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 una reacción química equilibrada, como la combustión de gas hidrógeno. Aquí, la relación cuantitativa entre los reactivos y los productos (H2, O2 y H2O) es que dos moléculas de H2 reaccionan con una molécula de O2 para producir dos moléculas de H2O. Esta relación cuantitativa se conoce como estequiometría.
En una reacción química, los reactivos interactúan entre sí para formar productos. El reactivo que se consume completamente es el reactivo limitante, y el reactivo presente en una cantidad mayor de la necesaria para reaccionar completamente con el reactivo limitante es el reactivo excedente.
Una reacción química implica la transformación química de los reactivos en productos. A medida que avanza la reacción, la concentración de reactivos disminuye mientras que la concentración de productos aumenta. Esta variación en las concentraciones de reactivos y productos se puede representar en un gráfico en función del tiempo.
La velocidad a la que progresa una reacción se denomina velocidad de reacción. Mide la tasa de desaparición de reactivos o la tasa de aparición de productos y se expresa en unidades de molaridad por segundo.
Una reacción química reversible es aquella en la que la conversión de reactivos en productos y la conversión de productos de nuevo en reactivos ocurren simultáneamente. Una doble flecha entre los reactivos y los productos significa su naturaleza reversible.
Cuando la velocidad de formación de productos, o la velocidad de la reacción directa, es igual a la tasa de formación de reactivos, o la velocidad de la reacción inversa, se logra el equilibrio químico.
La expresión de la constante de equilibrio se escribe como las concentraciones molares de los productos, C y D, sobre los reactivos, A y B, en equilibrio, cada uno elevado a sus respectivos coeficientes estequiométricos. Cuando se resuelve, la expresión es igual a la constante de equilibrio, Kc.
También se puede escribir una expresión en la misma forma para los reactivos y productos a cualquier concentración, y la cantidad calculada se conoce como cociente de reacción, Qc.

reacciones químicas

Una ecuación química balanceada proporciona la información de las fórmulas químicas de los reactivos y productos involucrados en el cambio químico. La estequiometría de una reacción ayuda a predecir qué cantidad de reactivo se necesita para producir la cantidad deseada de producto o, en algunos casos, cuánto producto se formará a partir de una cantidad específica de reactivo.


 Las cantidades relativas de reactivos y productos representados en una ecuación química balanceada a menudo se denominan cantidades estequiométricas. Sin embargo, en realidad, los reactivos no siempre están presentes en las cantidades estequiométricas indicadas por la ecuación balanceada.


 En una reacción química, el reactivo que se consume primero y limita la cantidad de producto formado es el reactivo limitante, mientras que la otra sustancia se convierte en el reactivo en exceso. A menudo se utiliza un exceso de uno o más reactivos para asegurar la conversión completa del otro reactivo en el producto.


 Considere la reacción para la formación de agua representada por la ecuación:


 <img alt="Eq1" src="/files/ftp_upload/11691/11691_Equation1.svg" style="text-align: center; height: 20px;">


 La estequiometría indica que dos moles de hidrógeno y un mol de oxígeno reaccionan para producir dos moles de agua; es decir, el hidrógeno y el oxígeno se combinan en una proporción de 2:1.


 Imagínese si estuvieran presentes 5 moles de hidrógeno y 2 moles de oxígeno. La proporción de los reactivos es ahora 5:2 (o 2,5:1), que es mayor que la proporción estequiométrica de 2:1. Por lo tanto, el hidrógeno está presente en exceso y el oxígeno es el reactivo limitante. La reacción de todo el oxígeno proporcionado (2 moles) consumirá 4 moles de los 5 moles de hidrógeno proporcionados, dejando 1 mol de hidrógeno sin reaccionar. Calcular las cantidades molares de cada reactivo proporcionado y compararlas con las cantidades estequiométricas representadas en la ecuación química balanceada es una forma de identificar los reactivos limitantes y en exceso.


 La velocidad de reacción es el cambio en la cantidad de un reactivo o producto por unidad de tiempo. Por lo tanto, las velocidades de reacción se determinan midiendo la dependencia del tiempo de alguna propiedad que puede relacionarse con las cantidades de reactivo o producto.


 <img alt="Figure1" src="/files/ftp_upload/11691/11691_Figure1.png" style="text-align: center; height: 300px;">


 La velocidad de una reacción química se puede representar en un gráfico como la variación de las concentraciones de reactivos y productos en función del tiempo.


 Una reacción química reversible representa un proceso químico que se desarrolla tanto en dirección directa (de izquierda a derecha) como inversa (de derecha a izquierda). El estado de una reacción reversible se evalúa convenientemente evaluando su cociente de reacción (Q). Para una reacción reversible descrita por


 <img alt="Eq2" src="/files/ftp_upload/11691/11691_Equation2.svg" style="text-align: center; height: 20px;">


 el cociente de reacción se obtiene directamente de la estequiometría de la ecuación balanceada como


 <img alt="Eq3" src="/files/ftp_upload/11691/11691_Equation3.svg" style="text-align: center; height: 44px;">


 donde el subíndice c denota el uso de concentraciones molares en la expresión.


 <img alt="Figure2" src="/files/ftp_upload/11691/11691_Figure2.png" style="text-align: center; height: 300px;">


 Cuando las velocidades de las reacciones directa e inversa son iguales, las concentraciones de las especies del reactivo y del producto permanecen constantes en el tiempo y el sistema está en equilibrio. Se utiliza una doble flecha especial para enfatizar la naturaleza reversible de la reacción.

Reacciones químicas

Una ecuación química equilibrada proporciona la información de las fórmulas químicas de los reactivos y productos involucrados en el cambio químico. Un...

