Name: battery power from redox reactions
Uploaded: 2020-03-26T07:22:26.000Z
Duration: 3 min 31 s
Channel: JoVE

Электрохимическая реакция — это процесс, при котором электроны передаются между двумя веществами, при этом одно теряет электроны, а другое их приобретает. Когда вещество теряет электроны, мы говорим, что оно подверглось окислению или что оно окислилось. Когда он набирает электроны, мы говорим, что он подвергся восстановлению или был восстановлен. Чтобы помочь вам различить эти два понятия, запомните фразу «нефтяная вышка», которая означает, что окисление — это потеря, а восстановление — это приобретение. Некоторые электрохимические реакции протекают спонтанно, что мы можем изучить в гальваническом элементе.
Гальванический элемент состоит из четырех основных частей. Во-первых, есть два металлических электрода, каждый из которых погружен в разный раствор электролита. Растворы состоят из ионов, необходимых для полуреакций, будь то реакция окисления или восстановления, которые происходят на каждом электроде. Растворы соединены солевым мостиком, который обеспечивает поток ионов и поддерживает зарядовую нейтральность растворов. Электроды соединены внешней цепью, по которой электроны проходят во время реакции.
Анод — это электрод, который отдает электроны в ходе реакции или окисляется. Катод — это электрод, который набирает электроны и восстанавливается. Чтобы помочь вам различить эти два ярлыка, запомните фразу «красная кошка», поскольку редукция происходит на катоде.
Так как же узнать, какой металл действует как анод, а какой — как катод? Мы можем использовать стандартный электродный потенциал металла, который является мерой склонности металла к потере электронов. Более отрицательный электродный потенциал означает, что металл имеет более высокую склонность к потере электронов. Электроны всегда перетекают из металла с меньшим стандартным потенциалом в металл с большим стандартным потенциалом.
В гальваническом элементе электроны перетекают от анода к катоду, таким образом, анод — это металл с более низким стандартным потенциалом, а катод — металл с более высоким стандартным потенциалом. Когда электроны перемещаются между двумя электродами, измеряется электрический потенциал. Величина создаваемого напряжения равна стандартному потенциалу катода за вычетом стандартного потенциала анода.
Таким образом, если в гальваническом элементе используются два металла с одинаковыми стандартными электродными потенциалами, такие как, например, свинец и олово, величина производимого напряжения будет низкой. Если металлы имеют разные стандартные потенциалы, такие как серебро и магний, создаваемое напряжение будет высоким.
В этой лаборатории вы сконструируете гальваническую ячейку с использованием двух металлических электродов и измерите потенциал, возникающий при протекании реакций окисления и восстановления. Затем вы определите неизвестные металлические электроды, используя известные стандартные потенциалы, и определите величину производимого напряжения.

