Name: oxidation states in redox reactions
Uploaded: 2020-03-26T07:22:24.000Z
Duration: 4 min 56 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry

CHEM.9

CHEM.9.B

Redox Reactions

Окислительно-восстановительный потенциал, что является сокращением от восстановительно-окислительного процесса, представляет собой тип химической реакции, классифицируемой по суммарному переносу электронов. В этой реакции одна молекула теряет электроны, что называется окислением, а другая молекула приобретает электроны, что называется восстановлением.
Чтобы помочь вам различить эти два понятия, запомните фразу «OIL-RIG», которая расшифровывается как «окисление — это потеря, восстановление — это приобретение». Окисляемая молекула называется восстановителем, потому что она восстанавливает другой реагент. Точно так же восстанавливаемая молекула называется окислителем, потому что она окисляет другую молекулу.
Теперь, когда мы разобрались с терминологией, давайте рассмотрим пример окислительно-восстановительной реакции, образования минерального оксида магния. В ходе реакции каждый атом магния теряет два электрона. Таким образом, магний окисляется. Каждый атом кислорода получает два электрона; Таким образом, снижается содержание кислорода.
Однако не все реакции являются окислительно-восстановительными реакциями. Например, реакция оксида кальция с углекислым газом с образованием карбоната кальция не является окислительно-восстановительной реакцией. Итак, как мы можем идентифицировать окислительно-восстановительную реакцию?
Для этого мы отслеживаем число окисления каждого элемента по мере его перехода от реагента к продукту. Число окисления — это гипотетический заряд, который атом имел бы, если бы его связи с различными элементами были ионными, что означает, что электроны относятся к более электроотрицательному атому. Сумма чисел окисления в молекуле равна ее общему заряду.
Давайте вернемся к оксиду магния. Это нейтральное соединение, поэтому сумма чисел окисления магния и кислорода равна нулю. Магний может давать два электрона, поэтому его число окисления равно плюс два. Кислород может принимать два электрона, поэтому его число окисления равно минус двум.
А как насчет реакции? Чистые нейтральные элементарные соединения имеют нулевое число окисления. Таким образом, и магний, и кислород начинаются с нулевого числа окисления. Числа окисления магния и кислорода изменялись во время реакции, так что это окислительно-восстановительная реакция.
Теперь давайте посмотрим на реакцию карбоната кальция, которую мы наблюдали ранее. Оба реагента нейтральны, поэтому сумма чисел окисления для обоих соединений равна нулю. Как мы видели на примере оксида магния, кальций имеет окислительное число плюс два, а кислород минус два. Тогда углерод в молекуле углекислого газа имеет число окисления плюс четыре, а каждый кислород минус два.
А как насчет продукта? Кальций – это плюс два, а углерод плюс четыре, как и в реагентах. Каждый кислород имеет значение минус два, что в сумме составляет минус шесть, а чистое число окисления равно нулю. Поскольку ни одно из чисел окисления не изменилось, это не окислительно-восстановительная реакция.
Теперь давайте представим четыре типа окислительно-восстановительных реакций. Первый — это реакция одиночного смещения, при которой один атом вытесняет другой. Вы увидите это в термитной реакции, когда один металл восстанавливается, а другой металл окисляется.
Следующий тип – это реакция горения, которая происходит между топливом и окислителем с образованием окисленных продуктов и тепла. Вы видите это в лаборатории во время сгорания метана с кислородом при использовании горелки Бунзена.
Третья реакция — это реакция синтеза, в которой два реагента объединяются в один продукт, как при синтезе аммиака, где азот соединяется с водородом с образованием аммиака.
Наконец, четвертый тип — это реакция разложения, при которой реагент поглощает достаточно энергии, чтобы разорвать свои связи и образовать более мелкие соединения. Так происходит с фейерверками, где хлорат калия после нагревания разлагается на хлорид калия и кислород.
В этой лабораторной работе вы проведете и определите различные типы окислительно-восстановительных реакций, которые превращают твердую медь в оксид меди, а затем обратно в твердую медь.

