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

Uma reação eletroquímica é um processo em que os elétrons são transferidos entre duas substâncias, com uma perdendo elétrons e a outra ganhando-os. Quando uma substância perde elétrons, dizemos que ela sofreu oxidação ou que foi oxidada. Quando ganha elétrons, dizemos que sofreu redução ou foi reduzido. Para ajudá-lo a distinguir entre os dois, lembre-se da frase 'plataforma de petróleo', que significa oxidação está perdendo, redução está ganhando. Algumas reações eletroquímicas ocorrem espontaneamente, que podemos estudar em uma célula galvânica.
A célula galvânica tem quatro partes principais. Primeiro, existem dois eletrodos de metal, cada um dos quais é imerso em uma solução eletrolítica diferente. As soluções consistem nos íons necessários para as semi-reações, seja a reação de oxidação ou redução, que ocorrem em cada eletrodo. As soluções são conectadas por uma ponte salina, que permite o fluxo de íons e mantém a neutralidade de carga das soluções. Os eletrodos são conectados por um circuito externo, pelo qual os elétrons viajam durante a reação.
O ânodo é o eletrodo que libera elétrons na reação ou é oxidado. O cátodo é o eletrodo que ganha elétrons e é reduzido. Para ajudá-lo a distinguir entre esses dois rótulos, lembre-se da frase 'gato vermelho', pois a redução ocorre no cátodo.
Então, como sabemos qual metal atua como ânodo e qual atua como cátodo? Podemos usar o potencial padrão do eletrodo do metal, que é uma medida da tendência do metal de perder elétrons. Um potencial de eletrodo mais negativo significa que o metal tem uma tendência maior a perder elétrons. Os elétrons sempre fluem do metal com o potencial padrão mais baixo para o metal com o maior potencial padrão.
Em uma célula galvânica, os elétrons fluem do ânodo para o cátodo, portanto, o ânodo é o metal com o potencial padrão mais baixo e o cátodo é o metal com o potencial padrão mais alto. À medida que os elétrons viajam entre os dois eletrodos, um potencial elétrico é medido. A magnitude da tensão produzida é igual ao potencial padrão do cátodo menos o potencial padrão do ânodo.
Assim, se dois metais com potenciais de eletrodo padrão semelhantes forem usados na célula galvânica, como chumbo e estanho, por exemplo, a magnitude da tensão produzida será baixa. Se os metais tiverem diferentes potenciais padrão, como prata e magnésio, a tensão produzida será alta.
Neste laboratório, você construirá uma célula galvânica usando dois eletrodos de metal e medirá o potencial produzido à medida que as reações de oxidação e redução ocorrem. Em seguida, você identificará eletrodos de metal desconhecidos usando potenciais padrão conhecidos e determinará a magnitude da tensão produzida.

