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This is a€?Electrochemistrya€?, chapter 19 from the book Principles of General Chemistry (v.
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For more information on the source of this book, or why it is available for free, please see the project's home page. DonorsChoose.org helps people like you help teachers fund their classroom projects, from art supplies to books to calculators. In oxidationa€“reduction (redox) reactions, electrons are transferred from one species (the reductant) to another (the oxidant). In any electrochemical process, electrons flow from one chemical substance to another, driven by an oxidationa€“reduction (redox) reaction. Because it is impossible to have a reduction without an oxidation and vice versa, a redox reaction can be described as two half-reactionsReactions that represent either the oxidation half or the reduction half of an oxidationa€“reduction (redox) reaction., one representing the oxidation process and one the reduction process.
Each half-reaction is written to show what is actually occurring in the system; Zn is the reductant in this reaction (it loses electrons), and Br2 is the oxidant (it gains electrons). In any redox reaction, the number of electrons lost by the reductant equals the number of electrons gained by the oxidant.
In most of our discussions of chemical reactions, we have assumed that the reactants are in intimate physical contact with one another. A galvanic cell (left) transforms the energy released by a spontaneous redox reaction into electrical energy that can be used to perform work. To illustrate the basic principles of a galvanic cell, leta€™s consider the reaction of metallic zinc with cupric ion (Cu2+) to give copper metal and Zn2+ ion. We can cause this reaction to occur by inserting a zinc rod into an aqueous solution of copper(II) sulfate.
When a zinc rod is inserted into a beaker that contains an aqueous solution of copper(II) sulfate, a spontaneous redox reaction occurs: the zinc electrode dissolves to give Zn2+(aq) ions, while Cu2+(aq) ions are simultaneously reduced to metallic copper.
This same reaction can be carried out using the galvanic cell illustrated in part (a) in Figure 19.3 "The Reaction of Metallic Zinc with Aqueous Copper(II) Ions in a Galvanic Cell". The electrolyte in the salt bridge serves two purposes: it completes the circuit by carrying electrical charge and maintains electrical neutrality in both solutions by allowing ions to migrate between them. A voltmeter can be used to measure the difference in electrical potential between the two compartments.
A galvanic (voltaic) cell converts the energy released by a spontaneous chemical reaction to electrical energy.
B From the direction of electron flow, assign each electrode as either positive or negative. B Electrons flow from the tin electrode through the wire to the platinum electrode, where they transfer to nitrate.
The Pt electrode in the permanganate solution is the cathode; the one in the tin solution is the anode.
The cathode (electrode in beaker that contains the permanganate solution) is positive, and the anode (electrode in beaker that contains the tin solution) is negative. Because it is somewhat cumbersome to describe any given galvanic cell in words, a more convenient notation has been developed.
This cell diagram does not include a double vertical line representing a salt bridge because there is no salt bridge providing a junction between two dissimilar solutions. A single-compartment galvanic cell will initially exhibit the same voltage as a galvanic cell constructed using separate compartments, but it will discharge rapidly because of the direct reaction of the reactant at the anode with the oxidized member of the cathodic redox couple. Using the symbols described, write the cell diagram beginning with the oxidation half-reaction on the left. The solution concentrations were not specified, so they are not included in this cell diagram.
Electrochemistry is the study of the relationship between electricity and chemical reactions.
A galvanic (voltaic) cell uses the energy released during a spontaneous redox reaction to generate electricity, whereas an electrolytic cell consumes electrical energy from an external source to force a reaction to occur. If two half-reactions are physically separated, how is it possible for a redox reaction to occur? One criterion for a good salt bridge is that it contains ions that have similar rates of diffusion in aqueous solution, as K+ and Cla?’ ions do. It is often more accurate to measure the potential of a redox reaction by immersing two electrodes in a single beaker rather than in two beakers. Consider the following spontaneous redox reaction: NO3a?’(aq) + H+(aq) + SO32a?’(aq) a†’ SO42a?’(aq) + HNO2(aq).
If the reaction is carried out in a galvanic cell using an inert electrode in each compartment, which electrode corresponds to which half-reaction?
If the reaction is carried out in a galvanic cell using an inert electrode in each compartment, which reaction occurs at the cathode and which occurs at the anode?