Гальванические элементы

Galvanic Cells

электрохимия
Электрохимия — это раздел химии, изучающий взаимосвязь между электрической энергией и химическими изменениями. Эти химические реакции включают в себя движение электронов от одного вида к другому. Это движение либо генерирует ток, либо приводится в движение приложенным током.
Ключевой реакцией в электрохимии является окислительно-восстановительная, или окислительно-восстановительная реакция. Окислительно-восстановительная реакция состоит из двух полуреакций; реакция окисления, при которой вещество теряет электроны, и реакция восстановления, при которой вещество приобретает электроны. Эта химическая реакция приводит к тому, что каждое вещество меняет свою степень окисления.
Атом или молекула, которая теряет электроны или окисляется, является восстановителем. Атом или молекула, которая принимает электроны или восстанавливается, является окислителем. Один из способов запомнить эти взаимосвязи — использовать фразу «нефтяная вышка», которая означает: «oxidation is losing — reduction is gaining».
<h4>Электрохимические ячейки</h4>
Электрохимическая ячейка — это устройство, которое генерирует электрический ток из энергии, выделяющейся в результате окислительно-восстановительной реакции. Электрохимическая ячейка состоит из реакционной камеры или камер и двух проводящих электродов: анода и катода. Анод и катод соединены электрическим путем, а реакционная камера заполнена электролитом. Между двумя камерами находится солевой мостик, который замыкает цепь и позволяет ионам передаваться между электродами.
Восстановление происходит на катоде, в то время как окисление происходит на аноде. Это можно легко запомнить с помощью мнемонического слова «красный кот», означающего, что красноевсасывание происходит на кошачьемучастке.
Существует два типа электрохимических ячеек. Одним из них является электролитическая ячейка, которая использует электрическую энергию для запуска неспонтанной реакции. В этом типе элемента электрическая энергия подается от внешнего источника питания. Другим типом элементов является гальванический элемент, который использует спонтанную реакцию для выработки электрической энергии. Аккумуляторы являются примером гальванических элементов.
<h4>Гальванические элементы</h4>
Гальванические элементы, также известные как вольтовые, состоят из двух полуэлементов. Каждая полуячейка содержит металлический электрод, погруженный в электролит. Внешняя цепь соединяет два электрода, а солевой мостик соединяет два раствора электролита. Электроны перетекают от анода к катоду. Окислительная полуреакция происходит на аноде, в то время как восстановительная полуреакция происходит на катоде. Например, в гальваническом элементе между медью и магнием на катоде происходит следующая полуреакция:
Cu2+ + 2e- → Cu
А у анода происходит следующая полуреакция:
Mg → Mg2+ + 2e-
Поскольку электроны теряются во время окисления на аноде, они проходят через внешнюю цепь, чтобы уменьшить катод, генерируя ток.
По мере окисления анода концентрация катионов в электролите увеличивается. Точно так же, по мере уменьшения катода, концентрация анионов в электролите увеличивается. Для поддержания электрической нейтральности ионы перемещаются по соляному мосту. Когда катионы образуются на аноде, анионы перемещаются из раствора на анодной стороне с помощью солевого моста. На катодной стороне создаются анионы, которые побуждают катионы перемещаться от солевого мостика к раствору на катодной стороне. Важно помнить, что электроны проходят по проводам внешней цепи, а ионы протекают через солевой мостик и растворы.
<h4>Потенциал снижения</h4>
Некоторые металлы имеют большую склонность к потере электронов, чем другие. Таким образом, величина электрического тока, производимого гальваническим элементом, зависит от типа металлических электродов. Стандартный электродный потенциал (Eo) вещества является мерой склонности вещества к потере электронов. Если использовать два металла с почти равными электродными потенциалами, то величина производимого тока будет небольшой. Если используются два металла с очень разными электродными потенциалами, то величина тока будет большой. Чем больше восстановительный потенциал, тем больше вероятность того, что металл будет восстановлен и действовать как окислитель.
Возвращаясь к медно-магниевому гальваническому элементу, стандартный электродный потенциал меди составляет 0,337 вольта, в то время как стандартный электродный потенциал магния составляет -2,370 вольта. В этом примере медь является катодом, а магний — анодом.
При проведении эксперимента с гальваническим элементом разность потенциалов между двумя электродами контролируется с помощью мультиметра. Измеряемое напряжение равно разности потенциалов между двумя полуреакциями.
ΔEº =катод Eº -анод Eº
Стандартный электродный потенциал предполагает, что оба полуэлемента находятся при стандартных условиях 1 М, 1 бар и 298,15 К (25 °C). Напряжение зависит от концентрации растворов электролитов, которую можно определить с помощью уравнения Нернста.
<img src="/CDNSource/lm/labs/52/52_Concepts_4.jpg">
Здесь E соответствует разности потенциалов или измеренному напряжению, Eo — стандартный восстановительный потенциал, R — универсальная газовая постоянная (8,314462618 Дж/моль·K), T — измеренная температура в Кельвинах, F — постоянная Фарадея (96,485,33212 C⋅моль-1), n — количество электронов, перенесенных в реакции, а Qh — коэффициент реакции. Коэффициент реакции — это электрохимический эквивалент константы равновесия.
<h4>Ссылки</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Химия и химическая реакционная способность. Бельмонт, Калифорния: Брукс/Коул, Cengage Learning.</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). Химия: молекулярная природа материи и изменения. Бостон, Массачусетс: Макгроу Хилл.</li>
	<li style="font-size:17px">Harris, D.C. (2015). Количественный химический анализ. Шестое издание. Нью-Йорк, Нью-Йорк: W.H. Freeman and Company.</li>
</ol>

Электрохимия
Электрохимия — это раздел химии, изучающий взаимосвязь между электрической энергией и химическими изменениями. Эти химические...

