Name: redox reactions and half-reaction balancing
Uploaded: 2020-09-24T15:06:52.000Z
Duration: 2 min 58 s
Channel: JoVE

K12 Standards

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

NC K-12 Science Standard Course of Study

9-12

Physical Science

Matter and its Interactions

PS.PSc.2.3

Balancing Redox Equations

电池如何为 便携式设备供电？为什么切开的鳄梨会变成褐色，或金属在水下会 生锈？这些过程由特定类型的化学反应驱动，涉及到电子 从一个原子转移到另一个原子。失去电子的原子被氧化，而得到电子的原子被还原。这些反应称为氧化还原作用或氧化 还原反应，其特征是一种或多种反应物 的氧化态发生变化。镁和浓盐酸之间的相互作用 是氧化还原反应的一个例子。这里，镁被氧化成 2 价镁离子，质子被还原成氢气。简单的化学方程式很容易配平。然而，由于大多数氧化还原方程很复杂，失去和得到的电子数必须是合理的。在配平氧化还原方程时，必须遵守质量守恒定律。每种元素的数量以及任何电子的获得或失去 都必须在反应两边达到平衡；因此，如果一种反应物被还原，另一种反应物必然被氧化。半反应法用于成功配平氧化还原 方程。请考虑高锰酸盐与草酸盐的反应。对于酸性水溶液，应指定 氧化态，并把反应分成其组成的 半反应。然后，配平每个半反应，省略氢和氧。这里，锰已经达到平衡，但是草酸盐的系数需要为 2；其次，通过增加水分子来配平 氧原子；第三，在需要时 通过添加质子来配平氢原子。通过添加电子来配平电荷。由于锰被还原，反应物一侧 添加了五个电子。相反，草酸盐被氧化；因此，生成物这边添加了两个电子。将半反应乘以一个整数，这样添加进去的电子数就相等了。最后，通过抵消两边的物质 来添加和简化平衡的半反应，从而得到平衡的氧化还原反应。对于碱性水溶液，过程 类似，但包括一个额外步骤。这里，在将平衡的半反应 相加之前，在反应的两边都加入 相等数量的氢氧根离子 来中和质子。