células galvânicas

Galvanic Cells

eletroquímica
A eletroquímica é um ramo da química que estuda a relação entre a energia elétrica e uma mudança química. Essas reações químicas envolvem o movimento de elétrons de uma espécie para outra. Esse movimento gera corrente ou é impulsionado pela corrente aplicada.
A principal reação em eletroquímica é a reação de oxidação-redução, ou redox. A reação redox é composta por duas meias-reações; a reação de oxidação, onde uma substância perde elétrons, e a reação de redução, onde uma substância ganha elétrons. Essa reação química resulta na mudança de cada substância em seu estado de oxidação.
O átomo ou molécula que perde elétrons, ou é oxidado, é o agente redutor. O átomo ou molécula que aceita os elétrons, ou é reduzido, é o agente oxidante. Um método para lembrar essas relações é através da frase 'plataforma de petróleo', que significa 'oxidation is losing - reduction is gaining'.
<h4>Células eletroquímicas</h4>
Uma célula eletroquímica é um dispositivo que gera uma corrente elétrica a partir da energia liberada por uma reação redox. Uma célula eletroquímica consiste em uma câmara ou câmaras de reação e dois eletrodos condutores: um ânodo e um cátodo. O ânodo e o cátodo são conectados eletricamente e a câmara de reação é preenchida com um eletrólito. Entre as duas câmaras há uma ponte de sal, que completa o circuito e permite que os íons sejam transferidos entre os eletrodos.
A redução ocorre no cátodo, enquanto a oxidação ocorre no ânodo. Isso pode ser facilmente lembrado usando o mnemônico "gato vermelho", o que significa que a ucção vermelhaocorre no casco do gato.
Existem dois tipos de células eletroquímicas. Uma é a célula eletrolítica, que usa energia elétrica para conduzir uma reação não espontânea. Nesse tipo de célula, a energia elétrica é fornecida por uma fonte de energia externa. O outro tipo de célula é a célula galvânica, que usa uma reação espontânea para gerar energia elétrica. As baterias são um exemplo de células galvânicas.
<h4>Células Galvânicas</h4>
As células galvânicas, também conhecidas como células voltaicas, consistem em duas meias-células. Cada meia célula contém um eletrodo de metal imerso em um eletrólito. Um circuito externo conecta os dois eletrodos e uma ponte de sal conecta as duas soluções eletrolíticas. Os elétrons fluem do ânodo para o cátodo. A semi-reação de oxidação ocorre no ânodo, enquanto a semi-reação de redução ocorre no cátodo. Por exemplo, em uma célula galvânica entre cobre e magnésio, a seguinte semi-reação ocorre no cátodo:
2+ + 2e- →
E a seguinte semi-reação ocorre no ânodo:
Mg → Mg2+ + 2e-
À medida que os elétrons são perdidos durante a oxidação no ânodo, eles viajam pelo circuito externo para reduzir o cátodo, gerando corrente.
À medida que o ânodo é oxidado, a concentração de cátions aumenta no eletrólito. Da mesma forma, à medida que o cátodo é reduzido, a concentração de ânions aumenta no eletrólito. Para manter a neutralidade elétrica, os íons viajam pela ponte de sal. Quando os cátions são criados no ânodo, os ânions viajam da solução no lado do ânodo usando a ponte salina. No lado do cátodo, os ânions são criados, levando os cátions a viajar da ponte de sal para a solução no lado do cátodo. É importante lembrar que os elétrons viajam através dos fios do circuito externo e os íons fluem através da ponte de sal e das soluções.
<h4>Potencial de redução</h4>
Alguns metais têm uma tendência maior do que outros a perder elétrons. Portanto, a magnitude da corrente elétrica produzida por uma célula galvânica depende dos tipos de eletrodos metálicos. O potencial padrão do eletrodo (E, o) de uma substância é a medida da tendência de uma substância de perder elétrons. Se dois metais com potenciais de eletrodo quase iguais forem usados, a magnitude da corrente produzida será pequena. Se dois metais com potenciais de eletrodo muito diferentes forem usados, a magnitude da corrente será grande. Quanto maior o potencial de redução, maior a probabilidade de o metal ser reduzido e atuar como agente oxidante.
Voltando à célula galvânica de cobre-magnésio, o potencial padrão do eletrodo de cobre é de 0,337 volts, enquanto o potencial padrão do eletrodo de magnésio é de -2,370 volts. Neste exemplo, o cobre é o cátodo e o magnésio é o ânodo.
Ao realizar um experimento com células galvânicas, a diferença de potencial entre os dois eletrodos é monitorada usando um multímetro. A tensão medida é igual à diferença de potencial entre as duas semi-reações.
ΔEº = Eºcátodo -Eº ânodo
O potencial padrão do eletrodo pressupõe que ambas as meias células estejam sob condições padrão de 1 M, 1 bar e 298,15 K (25 °C). A tensão depende da concentração das soluções eletrolíticas, que pode ser determinada usando a equação de Nernst.
<img src="/CDNSource/lm/labs/52/52_Concepts_4.jpg">
Aqui, E corresponde à diferença de potencial ou à tensão medida, Eo é o potencial de redução padrão, R é a constante universal do gás (8,314462618 J/mol·K), T é a temperatura medida em Kelvin, F é a constante de Faraday (96,485.33212 C⋅mol-1), n é o número de elétrons transferidos na reação, e Qh é o quociente de reação. O quociente de reação é o equivalente eletroquímico da constante de equilíbrio.
<h4>Referências</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Química e Reatividade Química. Belmont, CA: Brooks / Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). Química: A Natureza Molecular da Matéria e da Mudança. Boston, MA: Colina McGraw.</li>
	<li style="font-size:17px">Harris, D.C. (2015). Análise Química Quantitativa. Sexta edição. Nova York, NY: W.H. Freeman and Company.</li>
</ol>

Células Galvânicas

Eletroquímica
A eletroquímica é um ramo da química que estuda a relação entre a energia elétrica e uma mudança química. Estes produtos químicos ...