Окислительно-восстановительные реакции

<h4>Окисление и восстановление</h4>
Некоторые химические реакции можно классифицировать как восстановительно-окислительные реакции, или окислительно-восстановительные реакции. Окисление — это процесс, при котором вещество, подобно атому или ионной молекуле, теряет один или несколько электронов, а восстановление — это процесс, при котором вещество приобретает один или несколько электронов.
<h4>Степени окисления</h4>
Каждый атом в молекуле имеет свою степень окисления или число окисления. Степень окисления описывает, насколько окислена молекула по отношению к ее свободной элементарной форме. Степень окисления выражается в заряде, который имел бы атом, если бы каждая из его связей с другими элементами была чисто ионной. Это означает, что электроны в связи относятся к более электроотрицательному атому. Степень окисления атома в его свободной элементарной форме определяется как 0.
Существует несколько правил, которые соблюдаются для определения степени окисления. Элементы группы I и группы II обычно имеют степени окисления +1 и +2 соответственно. Водород и кислород обычно имеют степень окисления +1 и -2 соответственно, а галогены обычно имеют степень окисления -1. Кроме того, степени окисления атомов в молекуле всегда складываются в заряд молекулы. Таким образом, часто можно вывести степень окисления атома, не перечисленного выше. Например, рассмотрим углекислый газ (CO2), который является нейтральной молекулой. Если каждая из двух молекул кислорода вносит свой вклад в -2, степень окисления углерода должна быть +4, чтобы нейтрализовать -4 из кислорода.
Для более общего подхода нарисуйте структуру молекулы Льюиса, определите связи между различными атомами и присвойте каждую связь более электроотрицательному атому. Затем подсчитайте количество электронов на каждом атоме, при этом каждая связь вносит два электрона. Вычтите количество электронов, которые в данный момент находятся на атоме, из стандартного числа валентных электронов для этого атома, чтобы получить число окисления.
Снова рассмотрим углекислый газ. Каждый кислород имеет две одинокие пары электронов и связан с центральным углеродом двойной связью. Кислород более электроотрицателен, чем углерод, поэтому каждая связь C=O, на которую приходится четыре электрона, относится к кислороду. Таким образом, каждому кислороду присвоено в общей сложности восемь электронов (четыре от одиноких пар и четыре от двойной связи), а углероду не присвоено ни одного. По умолчанию число валентных электронов для кислорода равно шести, поэтому число окисления для каждого кислорода равно 6 – 8 = -2. По умолчанию число валентных электронов для углерода равно четырем, поэтому число окисления для углерода равно 4 – 0 = +4.
<h4>Окислительно-восстановительные реакции</h4>
Не все химические реакции классифицируются как окислительно-восстановительные реакции. Окислительно-восстановительная реакция — это любая реакция, в которой происходит изменение степени окисления атома. Таким образом, чтобы проверить, является ли реакция окислительно-восстановительной реакцией, определите степени окисления каждого атома в реагентах и продуктах и обратите внимание на любые изменения.
Многие окислительно-восстановительные реакции включают перенос электронов непосредственно от одной молекулы или атома к другой. В этих реакциях, если молекула или атом получает электрон, другая молекула или атом должны потерять электрон. Один из простых способов запомнить определения окисления и восстановления – это фраза OIL-RIG, которая расшифровывается как: Oxidation Is Losing – Reduction Is Gaining.
Молекула, получающая электрон, восстанавливается, но она называется окислителем или окислителем, потому что она окисляет другую молекулу. Точно так же молекула, которая теряет электрон, окисляется, но она называется восстановителем или восстановителем, потому что она восстанавливает другую молекулу.
Существует четыре основных типа реакций, которые обычно включают в себя окислительно-восстановительные процессы.
<ol>
	<li style="font-size:17px">Реакция одиночного смещения: атом вытесняет другой атом, входящий в состав соединения, и заменяет его.</li>
	<li style="font-size:17px">Реакция горения: Соединение восстанавливается сильным окислителем, обычно газообразным кислородом. В результате реакций горения, происходящих между углеводородами и органическими соединениями, обычно образуются углекислый газ и вода.</li>
	<li style="font-size:17px">Реакция синтеза: Два реагента образуют один продукт.</li>
	<li style="font-size:17px">Реакция разложения: один реагент распадается на два или более продуктов.</li>
</ol>
<h4>Ссылки</h4>
1. Харрис, Д. С. (2015). Количественный химический анализ. Нью-Йорк, Нью-Йорк: W. H. Freeman and Company.

Окисление и восстановление
Некоторые химические реакции можно классифицировать как восстановительно-окислительные реакции, или окислительно-восстановительные реакции. Окисление – это процесс...

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JoVE Lab Manual

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oxidation states in redox reactions

Oxidation states help identify redox reactions. In a redox reaction, oxidation is the loss of one or more electrons, and reduction is the gain of one or more electrons. These reactions are often tracked by changes in oxidation state, also called oxidation number.

