Name: stoichiometry, rates, and equilibrium
Uploaded: 2023-04-30T18:28:32.000Z
Duration: 2 min 26 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.8

IPC.8.A

Chemical Reactions

Considere uma reação química equilibrada, como a combustão do gás hidrogênio. Aqui, a relação quantitativa entre os reagentes e produtos - H2, O2 e H2O - é que duas moléculas de H2 reagem com uma molécula de O2 para produzir duas moléculas de H2O. Essa relação quantitativa é conhecida como estequiometria.
Em uma reação química, os reagentes interagem uns com os outros para formar produtos. O reagente que é completamente consumido é o reagente limitante, e o reagente presente em uma quantidade maior do que o necessário para reagir completamente com o reagente limitante é o excesso de reagente.
Uma reação química envolve a transformação química de reagentes em produtos. À medida que a reação progride, a concentração de reagentes diminui enquanto a concentração de produtos aumenta. Essa variação nas concentrações de reagentes e produtos pode ser plotada em um gráfico em função do tempo.
A velocidade com que uma reação progride é chamada de taxa de reação. Ele mede a taxa de desaparecimento de reagentes ou a taxa de aparecimento de produtos e é expresso em unidades de molaridade por segundo.
Uma reação química reversível é aquela em que a conversão de reagentes em produtos e a conversão de produtos de volta em reagentes ocorrem simultaneamente. Uma seta dupla entre os reagentes e os produtos significa sua natureza reversível.
Quando a taxa de formação de produtos, ou a taxa da reação direta, é igual à taxa de formação de reagentes, ou a taxa da reação reversa, o equilíbrio químico é alcançado.
A expressão da constante de equilíbrio é escrita como as concentrações molares dos produtos, C e D, sobre os reagentes, A e B, em equilíbrio, cada um elevado aos seus respectivos coeficientes estequiométricos. Quando resolvida, a expressão é igual à constante de equilíbrio, Kc.
Uma expressão na mesma forma também pode ser escrita para os reagentes e produtos em qualquer concentração, e a quantidade calculada é conhecida como quociente de reação, Qc.

reações químicas

Uma equação química balanceada fornece informações sobre as fórmulas químicas dos reagentes e produtos envolvidos na mudança química. A estequiometria de uma reação ajuda a prever quanto do reagente é necessário para produzir a quantidade desejada de produto ou, em alguns casos, quanto produto será formado a partir de uma quantidade específica do reagente.

As quantidades relativas de reagentes e produtos representados em uma equação química balanceada são frequentemente chamadas de quantidades estequiométricas. Porém, na realidade, os reagentes nem sempre estão presentes nas quantidades estequiométricas indicadas pela equação balanceada.

 Em uma reação química, o reagente que é consumido primeiro e limita a quantidade de produto formado é o reagente limitante, enquanto a outra substância se torna o reagente em excesso. Um excesso de um ou mais reagentes é frequentemente utilizado para garantir a conversão completa do outro reagente em produto.

Considere a reação de formação de água representada pela equação:

<img alt="Eq1" src="/files/ftp_upload/11691/11691_Equation1.svg" style="text-align: center; height: 20px;">

A estequiometria indica que dois mols de hidrogênio e um mol de oxigênio reagem para produzir dois mols de água; isto é, hidrogênio e oxigênio se combinam em uma proporção de 2:1.

Imagine se 5 mols de hidrogênio e 2 mols de oxigênio estivessem presentes. A proporção dos reagentes é agora 5:2 (ou 2,5:1), que é maior que a proporção estequiométrica de 2:1. O hidrogênio, portanto, está presente em excesso e o oxigênio é o reagente limitante. A reação de todo o oxigênio fornecido (2 mols) consumirá 4 mols dos 5 mols de hidrogênio fornecidos, deixando 1 mol de hidrogênio sem reagir. Calcular as quantidades molares de cada reagente fornecido e compará-las com as quantidades estequiométricas representadas na equação química balanceada é uma forma de identificar os reagentes limitantes e em excesso.

A taxa de reação é a mudança na quantidade de um reagente ou produto por unidade de tempo. As taxas de reação são, portanto, determinadas medindo a dependência do tempo de alguma propriedade que pode estar relacionada às quantidades de reagentes ou produtos.