平衡氧化还原方程

电化学是电气和化学反应相互转换的科学。 这种反应称为还原氧化或氧化还原反应。 这些重要反应是通过一个或多个反应物元素的氧化状态变化来定义的，其中包括一组反应，涉及反应物物种之间的电子传输。 电化学作为一个领域已发展到产率对氧化还原化学的基本原理有了充分的了解，并采用了多种技术，从工业规模的冶金工艺到用于电动车的坚固的可充电电池。 由于涉及电子传输的反应对电化学研究至关重要，因此对氧化还原化学的简要回顾包括以下内容。 
 氧化数值 
 根据定义，氧化还原反应涉及一个或多个反应物的氧化数值或氧化状态的变化。 元素的氧化数量是对原子的电子环境与纯原子的电子环境的差异的评估。 根据这一定义，元素中的原子的氧化值为零。 对于原子，如果化合物是离子，氧化数值等于化合物中原子的电荷。 因此，分子中所有原子的氧化数值之和等于该分子的电荷。 
 离子化合物 
 简单离子化合物是这种形式主义的最简单例子，因为元素的氧化值等于其离子电荷。 氯化钠 NaCl 由 Na+ 阳离子和 CL− 阴离子组成，钠和氯的氧化数值分别为 +1 和 −1。 氟化钙 CaF2 由 Ca2+ 阳离子和 F− 阴离子组成，钙和氟的氧化数值为 +2 和 −1。 
 共价化合物 
 共价化合物在使用形式主义方面更具挑战性。 水是一种共价化合物，由两个通过极性共价 O−H 键将键合原子与 O 原子组成。 制造 O−H 键的共享电子更受电子原子的强烈吸引。 因此，与元素氧中的 O 原子相比，氧会获得部分负电荷。 因此，与氢气中的氢原子相比，水分子中的 H 原子具有部分正电荷。 每个水分子的部分负荷和正荷总和为零，使水分子处于中性状态。 
 如果水的 O−H 键中共享电子的极化已完成—结果将是电子从 H 完全传输到 O ，而水将是由 O2− 阴离子和 H+ 阳离子组成的离子化合物。 因此，水中氧气和氢的氧化物分别为 −2 和 +1。 对四氯化碳 (CCl4) 应用同样的逻辑后，二氧化碳的氧化数值为 +4 ，氯的氧化数值为 −1。 在硝酸盐− 中，氮的氧化值为 +5 ，氧气的氧化值为 −2 ，等于分子上的 −1 电荷： 
 <img alt="Eq1" src="/files/ftp_upload/11432/11432_Equation_1.png" style="text-align: center; height: 70px;" />
 平衡氧化还原方程 
 下图所示的非平衡方程描述了氯化钠的分解： 
 <img alt="Eq2" src="/files/ftp_upload/11432/11432_Equation_2.png" style="text-align: center; height: 48px;" />
 这种反应符合氧化还原反应的标准，因为 Na 的氧化数值从 +1 下降到 0 (通过进行还原) ，而 CL 的氧化数值从 −1 上升到 0 (通过氧化)。 通过为 NaCl 和 Na 添加 2 的化学化学式化学式 (方程) 壳体，可以轻松实现平衡： 
 <img alt="Eq3" src="/files/ftp_upload/11432/11432_Equation_3.png" style="text-align: center; height: 48px;" />
 水溶液中发生的氧化还原反应通常发生在电化学中，许多反应涉及水或其离子 H+ (AQ) 和 OH− (AQ) ，可以是反应物或生成物。 
 在这种情况下，代表氧化还原反应的方程式很难仅仅通过检查来平衡，使用一种称为半反应方法的系统方法很有帮助。 这种方法包括以下步骤： 
 <ol>
 <li>将方程分解为其成分骨架氧化并减少半反应。 </li>
 <li>平衡 O 和 H 以外的所有元素的每个半反应 </li>
 <li>根据需要在方程中添加水分子，平衡 O 原子的每半反应。 </li>
 <li>根据方程的要求添加质子，平衡 H 原子的每半反应。 </li>
 <li>最后，通过根据需要添加电子来平衡元素上的电荷。 </li>
 <li>将半反应乘以所需的任何整数，以使氧化半反应中丢失的电子数与半反应减少中获得的电子数相等。 </li>
 <li>通过消除方程两侧的常见物种，添加半反应并进一步简化。 </li>
 <li>如果反应发生在碱性介质中，请将 OH− 离子添加到步骤 7 中获得的方程中，以中和质子 (在方程两侧以相等数量添加) 并简化。 </li>
 <li>检查方程以确保原子上的电荷平衡。 </li>
 </ol>
 本文改编自 <a href="https://openstax.org/books/chemistry-2e/pages/17-1-review-of-redox-chemistry"> Openstax, Chemistry 2e, Chapter 17: Introduction ，</a> and <a href="https://openstax.org/books/chemistry-2e/pages/17-1-review-of-redox-chemistry"> Openstax, 化学 2e, 第 17.2 节：氧化还原化学的回顾</a>。

电化学是研究电能与化学反应之间相互转化的科学。这类反应被称为还原-氧化反应，或简称...

Education

教育

JoVE Core

Chemistry

化学

Electrochemistry

电化学

KEY TERMS AND CONCEPTS

关键术语与概念

redox reactions and half-reaction balancing

Redox reactions involve electron transfer and changes in oxidation number. In electrochemistry, these reactions connect chemical changes with electrical energy. They also power technologies such as industrial metal processing and rechargeable batteries used in electric vehicles.

Oxidation number is a way to track how an atom's electronic environment changes compared with the pure element, where the oxidation number is zero. For a simple ionic compound, the oxidation number matches the ion charge. Sodium chloride has Na at +1 and Cl at -1, and calcium fluoride has Ca at +2 and F at -1.

Covalent compounds need a closer look because electrons are shared. In water, the O-H bonds are polar, so oxygen pulls shared electrons more strongly than hydrogen does. That gives oxygen an oxidation number of -2 and hydrogen +1. The same idea gives carbon +4 and chlorine -1 in carbon tetrachloride, and nitrogen +5 with oxygen -2 in the nitrate ion, which adds up to the ion's -1 charge.

A simple redox example is the decomposition of sodium chloride. Sodium changes from +1 to 0, so it is reduced. Chlorine changes from -1 to 0, so it is oxidized. The equation is balanced by placing a coefficient of 2 in front of NaCl and Na.