Each atom in a molecule has an oxidation state. This number shows how oxidized an atom is compared with its free elemental form. It is written as the charge an atom would have if every bond were purely ionic. In that model, bonding electrons are assigned to the more electronegative atom. An atom in its elemental form has an oxidation state of 0.

Several rules help determine oxidation states. Group I elements usually have +1, and Group II elements usually have +2. Hydrogen is typically +1, oxygen is typically -2, and halogens usually are -1. The oxidation states in a molecule or ion must add up to the total charge, so unknown values can often be found from the rest.

Carbon dioxide is a clear example. CO2 is neutral, so its total oxidation states must add to zero. Each oxygen is usually -2, so carbon must be +4 to balance the two oxygens. Another way to find oxidation numbers is to draw the Lewis structure, assign each bond to the more electronegative atom, and count electrons on each atom.

In CO2, oxygen is more electronegative than carbon, so each C=O bond is assigned to oxygen. Each oxygen gets eight electrons total, including lone pairs and bonding electrons. Oxygen has six valence electrons by default, so its oxidation number is 6 - 8 = -2. Carbon has four valence electrons by default and is assigned none, so its oxidation number is 4 - 0 = +4.

A reaction is a redox reaction when an atom’s oxidation state changes from reactants to products. To check for redox, compare oxidation states on both sides of the reaction. Many redox reactions also involve direct electron transfer between atoms or molecules. If one species gains an electron, another must lose one.

The phrase OIL-RIG helps remember the process: oxidation is losing, and reduction is gaining. The species that gains electrons is reduced, but it is also called the oxidizing agent or oxidant because it oxidizes the other species. The species that loses electrons is oxidized, but it is also called the reducing agent or reductant because it reduces the other species.

Four major reaction types often involve redox processes. In single displacement reactions, one atom replaces another atom in a compound. In combustion reactions, a compound is reduced by a strong oxidant, usually oxygen gas, and hydrocarbons or other organic compounds often produce carbon dioxide and water. In synthesis reactions, two reactants form one product. In decomposition reactions, one reactant breaks into two or more products.

Oxidation States in Redox Reactions

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thin-layer chromatography

TLC Separation by Polarity

Thin-layer chromatography (TLC) separates compounds in a mixture by polarity. It uses the same basic parts as other chromatography methods: a stationary phase, a mobile phase, and the solute. In TLC, the stationary phase is a thin coating on a plate instead of material packed into a column. The stationary phase in most TLC systems is polar silica gel, a form of silicon dioxide. Its surface has OH groups that can form hydrogen bonds. The sample is first spotted on a pencil start line with a...

column chromatography

Separating Mixtures by Polarity

Column chromatography separates compounds in an organic chemistry lab by using differences in physical properties. It is especially useful for microscale separations when less than 10 grams of material is available for analysis. The method can separate a mixture based on solubility, polarity, hydrophobicity, size, and charge. Any chromatography system has three main parts: the stationary phase, the mobile phase, and the sample mixture. In column chromatography, the stationary phase is usually...

hydrolysis of an ester

Soap Formation from Triglycerides

Triglycerides are lipids made of a glycerol backbone linked to three fatty acids. They are common in vegetable oils and animal fats. Plant lipids are the basis of vegetable oils, while animal lipids are used as lard or general sources of fats. A fatty acid has a long carbon chain, usually between 12 and 20 carbons, with a weak organic acid at one end. Some fatty acids contain double bonds. These are unsaturated fats. The double bond usually gives the chain a bent, cis shape. As a result,...

synthesis of luminol

Luminol Chemiluminescence in Forensics

Luminol chemiluminescence shows how a chemical reaction can produce light and help with blood detection. Chemical reactions may be endothermic or exothermic, depending on whether they absorb or release heat as reactants become products. The enthalpy of a reaction is the energy difference between reactants and products, or ΔH. A positive ΔH means the reaction is endothermic. A negative ΔH means it is exothermic. Many reactions do not happen in a single step. They can pass through intermediates...

esterification

Ester Formation and Everyday Fragrances

Esters are organic compounds with a carbonyl group attached to an oxygen that connects to another alkyl or aryl group. Their general formula is RCOOR'. They are often made by reacting a carboxylic acid with an alcohol in an esterification reaction. In this process, the hydroxyl group of the acid is replaced by an alkyl or aryl group. Many simple esters have familiar fruit and flower smells because their structures are small and less complex. Formic acid and methanol can combine to form methyl...

