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

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Physical Science

Matter and its Interactions

PS.PSc.2.3

Balancing Redox Equations

Как аккумулятор генерирует электричество для работы портативных устройств? Почему авокадо на срезе становится коричневым, а металл ржавеет под водой? Эти процессы вызваны определенными типами химических реакций, включающих перенос электрона от одного атома к другому.Атом, теряющий электроны, окисляется, а атом, получающий электроны, восстанавливается. Эти реакции называются окислительно-восстановительными или ОВР и характеризуются изменением степени окисления одного или нескольких исходных веществ. Взаимодействие между магнием и концентрированной соляной кислотой является примером окислительно-восстановительной реакции.Здесь магний окисляется до ионов магния 2, а протоны восстанавливаются до газообразного водорода. Простые химические уравнения легко сбалансировать. Однако, поскольку большинство уравнений ОВР сложны, количество потерянных и принятых электронов должно быть обосновано.При уравновешивании уравнений ОВР необходимо соблюдать закон сохранения массы. Количество каждого элемента и любой прирост или потеря электронов должны быть сбалансированы с обеих сторон реакции;таким образом, если одно исходное вещество восстанавливается, другое должно окисляться. Для успешного уравновешивания окислительно-восстановительных уравнений используется метод полуреакций.Рассмотрим реакцию перманганата с оксалатом. Для водных растворов кислот присвойте степени окисления и разделите реакцию на составляющие ее полуреакции. Затем сбалансируйте каждую полуреакцию, исключив водород и кислород.Здесь марганец уже сбалансирован, но оксалату нужен коэффициент два;во-вторых, необходимо уравновесить атомы кислорода с добавлением молекул воды;и в-третьих, необходимо уравновесить атомы водорода, добавляя протоны, где это необходимо. Уравновесим заряды, добавив электроны. Поскольку марганец восстанавливается, со стороны исходных веществ добавляются пять электронов.Наоборот, оксалат окисляется;таким образом, на стороне продукта добавляются два электрона. Умножьте полуреакции на целое число, чтобы количество добавленных электронов было равно. Наконец, добавьте и упростите сбалансированные полуреакции, исключив компоненты с обеих сторон, чтобы получить сбалансированную окислительно-восстановительную реакцию.Для основных водных растворов процедура аналогична, но включает один дополнительный шаг. Здесь равное количество ионов гидроксила добавляется к обеим сторонам реакции для нейтрализации протонов перед суммированием сбалансированных полуреакций.