<img alt="Figure1" src="/files/ftp_upload/11691/11691_Figure1.png" style="text-align: center; height: 300px;">

A taxa de uma reação química pode ser representada em um gráfico como a variação nas concentrações de reagentes e produtos em função do tempo.

Uma reação química reversível representa um processo químico que ocorre tanto na direção direta (da esquerda para a direita) quanto na inversa (da direita para a esquerda). O estado de uma reação reversível é convenientemente avaliado através do seu quociente de reação (Q). Para uma reação reversível descrita por

<img alt="Eq2" src="/files/ftp_upload/11691/11691_Equation2.svg" style="text-align: center; height: 20px;">

o quociente de reação é derivado diretamente da estequiometria da equação balanceada como

<img alt="Eq3" src="/files/ftp_upload/11691/11691_Equation3.svg" style="text-align: center; height: 44px;">

onde o subscrito c denota o uso de concentrações molares na expressão.

<img alt="Figure2" src="/files/ftp_upload/11691/11691_Figure2.png" style="text-align: center; height: 300px;">

Quando as taxas das reações direta e inversa são iguais, as concentrações das espécies reagentes e produtos permanecem constantes ao longo do tempo e o sistema está em equilíbrio. Uma seta dupla especial é usada para enfatizar a natureza reversível da reação.

Este texto foi adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/4-3-reaction-stoichiometry">OpenStax Chemistry 2e, Section 4.3: Reaction Stoichiometry</a>; <a href="https://openstax.org/books/chemistry-2e/pages/4-4-reaction-yields">Section 4.4: Reaction Yield</a>; <a href="https://openstax.org/books/chemistry-2e/pages/12-1-chemical-reaction-rates#12163">Section 12.1: Chemical Reaction Rates</a>; <a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria">Section 13.1 Chemical Equilibria</a>, <a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">Section 13.2 Equilibrium Constants</a><a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">.</a>

Reações Químicas

Uma equação química balanceada fornece as informações das fórmulas químicas dos reagentes e produtos envolvidos na mudança química. Um...

Education

JoVE Core

Organic Chemistry

Química Orgânica

Unit 1

Unidade 1

Thermodynamics and Chemical Kinetics

Termodinâmica e Cinética Química

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

stoichiometry, rates, and equilibrium

Stoichiometry, reaction rates, and equilibrium explain how chemical reactions are measured and predicted. A balanced chemical equation shows the chemical formulas of the reactants and products in a chemical change. It also gives the relative amounts of each substance involved.

Reaction stoichiometry helps predict how much reactant is needed to make a desired amount of product. It can also show how much product forms from a given amount of reactant. The amounts shown by a balanced equation are called stoichiometric amounts.

In real reactions, reactants are not always present in the exact amounts shown by the equation. The limiting reactant is the substance that gets used up first. It limits how much product can form. The other substance is the excess reactant. Extra reactant is often added so the other reactant can be fully converted into product.

Water formation is a clear example. The balanced equation shows that 2 moles of hydrogen react with 1 mole of oxygen to make 2 moles of water. This is a 2:1 ratio. If 5 moles of hydrogen and 2 moles of oxygen are available, the ratio is 5:2, or 2.5:1. Hydrogen is then in excess, and oxygen is the limiting reactant. All 2 moles of oxygen use 4 moles of hydrogen, leaving 1 mole of hydrogen unreacted.

Reaction rate is the change in the amount of a reactant or product per unit time. Scientists find reaction rates by measuring how a property changes over time if that property can be linked to reactant or product amounts. A rate graph can show how reactant and product concentrations vary with time.

Some reactions are reversible, which means they can proceed in both the forward and reverse directions. The reaction quotient, Q, helps assess the status of a reversible reaction. It is based on the stoichiometry of the balanced equation, and the subscript c shows that molar concentrations are used in the expression. When the forward and reverse rates are equal, concentrations stay constant and the system is at equilibrium. A double arrow is used to show that the reaction is reversible.