Redox reactions in aqueous solution are often harder to balance because water, H+ ions, or OH- ions can appear as reactants or products. A systematic half-reaction method helps. First, split the equation into oxidation and reduction half-reactions. Then balance all elements except O and H, add water for oxygen, add protons for hydrogen, and add electrons to balance charge. After that, multiply the half-reactions so the electrons lost and gained are equal, add them together, and cancel species that appear on both sides.

If the reaction occurs in an alkaline medium, add OH- ions to neutralize the protons and simplify the equation. Finish by checking that both atoms and charge are balanced. This method gives a clear path for balancing redox equations in electrochemistry.

Redox Reactions and Half-Reaction Balancing

redox-reactions-and-half-reaction-balancing

balancing redox equations

Redox reactions involve electron transfer and changes in oxidation number. In electrochemistry, these reactions connect chemical changes with electrical energy. They also power technologies such as industrial metal processing and rechargeable batteries used in electric vehicles. Oxidation number is a way to track how an atom's electronic environment changes compared with the pure element, where the oxidation number is zero. For a simple ionic compound, the oxidation number matches the ion...

electromotive force

Cell Voltage and Electron Flow

Electric current is the flow of electric charge through a conducting medium or a solution. In wires, that charge is carried by electrons. In some solutions, ions can also move and carry charge. When electrons move through a wire, they produce an electric current. This idea connects to redox reactions, where electrons are lost and gained. In the spontaneous reaction between zinc and copper ions, zinc is oxidized to zinc ions. Copper ions are reduced to solid copper. Even so, this direct...

voltaic/galvanic cells

Galvanic Cells and Battery Reactions

Galvanic cells use spontaneous redox reactions to produce electric current. A common example is a disposable AA battery, which powers devices by turning chemical energy into electrical energy. Another example is a copper wire placed in silver nitrate solution, where the solution slowly turns bright blue and a grey solid forms on the wire. In the copper and silver reaction, copper is oxidized and silver ions are reduced. Copper forms cuprous ions, which give the solution its blue color. At the...

standard electrode potentials

Using Electrode Potentials to Predict Reactions

Electrode potentials help predict whether a redox reaction will happen on its own. A redox reaction involves electron transfer. In electrochemistry, the measured value is called cell potential or cell voltage. It shows the energy linked to charge transfer and is measured in volts, where 1 volt equals 1 joule per coulomb. A single half-cell cannot be measured by itself because electron transfer needs both a donor and a receiver. For that reason, scientists measure the difference between two...

cell potential and free energy

Redox Cell Potential, ΔG°, and K

Redox cell potential connects chemical energy, free energy change, and the equilibrium constant in an electrochemical cell. In these reactions, chemical energy is converted into electrical energy. That link helps predict whether a reaction is spontaneous and how far it may proceed. Thermodynamics is the study of how heat and other forms of energy are related. In an electrochemical cell, cell potential can also reflect oxidizing strength or reducing strength. A similar idea appears in acid-base...

the nernst equation

Cell Potential Under Changing Conditions

Cell potential changes when a redox reaction does not run at standard conditions. Standard cell potentials connect with standard free energy change, ΔG°, and the equilibrium constant K, but those relationships only hold at 1 M concentration, 298 K, and 1 atmosphere. A zinc(II) and tin(II) redox cell with E°cell = +0.291 V and ΔG° = −56.2 kJ is spontaneous under those standard conditions. As a cell discharges, the reactant concentration changes and the cell potential steadily drops. Many redox...

concentration cells

Cell Potential from Concentration Differences

Concentration cells are voltaic cells that use two almost identical half-cells. The half-cells use the same electrode and the same half-reaction. They differ only in the concentration of one redox species, so the cell potential depends on that concentration difference. In a concentration cell, the Nernst equation can be used to find the cell potential. For the zinc example shown, the calculated cell potential is positive. A positive cell potential means the overall reaction is spontaneous. <br...

batteries and fuel cells

Primary, Rechargeable Batteries and Fuel Cells

Batteries and fuel cells are galvanic cells that supply electrical power for many applications. They range from tiny button batteries in wristwatches to large systems that can provide backup energy to municipal power grids. Some are primary cells, which are single-use and cannot be recharged. Others are secondary cells, which use reversible reactions and can be charged again by an external power source. A common primary battery is the dry cell. It uses zinc as both the container and the anode,...

