Name: using q to track equilibrium changes
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 35 s
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Reaction Quotient

Выражение константы равновесия записывается в виде молярных концентраций продуктов C и D, деленных на концентрации исходных веществ A и B, в состоянии равновесия, каждая из которых возведена в степень их соответствующих стехиометрических коэффициентов. После решения выражение равно константе равновесия Kc.Выражение в той же форме может быть записано для исходных веществ и продуктов реакции при любой концентрации, а вычисленное количество известно как коэффициент реакции Qc.Как и Qc, выражение Qp может быть записано для газовых реакций с использованием парциальных давлений. В то время как K остается постоянным при определенной температуре независимо от концентрации, значение Q изменяется по мере того, как реакция протекает в направлении продуктов или исходных веществ.Коэффициент реакции можно использовать для определения направления, в котором реакция будет протекать для достижения равновесия. В начале данной реакции, если концентрация продуктов равна нулю, коэффициент реакции равен нулю. Когда концентрация исходных веществ в знаменателе высока, так что Q меньше K, реакция будет двигаться вправо в сторону продуктов, пока система не достигнет равновесия.Если концентрация исходных веществ равна нулю, коэффициент реакции бесконечен. Когда концентрация продуктов в числителе высока, так что Q больше, чем K, реакция будет двигаться влево в сторону исходных веществ. Если Q равно K, система находится в равновесии, и скорости прямой и обратной реакций равны.Рассмотрим данную реакцию с константой равновесия 50. Если реакционная смесь содержит 0, 20 моль водорода, 0, 20 моль йода и 1, 7 моль иодистого водорода, направление показанной реакции можно определить, вычислив Q.Подставляя данные концентрации в выражение, Q равно 72, что больше K.Следовательно, равновесие реакции сместится влево.

<ol>
	<li>Ледерер, Роберт. "Коэффициент реакции не нужен для решения проблем равновесия: нет проблем с Q." Журнал химического образования 82, No8 (2005): 1149. <a href="https://pubs.acs.org/doi/pdf/10.1021/ed082p1149.2">https://pubs.acs.org/doi/pdf/10.1021/ed082p1149.2</a></li>
</ol>

Коэффициент реакции

Статус обратимой реакции удобно оценивается путем вычисления ее коэффициента реакции (Q). Для обратимой реакции, описанной m A + n B ⇌ x C + y D, коэффициент реакции выводится непосредственно из стехиометрии сбалансированного уравнения AS 
 <img alt=""Eq1"" src=""/files/ftp_upload/11389/11389_Equation_1.png"" style=""text-align: center; height: 90px;"" />
 где индекс c обозначает использование молярных концентраций в выражении. Если реагенты и продукты газообразны, коэффициент реакции может быть аналогичным образом получен с использованием парциального давления: 
 <img alt=""Eq2"" src=""/files/ftp_upload/11389/11389_Equation_2.png"" style=""text-align: center; height: 90px;"" />
 Обратите внимание, что приведенные выше уравнения реактивного коэффициента являются упрощением более строгих выражения, в которых используются относительные значения для концентраций и давлений, а не абсолютные значения. Эти относительные значения концентрации и давления не имеют размеров (они не имеют единиц), следовательно, так же, как и реактивные показатели. 
 Численное значение Q изменяется по мере того, как реакция идет к равновесию; поэтому оно может служить полезным индикатором состояния реакции. Чтобы проиллюстрировать этот момент, рассмотрите окисление диоксида серы: 
 <img alt=""Eq3"" src=""/files/ftp_upload/11389/11389_Equation_3.png"" style=""text-align: center; height: 50px;"" />
 Здесь возможны два различных экспериментальных сценария, один из которых инициируется с использованием смеси только реагенты, SO2 и O2, а другой, который начинается только с продукта SO3. Для реакции, начинающейся только с смеси реагенты, Q изначально равен нулю: 
 <img alt=""Eq4"" src=""/files/ftp_upload/11389/11389_Equation_4.png"" style=""text-align: center; height: 90px;"" />
 По мере того как реакция движется к равновесию в прямом направлении, концентрации реагент уменьшаются (как и знаменатель КК), концентрация продукта увеличивается (как и числитель КК), и соответственно возрастает коэффициент реакции. При достижении равновесия концентрации реагенты и продукта остаются неизменными, как и значение КК. 
 Если реакция начинается с наличия только продукта, значение КК изначально не определено (неизмеримо большое или бесконечное): 
 <img alt=""Eq5"" src=""/files/ftp_upload/11389/11389_Equation_5.png"" style=""text-align: center; height: 90px;"" />
 В этом случае реакция идет на равновесие в обратном направлении. Концентрация продукта и числитель КК со временем уменьшаются, концентрации реагент и знаменатель КК увеличиваются, и соответственно коэффициент реакции уменьшается до тех пор, пока он не станет постоянным в равновесии. Постоянное значение Q, отображаемое системой в равновесии, называется константой равновесия, K: 
 <img alt=""Eq6"" src=""/files/ftp_upload/11389/11389_Equation_6.png"" style=""text-align: center; height: 50px;"" />
 Оценка коэффициента реакции 
 Газообразный диоксид азота образует тетраоксид динитрогена по этому уравнению: 
 <img alt=""Eq7"" src=""/files/ftp_upload/11389/11389_Equation_7.png"" style=""text-align: center; height: 50px;"" />
 При добавлении 0.10 моль NО2 к фласке объемом 1.0 л при температуре 25 °C концентрация изменяется таким образом, что при равновесии, [NО2] = 0.016 м и [N2O4] = 0.042 М. до формирования любого продукта, [NО2] = 0.10 м и [N2O4] = 0 М. таким образом, 
 <img alt=""Eq8"" src=""/files/ftp_upload/11389/11389_Equation_8.png"" style=""text-align: center; height: 90px;"" />
 При равновесии, 
 <img alt=""Eq9"" src=""/files/ftp_upload/11389/11389_Equation_9.png"" style=""text-align: center; height: 90px;"" />
 Этот текст был адаптирован из<a href=""https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants""> Openstax, Химия 2е изд., раздел 13.2 равновесные константы</a><a href=""https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants"">.</a>

Состояние обратимой реакции удобно оценивать, оценивая ее коэффициент реакции (Q). Для обратимой реакции, описываемой m A + n B ...

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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 moves toward equilibrium, the value of Q changes, so it gives a snapshot of the system’s status.

The oxidation of sulfur dioxide shows how Q changes over time. If a reaction starts with only reactants, SO2 and O2, then Q is initially zero. As the reaction moves forward, reactant concentrations drop, the denominator of Qc gets smaller, product concentration rises, and Qc increases.

If the reaction starts with only product, SO3, then Qc is initially undefined and can be treated as immeasurably large or infinite. In that case, the reaction moves in reverse toward equilibrium. Product concentration and the numerator of Qc decrease, while reactant concentrations and the denominator increase. Qc keeps dropping until it becomes constant at equilibrium.

The constant value of Q for a system at equilibrium is the equilibrium constant, K. A nitrogen dioxide example shows how this works in practice. When 0.10 mol NO2 is placed in a 1.0-L flask at 25 °C, the initial concentrations are [NO2] = 0.10 M and [N2O4] = 0 M, so Qc can be found from the starting amounts. At equilibrium, [NO2] = 0.016 M and [N2O4] = 0.042 M, and Qc is calculated again from those concentrations.

Using Q to Track Equilibrium Changes

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

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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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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

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)

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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...

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 the reaction quotient Q shows whether a reversible reaction is moving forward, reversing, or reaching equilibrium using concentration and pressure examples.

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Using Q to Track Equilibrium Changes

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