Education

Educación

JoVE Core

Organic Chemistry

Química Orgánica

Unit 1

Unidad 1

Thermodynamics and Chemical Kinetics

Termodinámica y cinética química

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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...

enthalpy and heat of reaction

Combustion Heat and Hydrocarbon Stability

Combustion is a burning reaction that releases energy as heat, light, or sound. It happens when a substance reacts with an oxidizing agent, usually molecular oxygen. The heat of combustion is also called the enthalpy of combustion. The energy released when one mole of a substance undergoes complete combustion at constant pressure is the molar heat of combustion. Combustion reactions are exothermic, so they give off energy. For that reason, the sign of ΔH is negative. In 1772, Antoine Lavoisier...

energetics of solution formation

Why Some Solutions Form Spontaneously

Solution formation can happen spontaneously under the right conditions. A spontaneous process is one that occurs without energy from an outside source. During dissolution, solute and solvent particles separate and then attract each other in a new way. The energy change depends on the balance of intermolecular forces, which are the attractions between particles. If the solute-solute and solvent-solvent forces are about the same as the new solute-solvent forces, the solution forms with no energy...

entropy and solvation

Hydrophobic Effects in Solvation

Solvation is the process of surrounding a solute with solvent. It spreads the solute evenly through the solvent. A useful rule is that like dissolves like. A good solvent has molecular features similar to the substance being dissolved. Polar solvents dissolve polar solutes, and apolar solvents dissolve apolar solutes. A polar solvent has a high dielectric constant, with ε greater than or equal to 15. An apolar solvent has a low dielectric constant. The dielectric constant is defined by the...

gibbs free energy and thermodynamic favorability

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Temperature Effects on Gibbs Free Energy

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...

chemical and solubility equilibria

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Solubility depends on free energy changes when a solute dissolves in a solvent. The free energy change for this process is called the free energy of solution, ΔGsolution. When ΔGsolution is less than zero, solution formation is favorable. When it is greater than zero, solution formation is unfavorable. For complete dissolution to happen, the Gibbs energy change must be negative. This means the balance of forces in the mixture must favor solution formation. The amount a solute dissolves in a...

rate law and reaction order

How Reactant Concentration Sets Reaction Rate

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...

effect of temperature change on reaction rate

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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...

multi-step reactions

Reaction Mechanisms and Rate-Determining Steps

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...

bond dissociation energy and activation energy

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How Bond Strength Shapes Reaction Speed

Bond strength and activation energy help explain why organic reactions happen. Bond energy is the energy needed to break a bond homolytically, which means each atom takes one of the shared electrons. When a bond value is given for one specific bond, it is called bond dissociation energy. When it is given for a bond type across many compounds, it is called average bond energy. Bond dissociation energy usually follows a clear pattern. A single bond is weaker than a double bond, and a double bond...

energy diagrams, transition states, and intermediates

energy-diagrams-transition-states-and-intermediates

Reaction Coordinate Diagrams in Chemical Reactions

Reaction coordinate diagrams show how energy changes during a chemical reaction. These free-energy graphs use the horizontal axis to show reaction progress. Points near the reactants look like the reactants, while points near the products look like the products. The peaks on the diagram represent stable structures with measurable lifetimes. Other points on the curve are unstable and cannot be isolated. The high-energy, unstable peak is called the transition state, or activated complex. At the...

predicting reaction outcomes

How Temperature Changes Reaction Rate

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 Orgánica explica los conceptos básicos de la estereoquímica, las reacciones y sus mecanismos mediante lecciones de vídeo animadas, concisas y fáciles de entender. Además, los vídeos de científicos en acción muestran la aplicación de esos conceptos en experimentos de investigación clásicos y originales, desarrollados actualmente en laboratorios de todo el 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

C-PS1-6

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Oxidation-Reduction Reactions

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Oxidation Numbers

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Acids, Bases and Neutralization Reactions

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Chemical Equilibria: Redefining Equilibrium Constant

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The extent ...

Factors Affecting Activity Coefficient

The activity coefficient of an ion depends on the ionic strength of the solution.
As indicated by the Debye&#8211;H&#252;ckel theory, when the ionic ...

Thermochemical Equations

For a chemical reaction (the system) carried out at constant pressure &ndash; with the only work done caused by expansion or contraction &ndash; the ...

Develop and use models to balance chemical equations and support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

Nuclear Fission

Many heavier elements with smaller binding energies per nucleon can decompose into more stable elements that have intermediate mass numbers and larger ...

Nuclear Power

Controlled nuclear fission reactions are used to generate electricity. Any nuclear reactor that produces power via the fission of uranium or plutonium by ...

Nuclear Fusion

The process of converting very light nuclei into heavier nuclei is also accompanied by the conversion of mass into large amounts of energy, a process ...

Nuclear Transmutation

Nuclear transmutation is the conversion of one nuclide into another. It can occur by the radioactive decay of a nucleus, or the reaction of a nucleus with ...

Biological Effects of Radiation

All radioactive nuclides emit high-energy particles or electromagnetic waves. When this radiation encounters living cells, it can cause heating, break ...

Research and communicate the uses, advantages, and disadvantages of nuclear reactions in current technologies.

IPC.8.C

What is Climate?

Climate refers to the prevailing weather conditions in a specific area over an extended period. As the saying goes, &#8220;Climate is what you expect. ...

Global Climate Change

Throughout its ~4.5 billion year history, the Earth has experienced periods of warming and cooling. However, the current drastic increase in global ...

Climate Change

The certainty of climate change remains a public controversy despite the consensus among approximately 97% of active climate researchers, who not only ...

Sustainable Development

As the human population continues to grow and use resources, we must be mindful of our planet&rsquo;s natural limits. Sustainable development provides a ...

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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