corrosion

Rusting and Metal Protection

Corrosion is the natural breakdown of metals through electrochemical reactions. It can appear as rust on iron, tarnish on silver, or a blue-green patina on copper. In each case, the metal is oxidized. Some corrosion products can protect the surface, while others keep damaging the metal. Rusting is the harmful type of corrosion seen in iron. It happens when iron is exposed to oxygen and water. A galvanic cell forms on the iron surface, and this drives oxidation at the anodic region and...

electrolysis

Electrolysis in Batteries and Metal Production

Electrolysis uses electrical energy to drive a nonspontaneous redox reaction. In a galvanic cell, a spontaneous redox reaction does electrical work on the surroundings by sending electrons through an external circuit. In electrolysis, an external circuit does work on the redox system by applying a voltage large enough to force the reaction in the opposite direction. This process is used in several real-world settings. Recharging a battery is one example, because an outside power source drives...

key terms and concepts

Learn how oxidation numbers, redox changes, and the half-reaction method help balance redox equations in aqueous and alkaline solutions.

Redox Reactions and Half-Reaction Balancing | JoVE K12

chemistry

JoVE Core 化学通过简单明了且通俗易懂的动画视频课程，将普通化学概念带入到现实生活当中。此外，在科学家行动视频中，我们演示了在实验室中进行的经典和原始研究实验。

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.8

IPC.8.B

Tennessee Academic Standards

Matter and Its Interactions

CHEM1.PS1.2

Virginia  Standards of Learning

HIgh School

PS.3

PS.3.d

CHEM.9

CHEM.9.A

Alabama Course of Study Science

Matter and Its Interactions: Energy

9-12.C.3.3.a

9-12.PS.3.6.d

Next Generation Science Standards

HS-PS1-2

HS-PS1-7

Florida Department of Education

SC.912.P.8

SC.912.P.8.10

Georgia Standards of Excellence

SC3.

balancing-chemical-equations-and-reaction-outcomes

Balancing Chemical Equations and Reaction Outcomes

Classifying Matter by State

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Classifying Matter by Composition

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Elements: Chemical Symbols and Isotopes

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Molecules and Compounds

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Solution Formation

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Colloids

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Construct an explanation to classify matter as a pure substance or mixture; homogeneous or heterogeneous; element or compound; solution, colloid or suspension.

PS.PSc.1.1

Phase Transitions

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Phase Transitions: Vaporization and Condensation

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Vapor Pressure

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Clausius-Clapeyron Equation

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Phase Transitions: Melting and Freezing

Heating a crystalline solid increases the average energy of its atoms, molecules, or ions, and the solid gets hotter. At some point, the added energy ...

phase-transitions-sublimation-and-deposition

Phase Transitions: Sublimation and Deposition

Some solids can transition directly into the gaseous state, bypassing the liquid state, via a process known as sublimation. At room temperature and ...

Heating and Cooling Curves

When a substance&#8212;isolated from its environment&#8212;is subjected to heat changes, corresponding changes in temperature and phase of the substance ...

Phase Diagrams

A phase diagram combines plots of pressure versus temperature for the liquid-gas, solid-liquid, and solid-gas phase-transition equilibria of a substance. ...

Use models to compare the phases of matter and the physical changes they undergo.

PS.PSc.1.2

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Physical and Chemical Properties of Matter

The characteristics that enable us to distinguish one substance from another are called properties.

Physical Properties of Matter

A physical property is ...

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Molecular Comparison of Gases, Liquids, and Solids

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Intermolecular vs Intramolecular Forces

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Intermolecular Forces

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comparing-intermolecular-forces-melting-point-boiling-point

Comparing Intermolecular Forces: Melting Point, Boiling Point, and Miscibility

Intermolecular forces are attractive forces that exist between molecules. They dictate several bulk properties, such as melting points, boiling points, ...

Carry out investigations to compare physical and chemical properties of matter.

PS.PSc.1.3

The Atomic Theory of Matter

The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers. Leucippus and Democritus argued that all matter ...

Subatomic Particles

Dalton was only partially correct about the particles that make up matter. All matter is composed of atoms, and atoms are composed of three smaller ...