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JoVE Lab Manual

Chemistry

Химия

battery power from redox reactions

Electrochemistry studies how chemical change and electrical energy are linked. In this branch of chemistry, electrons move from one species to another. That movement can either produce current or be driven by an applied current. The main reaction in electrochemistry is the oxidation-reduction, or redox, reaction. A redox reaction has two half-reactions. Oxidation is the loss of electrons, and reduction is the gain of electrons. These changes also shift the oxidation state of each substance. The species that loses electrons is the reducing agent, and the species that gains electrons is the oxidizing agent. A common memory aid is “oil-rig,” which means oxidation is losing and reduction is gaining.

An electrochemical cell is a device that turns the energy from a redox reaction into electric current. It includes reaction chamber or chambers and two conductive electrodes, an anode and a cathode. The electrodes are connected electrically, and the chamber contains an electrolyte. A salt bridge sits between the two chambers. It completes the circuit and lets ions move between the electrodes. Reduction occurs at the cathode, and oxidation occurs at the anode. Students often remember this with “red cat,” which means reduction occurs at the cathode. The two main kinds of electrochemical cells are electrolytic cells and galvanic cells. An electrolytic cell uses external electrical energy to drive a nonspontaneous reaction. A galvanic cell uses a spontaneous reaction to generate electrical energy. Batteries are examples of galvanic cells.

Galvanic cells, also called voltaic cells, have two half-cells. Each half-cell contains a metal electrode immersed in an electrolyte. An external circuit connects the electrodes, and a salt bridge connects the two electrolyte solutions. Electrons flow from the anode to the cathode. Oxidation happens at the anode, and reduction happens at the cathode. In a copper-magnesium galvanic cell, copper is reduced at the cathode through the half-reaction Cu2+ + 2e- → Cu. Magnesium is oxidized at the anode through the half-reaction Mg → Mg2+ + 2e-. As electrons move through the external circuit, they reduce the cathode and generate current. As the anode is oxidized, the concentration of cations increases in that electrolyte. As the cathode is reduced, the concentration of anions increases in that electrolyte.

The salt bridge keeps the cell electrically neutral. When cations are created at the anode, anions move from the anode side through the salt bridge. On the cathode side, anions are created, so cations move from the salt bridge into that solution. It is important to separate electron flow from ion flow. Electrons travel through the external wires. Ions move through the salt bridge and the solutions.

Some metals lose electrons more easily than others. For that reason, the current produced by a galvanic cell depends on the metals used for the electrodes. The standard electrode potential, E°, measures this tendency. If two metals have nearly equal electrode potentials, the current is small. If their electrode potentials are very different, the current is larger. A larger reduction potential means a metal is more likely to be reduced and act as the oxidizing agent. In the copper-magnesium cell, copper has a standard electrode potential of 0.337 volts, and magnesium has a standard electrode potential of -2.370 volts. That is why copper is the cathode and magnesium is the anode.

In a galvanic cell experiment, a multimeter is used to measure the potential difference between the two electrodes. The measured voltage equals the difference between the two half-reactions, written as ΔE° = E° cathode - E° anode. Standard electrode potential assumes standard conditions of 1 M, 1 bar, and 298.15 K, or 25 °C. The voltage also depends on electrolyte concentration. That dependence can be found with the Nernst equation. In the equation, E is the measured potential difference, E° is the standard reduction potential, R is the universal gas constant, T is temperature in Kelvin, F is Faraday’s constant, n is the number of electrons transferred, and Qh is the reaction quotient. The reaction quotient is the electrochemical equivalent of the equilibrium constant.

Battery Power from Redox Reactions

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Boiling Point and Vapor Pressure

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steam distillation

Steam Distillation and Low-Boiling Separation

Steam distillation is a separation method for temperature-sensitive organic compounds. It works best when the organic compound is immiscible in water, meaning the two liquids do not mix. The method helps remove the organic molecule from a non-volatile contaminant without using the higher temperatures needed for many other separations. In steam distillation, the immiscible mixture is heated until both water and the volatile organic compound distill together. The vapor rises to a condenser,...

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

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Battery Power from Redox Reactions

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