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

考虑一个平衡的化学反应，例如氢气的燃烧反应。在此反应中，反应物与生成物——H2、O2 和 H2O——之间的定量关系是：两个 H2 分子与一个 O2 分子反应生成两个 H2O 分子。这种定量关系称为化学计量学。
在化学反应中，反应物相互作用生成产物。完全被消耗的反应物称为限量反应物，而相对于限量反应物过量存在的反应物则称为过量反应物。
化学反应涉及反应物向生成物的化学转化。随着反应的进行，反应物的浓度降低，而生成物的浓度升高。反应物和生成物浓度的这种变化可以用浓度随时间变化的曲线图来表示。
反应进行的快慢称为反应速率。它衡量反应物消失的速率或产物生成的速率，单位为摩尔每秒（molarity-per-second）。
可逆化学反应是指反应物转化为产物，同时产物也能重新转化为反应物的化学反应。反应物与产物之间用双箭头表示其可逆性。
当产物的生成速率（即正反应速率）等于反应物的生成速率（即逆反应速率）时，反应达到化学平衡。
平衡常数表达式写作生成物 C 和 D 的摩尔浓度除以反应物 A 和 B 的摩尔浓度，所有浓度均为平衡时的数值，并各自升高到其相应的化学计量系数次幂。当计算该表达式时，其值等于平衡常数 Kc。
相同形式的表达式也可用于表示任意浓度下的反应物和生成物，所计算出的量称为反应商，Qc。

化学反应

平衡化学方程式提供了参与化学变化的反应物和产物的化学式信息。 反应的化学计量有助于预测产生所需量的产物需要多少反应物，或者在某些情况下，预测特定量的反应物将形成多少产物。


 平衡化学方程式中表示的反应物和产物的相对量通常称为化学计量。 然而，实际上，反应物并不总是以平衡方程所示的化学计量存在。


 在化学反应中，首先被消耗并限制生成产物量的反应物是限量反应物，而其他物质则成为过量反应物。 通常使用过量的一种或多种反应物来确保另一种反应物完全转化为产物。


 考虑由以下方程式表示的水形成反应:


 <img alt="Eq1" src="/files/ftp_upload/11691/11691_Equation1.svg" style="text-align: center; height: 20px;">


 化学计量表示两摩尔氢和一摩尔氧反应生成两摩尔水; 也就是说，氢和氧以 2:1 的比例结合。


 想象一下如果存在 5 摩尔氢和 2 摩尔氧。 现在反应物的比例为 5:2（或 2.5:1），大于 2:1 的化学计量比。 因此，氢气过量存在，而氧气是限制反应物。 所有提供的氧气（2 摩尔）的反应将消耗所提供的 5 摩尔氢气中的 4 摩尔，留下 1 摩尔未反应的氢气。 计算所提供的每种反应物的摩尔量并将它们与平衡化学方程式中表示的化学计量量进行比较是识别限制和过量反应物的一种方法。


 反应速率是单位时间内反应物或产物量的变化。 因此，反应速率是通过测量与反应物或产物量相关的某些性质的时间依赖性来确定的。


 <img alt="Figure1" src="/files/ftp_upload/11691/11691_Figure1.png" style="text-align: center; height: 300px;">


 化学反应的速率可以绘制在图表中，即反应物和产物的浓度随时间的变化。


 可逆化学反应表示沿正向（从左到右）和反向（从右到左）方向进行的化学过程。 可逆反应的状态可以通过评估其反应商 (Q) 来方便地评估。 对于以下描述的可逆反应，


 <img alt="Eq2" src="/files/ftp_upload/11691/11691_Equation2.svg" style="text-align: center; height: 20px;">


 反应商直接从平衡方程的化学计量中得出:


 <img alt="Eq3" src="/files/ftp_upload/11691/11691_Equation3.svg" style="text-align: center; height: 44px;">


 其中下标 c 表示表达式中使用的摩尔浓度。


 <img alt="Figure2" src="/files/ftp_upload/11691/11691_Figure2.png" style="text-align: center; height: 300px;">


 当正向反应和逆向反应的速率相等时，反应物和产物物质的浓度随时间保持恒定，系统处于平衡状态。 使用特殊的双箭头来强调反应的可逆性质。


本文改编自 <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>

平衡的化学方程式提供了参与化学变化的反应物和生成物的化学式信息。A ...

Education

教育

JoVE Core

Organic Chemistry

有机化学

Unit 1

第一单元

Thermodynamics and Chemical Kinetics

热力学与化学动力学

KEY TERMS AND CONCEPTS

关键术语与概念

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

gibbs-free-energy-and-thermodynamic-favorability

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

How Solubility Depends on Free Energy

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

effect-of-temperature-change-on-reaction-rate

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 有机化学（Organic Chemistry）使用简明易懂的动画视频课程解释立体化学、反应和机制的基本概念。 此外，科学家在行动视频展示了相关概念在当今世界各地实验室进行的经典和原创研究实验中的应用。

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

A balanced chemical equation provides the information of chemical formulas of the reactants and products involved in the chemical change. A ...

Enthalpy and Heat of Reaction

Combustion, commonly known as burning, is a reaction in which a substance reacts with an oxidizing agent, which in most cases is molecular oxygen, to ...

Oxidation-Reduction Reactions

Oxidation&ndash;Reduction Reactions

Earth&rsquo;s atmosphere contains about 20% molecular oxygen, O2, a chemically reactive gas that plays an essential ...

Oxidation Numbers

In redox reactions, the transfer of electrons occurs between reacting species. Electron transfer is described by a hypothetical number called the ...

Acids, Bases and Neutralization Reactions

An acid-base reaction is one in which a hydrogen ion, H+, is transferred from one chemical species to another. Such reactions are of central importance to ...

combustion-energy-a-measure-of-stability-in-alkanes-and-cycloalkanes

Combustion Energy: A Measure of Stability in Alkanes and Cycloalkanes

The low reactivity in alkanes can be attributed to the non-polar nature of C&#8211;C and C&#8211;H &#963; bonds. Alkanes, therefore, were&#160; initially ...

Investigate how changes in properties are indicative of chemical reactions such as hydrochloric acid with a metal, oxidation of metal, combustion, and neutralizing an acid with a base.

Balancing Redox Equations

Electrochemistry is the science involved in the interconversion of electrical and chemical reactions. Such reactions are called reduction-oxidation, or ...

chemical-equilibria-redefining-equilibrium-constant

Chemical Equilibria: Redefining Equilibrium Constant

Recall that the solubility of a sparingly soluble salt increases with the addition of an inert salt in a phenomenon known as the salt effect.
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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