Stoichiometry, Rates, and Equilibrium

chemical reactions

Stoichiometry, reaction rates, and equilibrium explain how chemical reactions are measured and predicted. A balanced chemical equation shows the chemical formulas of the reactants and products in a chemical change. It also gives the relative amounts of each substance involved. Reaction stoichiometry helps predict how much reactant is needed to make a desired amount of product. It can also show how much product forms from a given amount of reactant. The amounts shown by a balanced equation are...

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energetics of solution formation

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gibbs free energy and thermodynamic favorability

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Temperature changes can decide whether a process is spontaneous. This idea applies to phase changes and to some chemical reactions. The key link is Gibbs free energy, which connects free energy change with enthalpy and entropy. The sign of the Gibbs free energy change, or ΔG, depends on the signs of enthalpy change (ΔH) and entropy change (ΔS), and sometimes on the absolute temperature. Because temperature is measured in kelvin, it is always a positive value. That means there are four main...

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rate law and reaction order

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Reaction rate depends on reactant concentration, and rate laws show that relationship with a mathematical equation. A rate law, also called a differential rate law or rate equation, links the rate of a chemical reaction to the concentrations of its reactants. For a generic reaction, a A + b B  products, the rate law can be written as rate = k [A]^m [B]^n. Here, [A] and [B] are the molar concentrations of the reactants. The constant k is the rate constant, and it is specific to one reaction at...

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Arrhenius Equation in Reaction Speed

The Arrhenius equation connects activation energy, temperature, and the rate constant, k, for many chemical reactions. It helps explain why some reactions move slowly while others happen faster. In this equation, R is the ideal gas constant, with a value of 8.314 J/mol·K. T is the temperature in kelvin, Ea is the activation energy in joules per mole, e is the constant 2.7183, and A is the frequency factor. The frequency factor, A, describes how often reacting molecules collide and how well...

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Chemical reactions can happen in a sequence of steps. A balanced chemical equation shows the reactants and products, but it does not show how the reaction occurs at the molecular level. The reaction mechanism, or reaction path, gives that step-by-step detail. Each step in a mechanism is an elementary reaction. These elementary reactions happen in order, and together they add up to the balanced equation for the overall reaction. In a multistep mechanism, one step is often much slower than the...

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energy diagrams, transition states, and intermediates

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Reaction Coordinate Diagrams in Chemical Reactions

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Reaction rate and reaction pathway are key ideas in chemical kinetics. Thermodynamics, by contrast, focuses on the state of a system. It describes the properties, behavior, and components of reactants and products. It does not describe the path of the process or the speed of the reaction. Kinetics gives information on the atomic or molecular level, while thermodynamics focuses on the energetics of the reactants and products. Thermodynamics can show what may happen in a reaction, but it does...

key terms and concepts

Learn how balanced equations, limiting reactants, reaction rates, and equilibrium help predict product formation and track chemical change in high school chemistry.

Stoichiometry, Rates, and Equilibrium | JoVE K12

organic chemistry

JoVE Core - Química Orgânica explica conceitos básicos de estereoquímica, reações e mecanismos usando vídeos de animação concisos e fáceis de entender. Além disso, os vídeos de Cientistas em ação demonstram a aplicação de conceitos relacionados em experimentos de pesquisa clássicos e originais realizados nos laboratórios atuais em todo o mundo.

IPC.8.B

Louisiana Student Standards for Science

Chemistry

Matter And Its Interactions

HS-PS1-6

Tennessee Academic Standards

Matter and Its Interactions

CHEM1.PS1.3

CHEM1.PS1.4

Virginia  Standards of Learning

HIgh School

CH.3

CH.3.f

Florida Department of Education

Physical Science

SC.912.P.10

SC.912.P.10.6

SC.912.P.10.12

SC.912.P.12

SC.912.P.12.12

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

South Carolina College and Career Ready Science Standards

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IPC.8.C

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Construct and communicate an evidence-based explanation of the environmental impact of the end-products of chemical reactions such as those that may result in degradation of water, soil, air quality, and global climate change.

IPC.8.D

Science concepts. The student knows that changes in matter affect everyday life. The student is expected to:

Stoichiometry, Rates, and Equilibrium

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