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

K12 Standards

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

North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.2

Reaction Quotient

A expressão de equilíbrio constante é escrita como as concentrações molares dos produtos, C e D, sobre os reagentes, A e B, em equilíbrio, cada um elevado aos seus respetivos coeficientes estequiométricos. Quando resolvida, a expressão é igual ao equilíbrio constante, Kc.Da mesma forma uma expressão também pode ser escrita para os reagentes e produtos em qualquer concentração, e a quantidade calculada é conhecida como o quociente de reação, Qc.Tal como o Qc, a expressão Qp pode ser escrita para reações gasosas utilizando pressões parciais. Enquanto K permanece constante a uma temperatura específica independentemente da concentração, o valor de Q muda à medida que a reação prossegue para os produtos ou os reagentes.O quociente da reação pode ser utilizado para determinar a direção que a reação irá prosseguir para alcançar o equilíbrio. No início de uma dada reação, se a concentração dos produtos for zero, o quociente de reação é zero. Sempre que a concentração dos reagentes no denominador é elevado, de tal forma que Q é mais pequeno que K, a reação irá mover-se para a direita para sintetizar mais produtos até que o sistema atinja o equilíbrio.Se a concentração dos reagentes for zero, o quociente da reação é infinito. Sempre que a concentração dos produtos no numerador é elevada, de tal forma que Q é maior que K, a reação irá deslocar-se para a esquerda para produzir mais reagentes. Se Q for igual a K, o sistema está em equilíbrio, e a taxa das reações avançadas e inversas são iguais.Consideremos a reação dada com uma constante de equilíbrio de 50. Se a mistura da reação contém 0, 20 molar de hidrogénio, 0, 20 molar de iodo e 1, 7 molar de iodeto de hidrogénio, a direção da reação mostrada pode ser determinada calculando Q.Substituindo as Concentrações dadas na expressão, Q é igual a 72, que é maior do que K.Portanto, a reação irá deslocar-se para a esquerda.

<ol>
	<li>Lederer, Robert. "O quociente de reação é desnecessário para resolver problemas de equilíbrio: sem problemas com Q." Journal of Chemical Education 82, nº 8 (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>

quociente da reação

O estado de uma reação reversível é convenientemente avaliado através do seu quociente de reação (Q). Para uma reação reversível descrita por m A + n B ⇌ x C + y D, o quociente de reação é derivado diretamente da estequiometria da equação equilibrada como
<img alt="Eq1" src="/files/ftp_upload/11389/11389_Equation_1.png" style="text-align: center; height: 90px;" />
onde o subscrito c denota o uso de concentrações molares na expressão. Se os reagentes e os produtos forem gasosos, um quociente de reação pode ser derivado de forma semelhante utilizando pressões parciais:
<img alt="Eq2" src="/files/ftp_upload/11389/11389_Equation_2.png" style="text-align: center; height: 90px;" />
Observe que as equações de quociente de reação acima são uma simplificação de expressões mais rigorosas que usam valores relativos para concentrações e pressões em vez de valores absolutos. Estes valores relativos de concentração e pressão são não dimensionáveis (não têm unidades); consequentemente, também os quocientes de reação.
O valor numérico de Q varia à medida que uma reação prossegue em direção ao equilíbrio; portanto, pode servir como um indicador útil do estado da reação. Para ilustrar este ponto, considere a oxidação do dióxido de enxofre:
<img alt="Eq3" src="/files/ftp_upload/11389/11389_Equation_3.png" style="text-align: center; height: 50px;" />
Dois cenários experimentais diferentes são possíveis aqui, um em que essa reação é iniciada apenas com uma mistura de reagentes, SO2 e O2, e outro que começa apenas com produto, SO3. Para a reação que começa apenas com uma mistura de reagentes, Q é inicialmente igual a zero:
<img alt="Eq4" src="/files/ftp_upload/11389/11389_Equation_4.png" style="text-align: center; height: 90px;" />
À medida que a reação prossegue para o equilíbrio na direção direta, as concentrações de reagentes diminuem (tal como o denominador de Qc), a concentração do produto aumenta (tal como o numerador de Qc), e o quociente de reação consequentemente aumenta. Quando se atinge o equilíbrio, as concentrações de reagentes e produto permanecem constantes, assim como o valor de Qc.
Se a reação começar apenas com o produto presente, o valor de Qc é inicialmente indefinido (incomensuravelmente grande, ou infinito):
<img alt="Eq5" src="/files/ftp_upload/11389/11389_Equation_5.png" style="text-align: center; height: 90px;" />
Neste caso, a reação prossegue para o equilíbrio na direção inversa. A concentração do produto e o numerador de Qc diminuem com o tempo, as concentrações de reagentes e o denominador de Qc aumentam, e o quociente de reação consequentemente diminui, até que se torna constante em equilíbrio. O valor constante de Q exibido por um sistema em equilíbrio é chamado de constante de equilíbrio, K:
<img alt="Eq6" src="/files/ftp_upload/11389/11389_Equation_6.png" style="text-align: center; height: 50px;" />
Avaliação de um Quociente de Reação
O dióxido de nitrogénio gasoso forma tetróxido de dinitrogénio de acordo com esta equação:
<img alt="Eq7" src="/files/ftp_upload/11389/11389_Equation_7.png" style="text-align: center; height: 50px;" />
Quando se adiciona 0,10 mol de NO2 a um frasco de 1,0 L a 25 °C, a concentração muda de modo a que, em equilíbrio, [NO2] = 0,016 M e [N2O4] = 0,042 M. Antes da formação de qualquer produto, [NO2] = 0,10 M e [N2O4] = 0 M. Assim,
<img alt="Eq8" src="/files/ftp_upload/11389/11389_Equation_8.png" style="text-align: center; height: 90px;" />
Em equilíbrio,
<img alt="Eq9" src="/files/ftp_upload/11389/11389_Equation_9.png" style="text-align: center; height: 90px;" />
Este texto foi adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">Openstax, Chemistry 2e, Section 13.2 Equilibrium Constants</a><a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">.</a>

Quociente da Reação

O status de uma reação reversível é convenientemente avaliado avaliando seu quociente de reação (Q). Para uma reação reversível descrita por m A + n B ...

Education

JoVE Core

Química

Chemical Equilibrium

Equilíbrio Químico

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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

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

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

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

Using Q to Track Equilibrium Changes | JoVE K12

chemistry

JoVE Core - Química dá vida a conceitos gerais de química usando vídeos de animação fáceis de entender. Além disso, nos vídeos Cientistas em ação, demonstramos experimentos de pesquisa clássicos e originais realizados atualmente por cientistas em laboratórios em todo o mundo.

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.F

Louisiana Student Standards for Science

High School

Matter And Its Interactions

HS-PS1-6

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Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Arizona Science Standards

HS+C.P1

HS+C.P1U1.6

Next Generation Science Standards

Matter and Its Interactions

Florida Department of Education

SC.912.P.12

SC.912.P.12.13

new-york-state-science-learning-standards

New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-6.

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Using Q to Track Equilibrium Changes

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