Education

JoVE Lab Manual

Chemistry

Química

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

lab techniques

Measuring with Accuracy in the Lab

Measuring with accuracy in the lab starts with careful records, the right tools, and correct reading of data. Scientists use lab notebooks to record how an experiment was done. These notes include the amounts of materials used and each step of the procedure. Without that record, an experiment cannot be repeated in the same way. Balances and glassware are used to measure solids and liquids for experiments. The precision of a measurement depends on the equipment used. Digital balances show the...

scientific measurement and lab skills

Graphs, Curve Fit, and Standard Deviation

Two-dimensional graphs help students compare an independent variable with a dependent variable in science data. In a chemistry example, temperature is the independent variable and volume is the dependent variable. After data are collected, the relationship between the two measurements must be quantified so it can be evaluated and compared with other relationships. A graph can also show when no relationship exists between the two variables. To make a two-dimensional graph, each data point needs...

stoichiometry, product yield, and limiting reactants

stoichiometry-product-yield-and-limiting-reactants

Balancing Equations and Reactant Limits

Chemical equations show how reactants change into products during a reaction. Stoichiometry is the study of the relative amounts of reactants and products. It is based on the Law of Conservation of Mass, which says matter is neither created nor destroyed. Because atoms are only rearranged, the number and identity of atoms must match on both sides of a reaction. That is why a chemical equation must be balanced. A balanced equation has the same number of atoms for each element in the reactants...

redox reactions

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...

ideal gas law

Gas Laws Behind the Ideal Gas Equation

The ideal gas equation brings together the main gas laws used in chemistry. Gases have molecules that are far apart, so their behavior is described with four measurable variables: pressure, volume, number of moles, and temperature. The ideal gas law links these variables with the gas constant, R, which equals 8.314 J·K−1 mol−1. Boyle’s law describes the link between pressure and volume. When temperature and the number of moles stay constant, pressure and volume change in opposite directions.

acid and base concentrations

Acids, Bases, and pH in Titration

Acids, bases, and pH explain how acidic or basic a solution is. An Arrhenius acid releases hydrogen ions when it dissolves in water, while an Arrhenius base releases hydroxide ions. These reactions also create conjugate partners: an acid forms a conjugate base after losing hydrogen, and a base forms a conjugate acid after losing hydroxyl. Every acid has a conjugate base, and every base has a conjugate acid. pH measures the amount of hydrogen ions in a solution. The pH scale runs from 0 to 14...

buffers

Buffer pH and the Common Ion Effect

Buffer solutions help keep pH steady when acids or bases are added. In water, an Arrhenius acid (HA) dissociates into hydrogen ions, H+, and a conjugate base, A-. More hydrogen ions make a solution more acidic and lower its pH. An Arrhenius base (BOH) dissociates into a conjugate acid, B+, and hydroxide ions, OH-. Strong acids and strong bases usually change pH a lot because they react completely in water and raise the amount of H+ or OH-. Buffers reduce those pH changes. A buffer usually...

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Reaction Heat, Hess’s Law, and Calorimetry

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solubility

Solubility in Water and Other Solvents

Solubility describes how much of a solute can dissolve in a specific solvent. It is usually reported as solute mass per solvent volume or as solute mass per solvent mass. For example, sodium chloride has a solubility of 36 g per 100 mL of water at room temperature. If solubility is given by solvent mass, that mass must be converted to volume for later calculations. Temperature changes solubility. Sodium carbonate dissolves at about 7 g per 100 mL of water at 0 °C, 22 g per 100 mL at room...

metal flame emission

Flame Test Metal Color Clues

Flame testing uses a metal sample and a flame to identify a substance by its color. Each element gives off a characteristic light emission in the flame, so the color can help distinguish one element from another. This same idea appears in fireworks, where different metal components create different colors. The color comes from energy absorbed by the metallic component. When a species absorbs energy, that energy can go into translation, vibration, rotation, or electron excitement. Electron...

balmer series

Hydrogen Emission Lines

Hydrogen emission lines show how electrons move between fixed energy levels in the Bohr model. Niels Bohr proposed this model for the hydrogen atom in 1913. In this model, an electron can only occupy certain orbits, and it cannot exist between them. These energy states are labeled with the quantum number n. The ground state is n = 1. Higher values, such as n = 2, 3, 4, and so on, are excited states. An electron changes state only by absorbing or emitting energy equal to the difference between...