The Bohr Model

Following the work of Ernest Rutherford and his colleagues in the early twentieth century, the picture of atoms consisting of tiny dense nuclei surrounded ...

Emission Spectra

When solids, liquids, or condensed gases are heated sufficiently, they radiate some of the excess energy as light. Photons produced in this manner have a ...

The de Broglie Wavelength

In the macroscopic world, objects that are large enough to be seen by the naked eye follow the rules of classical physics. A billiard ball moving on a ...

The Uncertainty Principle

Werner Heisenberg considered the limits of how accurately one can measure properties of an electron or other microscopic particles. He determined that ...

Use models to interpret the data presented in Bohr diagrams and electron dot diagrams for neutral atoms of elements 1 through 18.

PS.PSc.1.4

Ions and Ionic Charges

In ordinary chemical reactions, the nucleus &mdash; which contains the protons and neutrons of each atom and thus identifies the element &mdash; remains ...

Use models to compare representations of atoms, ions, and isotopes.

PS.PSc.1.5

The Periodic Table

As early chemists discovered more elements, they realized that various elements could be grouped by their similar chemical behaviors. One such grouping ...

Periodic Classification of the Elements

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their ...

atomic-radii-and-effective-nuclear-charge

Atomic Radii and Effective Nuclear Charge

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number ...

Ionic Radii

Ionic radius is the measure used to describe the size of an ion. A cation always has fewer electrons and the same number of protons as the parent atom; it ...

Ionization Energy

The amount of energy required to remove the most loosely bound electron from a gaseous atom in its ground state is called its first ionization energy ...

Electron Affinity

The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



This process can be either ...

Alkali Metals

Group 1 elements are soft and shiny metallic solids. They are malleable, ductile, and good conductors of heat and electricity. The melting points of the ...

Halogens

Group 17 elements, known as halogens, are nonmetals. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine a solid. ...

Noble Gases


The elements in group 18 are noble gases (helium, neon, argon, krypton, xenon, and radon). They earned the name &ldquo;noble&rdquo; because they were ...

Use the Periodic Table as a model to predict the relative properties (metallic/nonmetallic character, ionic charge, and reactivity) and arrangement of elements based on the pattern of valence electrons in the outermost energy levels of atoms.

PS.PSc.1.6

Types of Chemical Bonds

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Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence ...

Covalent Bonding and Lewis Structures

Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual ...

Bonding in Metals

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Construct an explanation to classify the type of chemical bond that occurs (covalent, ionic, or metallic) in a given substance.

PS.PSc.2.1

Nomenclature of Alkanes

In the late 19th-century, the number of new chemical compounds discovered increased tremendously. Hence, the necessity arose to develop a naming system ...

Nomenclature of Alkenes

The IUPAC naming system for alkenes replaces -an- with -en- in the corresponding parent alkanes. Accordingly, a simple alkene replaces the -ane suffix of ...

Nomenclature of Alkynes

Alkynes are unsaturated hydrocarbons characterized by the presence of carbon-carbon triple bonds and have a general formula CnH2n-2. The nomenclature of ...

structure-and-nomenclature-of-alcohols-and-phenols

Structure and Nomenclature of Alcohols and Phenols

Overview 
Alcohols are one of the most important functional groups in organic chemistry. The name of alcohol comes from the hydrocarbon from which it is ...

Structure and Nomenclature of Ethers

Structure and Bonding
Ethers are organic compounds with an ether functional group which is characterized by an oxygen atom connected to two &#8212; ...

IUPAC Nomenclature of Aldehydes

Aldehydes are named based on the systematic nomenclature rules set by the IUPAC. For acyclic aldehydes, the longest carbon chain containing the aldehydic ...

IUPAC Nomenclature of Ketones

Like aldehydes, ketones are named using IUPAC rules; in this case, by replacing &#8220;e&#8221; in the name of the longest hydrocarbon chain with ...

IUPAC Nomenclature of Carboxylic Acids

IUPAC names of carboxylic acids are systematically derived following a few rules discussed below.
For acyclic saturated monocarboxylic acids, the longest ...

Use models to apply International Union of Pure and Applied Chemistry (IUPAC) conventions to name and write formulas for simple compounds.

PS.PSc.2.2

Understand types, properties, and structure of matter.

Redox Reactions and Half-Reaction Balancing

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