Балансировка окислительно-восстановительных уравнений

Электрохимия — это наука, участвующее в преобразовании электрических и химических реакций. Такие реакции называются восстановительно-окислительной или окислительно-восстановительной реакцией. Эти важные реакции определяются изменениями в окислительных состояниях одного или нескольких элементов реагент и включают в себя подмножество реакций, связанных с передачей электронов между видами реагент. Электрохимия как область развития стала давать достаточно информации об основных принципах химии редокс и множественных технологиях, начиная от промышленных металлургических процессов и заканчивая надежными перезаряжаемыми батареями для электромобилей. Поскольку реакции, связанные с передачей электронов, имеют важное значение для изучения электрохимии, краткий обзор химии редокс включает следующее. 
 Окисление 
 По определению, окислительно-восстановительная реакция включает изменение либо окислительного числа, либо состояния окисления для одного или нескольких реагенты. Окислительный номер элемента — это оценка того, как электронная среда его атомов отличается от атомов чистого элемента. По этому определению атом в элементе имеет нулевое окисление. Для атома окислительный номер равен заряду атома в соединении, если соединение было ионным. Таким образом, сумма чисел окисления для всех атомов в молекуле равна заряду молекулы. 
 Ионные соединения 
 Простые ионные соединения являются простейшими примерами этого формализма, так как элементы имеют окислительные числа, равные их ионным зарядам. Хлорид натрия, NaCl, состоит из катионов Na+ и Cl- анионов, при этом окисление натрия и хлора составляет +1 и -1 соответственно. 1. Фторид кальция, CaF2, состоит из катионов Ca2+ и ионов F- с окислением кальция и фтора, +2, и -1. 
 Ковалентные соединения 
 Ковалентные соединения более сложны в использовании формализма. Вода представляет собой ковалентное соединение, состоящее из двух атомов H, связанных с атомом O через полярные ковалентные связи O-H. Общие электроны, делающие O-H связь, более сильно притягиваются к более электрикативному атому O. Таким образом, кислород приобретает частичный отрицательный заряд по сравнению с атомом O в элементном кислороде. В результате атомы H в молекуле воды демонстрируют частичный положительный заряд по сравнению с атомами водорода в газообразном водороде. Сумма частичных отрицательных и положительных зарядов для каждой молекулы воды равна нулю, что делает молекулу воды нейтральной. 
 Если поляризация общих электронов в молекунах O-H воды будет завершена, результатом будет полная передача электронов от H к O, а вода будет ионным соединением, состоящим из анионов O2 и H+ катионов. Таким образом, количество окислов кислорода и водорода в воде составляет 2 и 1, соответственно. Применение этой же логики к тетрахлорметану, CCl4, приводит к окислению +4 для углерода и -1 для хлора. В ионе нитратов NО3- окисление азота составляет +5, а для кислорода — -2, суммируя для молекулы заряд -2 1: 
 <img alt="Eq1" src="/files/ftp_upload/11432/11432_Equation_1.png" style="text-align: center; height: 70px;" />
 Балансирование редокс-уравнений 
 Несбалансированное уравнение, приведенное ниже, описывает разложение хлорида натрия: 
 <img alt="Eq2" src="/files/ftp_upload/11432/11432_Equation_2.png" style="text-align: center; height: 48px;" />
 Эта реакция соответствует критерию окислительно-восстановительной реакции, так как окисление Na уменьшается с +1 до 0 (за счет снижения) и что для CL увеличивается с 1 до 0 (за счет окисления). Случай уравнения легко сбалансирован путем добавления стехиометрического коэффициента 2 для NaCl и Na: 
 <img alt="Eq3" src="/files/ftp_upload/11432/11432_Equation_3.png" style="text-align: center; height: 48px;" />
 Окислительно-восстановительные реакции, возникающие в водных растворах, обычно встречаются в электрохимии, и многие из них включают воду или ее ионы, H+ (aq) и OH- (aq), как реагенты или продукты. 
 В этих случаях уравнения, представляющие окислительные реакции, могут быть очень сложными для балансировки только путем осмотра, и использование систематического подхода, известного как метод полуреакции, полезно. Этот подход включает в себя следующие шаги: 
 <ol>
 <li>Разделите уравнение на его компонент, окисление скелета и половинные реакции восстановления. </li>
 <li>Сбалансируйте каждую половинную реакцию для всех элементов, кроме O и H. </li>
 <li>Сбалансируйте каждую половинную реакцию атомов O, добавив молекулы воды по мере необходимости в уравнение. </li>
 <li>Сбалансируйте каждую половинную реакцию атомов H, добавив протоны в соответствии с требованиями уравнения. </li>
 <li>Наконец, сбалансируйте заряды на элементах, добавляя электроны по мере необходимости. </li>
 <li>Умножьте полуреакции на любое целое число, необходимое для выравнивания количества электронов, потерянных в окислительной полуреакции, на количество электронов, полученных в результате уменьшения полуреакции. </li>
 <li>Добавьте обе половинные реакции и упростите их, отменив общие виды с обеих сторон уравнения. </li>
 <li>Если реакция происходит в щелочной среде, добавьте ионы OH в уравнение, полученное на шаге 7, чтобы нейтрализовать протоны (добавленные равными числами к обеим сторонам уравнения) и упростить. </li>
 <li>Проверьте уравнение, чтобы убедиться, что заряды атомов сбалансированы. </li>
 </ol>
 Этот текст адаптирован к <a href="https://openstax.org/books/chemistry-2e/pages/17-1-review-of-redox-chemistry"> Openstax, Химия 2е изд., Chapter 17: Введение,</a> и <a href="https://openstax.org/books/chemistry-2e/pages/17-1-review-of-redox-chemistry"> Openstax, Химия 2е изд., Раздел 17.2: Обзор химического состава редокс</a>.

Электрохимия — это наука, изучающая взаимопревращение электрических и химических реакций. Такие реакции называются восстановительно-окислительными, или...

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KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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

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balancing-chemical-equations-and-reaction-outcomes

Balancing Chemical Equations and Reaction Outcomes

Classifying Matter by State

Chemistry is the study of matter and the changes it undergoes. Matter is anything that has mass and occupies space. Matter is all around us; the air, ...

Classifying Matter by Composition

Matter: Pure Substances and Mixtures
According to its composition, the matter can be classified into two broad categories &mdash; pure substances and ...

Elements: Chemical Symbols and Isotopes

A chemical symbol is an abbreviation used to indicate an element or an atom of an element. For example, the symbol for mercury is Hg. The same symbol is ...

Molecules and Compounds

Atoms and Molecules

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Solution Formation

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Colloids

Children at play often make suspensions such as mixtures of mud and water, flour and water, or a suspension of solid pigments in water known as tempera ...

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

Whether solid, liquid, or gas, a substance&#39;s state depends on the order and arrangement of its particles (atoms, molecules, or ions). Particles in the ...

phase-transitions-vaporization-and-condensation

Phase Transitions: Vaporization and Condensation

The physical form of a substance changes on changing its temperature. For example, raising the temperature of a liquid causes the liquid to vaporize ...

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

physical-and-chemical-properties-of-matter

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

Particles in a solid are tightly packed together (fixed shape) and often arranged in a regular pattern; in a liquid, they are close together with no ...

Intermolecular vs Intramolecular Forces

Intermolecular forces (IMF) are electrostatic attractions arising from charge-charge interactions between molecules. The strength of the intermolecular ...

Intermolecular Forces

Atoms and molecules interact through bonds (or forces): intramolecular and intermolecular. The forces are electrostatic as they arise from interactions ...

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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

Chemical bonding theories were pioneered by American chemist Gilbert&#xA0;N. Lewis. He developed a model called the Lewis model to explain the type and ...

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

Metallic bonds are formed between two metal atoms. A simplified model to describe metallic bonding has been developed by Paul Dr&#xFC;de called the ...

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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