beer's law

Absorbance Spectra and Beer-Lambert Analysis

Absorbance spectra and the Beer-Lambert Law show how light can be used to study molecules in solution. When light hits a substance, it may be absorbed, transmitted, or reflected. A substance often interacts with a range of wavelengths, and each wavelength can affect the molecules or atoms in a different way. When a molecule absorbs light, that energy can be used for translation, vibration, rotation, or electron excitement. Electron excitement is especially important in spectroscopy because it...

concentration dependence

Reaction Order and Rate Constants

Chemical kinetics describes the speed of a chemical reaction. The rate depends on the reaction mechanism, the complexity of the reaction, and the number of reactants involved. Reactant concentration also affects how fast a reaction happens. The rate law is the experimental relationship that connects these factors to reaction rate. In the rate law, each reactant contributes through a reaction order. The general form is r = k[A]^m[B]^n for aA + bB + ... → cC. Here, r is the reaction rate, k is...

temperature dependence

Reaction Rate, Arrhenius, and Catalysts

Chemical kinetics explains how fast a reaction happens and why that speed changes. The reaction rate is the speed of a chemical reaction. It is defined as the change in concentration of a reactant or product over time. Reaction speed depends on several factors, including the concentration of the reactants and the temperature of the reaction. Each reactant affects the rate in a specific way. That relationship is described by the rate law. The rate law is an equation that connects the...

galvanic cells

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...

electrolytic cells

Electroplating with Electrolytic Cells

Electrochemistry links electrical energy with chemical change, and electrolytic cells show how electricity can drive a nonspontaneous reaction. In these reactions, electrons move from one species to another. The key chemistry is a redox reaction, or oxidation-reduction reaction, which happens in two half-reactions. Oxidation is the loss of electrons, and reduction is the gain of electrons. The substance that loses electrons is the reducing agent, and the substance that gains electrons is the...

proper lab notebook keeping

Recording a Reproducible Lab Experiment

Accurate lab notebook records are essential in science and organic chemistry labs. Without clear notes, descriptions, quantities, and observations, an experiment cannot be repeated or checked. Good record keeping supports reproducibility, which means other people can follow the same work and get similar results. A laboratory notebook is useful in school labs, research settings, and industry. It can also become a legal record when an idea, process, substance, or technique is patented and...

basic organic chemistry techniques

Organic Lab Glassware and Safe Handling

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melting points

How Impurities Change Melting Points

Melting point helps identify an organic compound by showing the temperature where a solid turns into a liquid at 1 atmosphere of pressure. It is a physical property, like solubility, density, color, and electronegativity. Because the exact start of melting is hard to measure, the result is reported as a range. The lower end is when the first drops of liquid appear. The upper end is when the last solid has melted. Scientists compare this range with literature values to help identify a compound. <br...

boiling points

Boiling Point and Vapor Pressure

Boiling point and vapor pressure are key physical properties of a liquid. For a pure compound, the boiling point can help identify the substance. In experiments, the boiling point is usually recorded as a range because exact values are hard to measure and may differ by a few degrees from the literature value. Vapor pressure explains why a liquid boils. In a sealed container, some molecules at the liquid surface gain enough energy to escape into the gas phase. At the same time, some gas...

recrystallization

Purifying Crystals by Solubility

Purifying crystals by solubility is a key organic chemistry technique. Recrystallization uses differences in solubility to separate a desired solid product from starting materials, solvent, and impurities. It is one of the three standard purification methods, along with distillation and extraction. Solubility is the maximum amount of a substance that can dissolve in a fixed volume of solvent at a given temperature. Different solutes dissolve in different solvents, and their behavior can vary...

extraction

Separating Mixtures with Solvent Extraction

Solvent extraction is a common way to separate and isolate a target compound in organic chemistry. In this process, a solute moves from one phase to another so it can be separated from unreacted starting materials or impurities. Extraction is also useful when a product is dissolved in a reaction solvent that is hard to remove by evaporation, such as a solvent with a high boiling point. There are three main types of extraction. In solid-liquid extraction, the solute moves from a solid phase...

simple distillation

Separating Liquids by Boiling Point

Separating liquids by boiling point is the goal of distillation. A miscible mixture contains liquids that mix evenly, and it can be heated so that the more volatile liquid evaporates first. Volatility is a liquid’s tendency to vaporize, and boiling begins when bubbles form in the liquid phase. In a closed container, liquid and vapor can reach equilibrium, which means the rate of vaporization equals the rate of condensation. The pressure from the vapor above the liquid is called vapor pressure.

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

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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

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identifying alcohols

Alcohol Tests for Primary, Secondary, and Tertiary

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chemistry

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Battery Power from Redox Reactions

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