓度 发生变化，例如通过添加额外的分子 A 或去除一些分子 C，平衡就会受到干扰。这种变化是对系统的压力。根据勒沙特列原理，当一个处于平衡状态的体系由于浓度的 变化、体积或压力的变化 或温度的变化而受到压力时，平衡位置会朝着 使压力最小化和恢复平衡的方向移动。要了解一个体系对浓度变化的响应，请考虑一下两个相连的水箱 A 和 B。最初，两个水箱中的水位 相等，体系处于平衡状态。然后，向水箱 B 加水。这种压力提高了水箱 B 的水位，扰乱了平衡。为了抵消这种压力，一些水流入水箱 A 以恢复平衡。相反，如果水箱 B 中的水被排出，水位下降也会引发压力。因此，水会从水箱 A 流入水箱 B 以恢复平衡。对于一个处于平衡状态的体系，反应商 等于平衡常数。向平衡混合物中加入更多的反应物会降低 Q。根据勒沙特列原理，体系通过将平衡位置移向 产物以消耗添加的反应物 并重新建立平衡来最小化这种应力。同样地，从平衡混合物中除去一种产物 将导致类似的反应 以恢复平衡。相反地，添加更多的产物会增加 Q。因此，平衡位置向反应物移动，以消耗添加的产物并恢复平衡。浓度的变化会在不改变 K 值的情况下 移动平衡位置。

勒夏特列原理（le chatelier's principle） ：浓度变化

平衡系统处于动态平衡状态，正向和反向反应以相等的速率发生。 如果平衡系统的条件发生变化，对这些反应速度的影响也不同 (应力) ，则速度不再相等，系统也不处于平衡状态。 该系统随后将在更大的率值 (换档) 方向发生净反应，该转换将重新建立平衡。 勒夏特列原理（LeChatelier's Principle）概括了这一现象：如果强调平衡系统，该系统将经历一场转变，以应对重新建立平衡的压力。 
 反应速率主要受到反应的速率定律所述的浓度和阿伦尼乌斯方程所述的温度的影响。 因此，浓度和温度的变化是可以改变平衡的两个应力。 
 浓度更改的影响 
 如果平衡系统受到反应物或生成物物种浓度变化的影响，正向或反向反应的率值将发生变化。 以平衡反应为例： 
 <img alt="Eq1" src="/files/ftp_upload/11391/11391_Equation 1v2.png" style="text-align: center; height: 48px;" />
 当该系统处于平衡状态时，正向和反向反应速率相等。 
 <img alt="Eq2" src="/files/ftp_upload/11391/11391_Equation 2.png" style="text-align: center; height: 50px;" />
 如果通过添加氮气或氧气来对系统进行压力，则浓度的增加会导致正向反应的率值增加，超过反向反应的反应物： 
 <img alt="Eq3" src="/files/ftp_upload/11391/11391_Equation 3.png" style="text-align: center; height: 50px;" />
 该系统将在前进方向发生临时净反应，以重新建立平衡 (平衡将向右转移)。 如果系统中的某些生成物 NO 被移除，则会产生相同的偏移，这会降低反向反应的率值，再次导致速率失衡。 
 同样的逻辑也可用于解释从删除反应物或将生成物添加到平衡系统中所产生的左移。 这两种应力都会导致反向反应的率值增加 
 <img alt="Eq4" src="/files/ftp_upload/11391/11391_Equation 4.png" style="text-align: center; height: 50px;" />
 以及反向的临时净反应，以重新建立平衡。 
 作为这种动力学解释的替代方法，浓度变化对平衡的影响可以从反应商的角度加以合理化。 当系统处于平衡状态时， 
 <img alt="Eq5" src="/files/ftp_upload/11391/11391_Equation 5.png" style="text-align: center; height: 70px;" />
 如果添加了反应物 (增加反应商的分母) 或删除了生成物 (减少分子) ，则 QC < KC 和平衡将向右移动。 请注意，诱发这种压力的三种不同方式会导致平衡混合物的组成发生三种不同的变化。 如果添加 N2 ，则右移将消耗 O2 ，并在重新建立平衡时产生 NO ，产生的混合物中 N2的浓度越大， O2 的浓度越少，越少。 如果添加了 O2 ，新的平衡混合物的 O2 浓度将更高， NO 浓度将更低，浓度 N2 浓度将更低。 最后，如果不去除，新的平衡混合物的氮气和氧气浓度将会更高，浓度值将会更低。 尽管成分存在这些差异，但平衡常数的值在应力结束后将与以前相同 (根据质量作用定律)。 对于涉及移除反应物或添加生成物的应力，也可以应用相同的逻辑，在这种情况下Qc > KC 平衡将向左移动。 
 本文改编自<a href="https://openstax.org/books/chemistry-2e/pages/13-3-shifting-equilibria-le-chateliers-principle"> Openstax, 化学 2e, 第13.3节： 平衡位移：勒夏特列原理（LeChatelier's Principle）</a>。

勒夏特列原理（Le Chatelier's Principle） ：浓度变化

处于平衡状态的系统具有动态平衡的特征，其正向反应与逆向反应以相等的速率进行。如果一个平衡系统受到扰动……

Education

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JoVE Core

化学

Chemical Equilibrium

化学平衡

KEY TERMS AND CONCEPTS

关键术语与概念

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 reactant or product can move the equilibrium in one direction or the other.

Adding a reactant, such as N2 or O2 in the example reaction, increases the forward reaction rate. The forward rate then becomes greater than the reverse rate, so the equilibrium shifts right. Removing the product NO has the same effect because it lowers the reverse reaction rate and creates the same imbalance.

The opposite changes cause a left shift. Removing reactant or adding product increases the reverse reaction rate compared with the forward rate. The system then moves in the reverse direction until equilibrium is reached again.

This shift can also be explained with the reaction quotient, Qc, and the equilibrium constant, Kc. When reactant is added, Qc becomes less than Kc. When product is removed, Qc also becomes less than Kc, so the equilibrium shifts right. If reactant is removed or product is added, then Qc is greater than Kc and the equilibrium shifts left.

Although the final composition changes, the equilibrium constant stays the same after the stress is removed and the system rebalances. If N2 is added, the new mixture has more N2 and NO and less O2. If O2 is added, the mixture has more O2 and NO and less N2. If NO is removed, the mixture ends with more N2 and O2 and less NO. The same idea applies to shifts caused by removing reactant or adding product.

Shifting Equilibrium by Changing Concentration

shifting-equilibrium-by-changing-concentration

the equilibrium constant

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...

homogeneous equilibria for gaseous reactions

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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

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...

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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...

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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)

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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

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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...

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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 changing concentration shifts chemical equilibrium, including why adding or removing reactants or products moves the reaction left or right.

Shifting Equilibrium by Changing Concentration | JoVE K12

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JoVE Core 化学通过简单明了且通俗易懂的动画视频课程，将普通化学概念带入到现实生活当中。此外，在科学家行动视频中，我们演示了在实验室中进行的经典和原始研究实验。

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Shifting Equilibrium by Changing Concentration

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