identification of unknown aldehydes and ketones

identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

Aldehydes and ketones are carbonyl compounds, which means they contain a C=O functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon. Acetone is the simplest ketone, with two methyl groups. An aldehyde is similar, but it has at least one hydrogen attached to the carbonyl carbon. Formaldehyde is the simplest aldehyde, and acetaldehyde has one hydrogen and one methyl group. The carbonyl carbon in both aldehydes and ketones is electrophilic because the oxygen atom...

uv-vis spectroscopy of dyes

How Dyes Absorb Visible Light

Dyes absorb visible light because their molecular structure lets them interact with specific wavelengths. When light hits a substance, some light is absorbed and the rest is reflected or transmitted. The color we see depends on the wavelength that is reflected. A blue substance reflects blue light, around 430 to 480 nm, and absorbs the complementary orange region, around 590 to 630 nm. Compounds that do not absorb in the visible range appear colorless to the human eye. Light also has energy and...

identifying alcohols

Alcohol Tests for Primary, Secondary, and Tertiary

Alcohol tests can help sort alcohols by structure and reactivity. Alcohols are organic molecules with a hydroxyl group attached to an alkyl or aryl group, written as ROH. If the carbon with the hydroxyl group has one R group, the alcohol is primary. If it has two R groups, it is secondary. If it has three R groups, it is tertiary. Alcohols can also be aromatic when they contain a benzene ring. Phenol is the simplest aromatic alcohol. To identify an alcohol, chemists can use its chemical...

chemistry

Comprehensive, curriculum-focused videos for both instructors and students in introductory chemistry lab courses.

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Learn how oxidation states reveal redox reactions, how to assign oxidation numbers, and why oxidation, reduction, and agents matter in chemistry.

Oxidation States in Redox Reactions | JoVE K12

Florida Department of Education

Physical Science

SC.912.P.8

SC.912.P.8.10

Georgia Standards of Excellence

9-12

SC3.

Indicators of Chemical Reactions

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Balancing Redox Equations

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

ionic-compounds-formulas-and-nomenclature

Ionic Compounds: Formulas and Nomenclature

An element composed of atoms that readily lose electrons (a metal) can react with an element composed of atoms that readily gain electrons (a nonmetal) to ...

molecular-compounds-formulas-and-nomenclature

Molecular Compounds: Formulas and Nomenclature

Molecular compounds or covalent compounds result when atoms share electrons to form covalent bonds. Since there is no electron transfer, molecular ...

formula-mass-and-mole-concepts-of-compounds

Formula Mass and Mole Concepts of Compounds

Formula Mass of Covalent Compounds

Chemical formulas represent the elemental makeup of substances. For covalent compounds, the formula represents the ...

experimental-determination-of-chemical-formula

Experimental Determination of Chemical Formula

The elemental makeup of a compound defines its chemical identity, and chemical formulas are the most concise way of representing this elemental makeup. ...

Chemical Equations

Chemical equations represent the identities and relative quantities of substances involved in a chemical reaction. The substances undergoing reaction are ...

CHEM.9.A

Chemical Reactions in Aqueous Solutions

Chemical substances interact in many different ways. Certain chemical reactions exhibit common patterns of reactivity. Due to the vast number of chemical ...

Precipitation Reactions

In a precipitation reaction, aqueous solutions of soluble salts react to give an insoluble ionic compound &#x2013; the precipitate. The reaction occurs ...

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 ...

Oxidation and Reduction
Some chemical reactions can be classified as reduction-oxidation reactions, or redox reactions. Oxidation is the process of ...

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 ...

Reaction Stoichiometry

A balanced chemical equation provides a great deal of information in a very succinct format. Chemical formulas provide the identities of the reactants and ...

Limiting Reactant

The relative amounts of reactants and products represented in a balanced chemical equation are often referred to as stoichiometric amounts. However, in ...

Reaction Yield

The theoretical yield of a reaction is the amount of product estimated to form based on the stoichiometry of the balanced chemical equation. The ...

Stoichiometry, Product Yield, and Limiting Reactants

Chemical equations represent how a chemical reaction proceeds from reactants to products through physical or chemical change using chemical formulas.
...

CHEM.9.C

Science concepts. The student understands how matter is accounted for in chemical reactions. The student is expected to:

Oxidation States in Redox Reactions

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