Write the spontaneous half-reactions and the overall reaction for each proposed cell diagram. For each galvanic cell represented by these cell diagrams, determine the spontaneous half-reactions and the overall reaction.
For each redox reaction, write the half-reactions and draw the cell diagram for a galvanic cell in which the overall reaction occurs spontaneously.
Write the half-reactions for each overall reaction, decide whether the reaction will occur spontaneously, and construct a cell diagram for a galvanic cell in which a spontaneous reaction will occur. In a galvanic cell, current is produced when electrons flow externally through the circuit from the anode to the cathode because of a difference in potential energy between the two electrodes in the electrochemical cell. The potential energy of a system consisting of metallic Zn and aqueous Cu2+ ions is greater than the potential energy of a system consisting of metallic Cu and aqueous Zn2+ ions. Because the potential energy of valence electrons differs greatly from one substance to another, the voltage of a galvanic cell depends partly on the identity of the reacting substances. The measured potential of a cell also depends strongly on the concentrations of the reacting species and the temperature of the system.
Measured redox potentials depend on the potential energy of valence electrons, the concentrations of the species in the reaction, and the temperature of the system.
It is physically impossible to measure the potential of a single electrode: only the difference between the potentials of two electrodes can be measured. This cell diagram corresponds to the oxidation of a cobalt anode and the reduction of Cu2+ in solution at the copper cathode. All tabulated values of standard electrode potentials by convention are listed for a reaction written as a reduction, not as an oxidation, to be able to compare standard potentials for different substances. In contrast, recall that half-reactions are written to show the reduction and oxidation reactions that actually occur in the cell, so the overall cell reaction is written as the sum of the two half-reactions.
The overall cell reaction is the sum of the two half-reactions, but the cell potential is the difference between the reduction potentials: EA°cell = EA°cathode a?’ EA°anode. Although it is impossible to measure the potential of any electrode directly, we can choose a reference electrode whose potential is defined as 0 V under standard conditions.
One especially attractive feature of the SHE is that the Pt metal electrode is not consumed during the reaction. The SHE consists of platinum wire that is connected to a Pt surface in contact with an aqueous solution containing 1 M H+ in equilibrium with H2 gas at a pressure of 1 atm. Figure 19.7 "Determining a Standard Electrode Potential Using a Standard Hydrogen Electrode" shows a galvanic cell that consists of a SHE in one beaker and a Zn strip in another beaker containing a solution of Zn2+ ions. Although the reaction at the anode is an oxidation, by convention its tabulated EA° value is reported as a reduction potential. Because electrical potential is the energy needed to move a charged particle in an electric field, standard electrode potentials for half-reactions are intensive properties and do not depend on the amount of substance involved.
Moles, or nevi, are sets of pigmented cells that will show up on the epidermis as brown or black moles on skin or possibly as groups or dots. While they are generally black or brown, they could appear in numerous tones of a tan, red mole, or pink mole, and may manifest just about anyplace on the human body. Nobody understands precisely why a mole on skin actually manifests, nonetheless they have a concept of the way they develop. These types of measures comprise frequent appointments to your doctor or skincare specialist to get full body testing. In every case, these skin moles should be observed to ensure that modifications in measurement, contour, coloration, and various aspects could be identified in future appointments.


Nearly all skin moles tend to be benign, however , particular moles are more inclined to become melanoma or skin cancer moles. Each time a doctor records a unique skin mole, they will most likely carry out a biopsy, eliminating some of the mole or perhaps the complete skin mole, and delivering the tissue to a lab to get screened.
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This transfer of electrons provides a means for converting chemical energy to electrical energy or vice versa. We then explore the relationships among the electrical potential, the change in free energy, and the equilibrium constant for a redox reaction, which are all measures of the thermodynamic driving force for a reaction. The patina is formed by corrosion of the copper skin of the statue, which forms a thin layer of an insoluble compound that contains copper(II), sulfate, and hydroxide ions. As we described in Chapter 3 "Chemical Reactions", a redox reaction occurs when electrons are transferred from a substance that is oxidized to one that is being reduced. Acida€“base reactions, for example, are usually carried out with the acid and the base dispersed in a single phase, such as a liquid solution.
The oxidative and reductive half-reactions usually occur in separate compartments that are connected by an external electrical circuit; in addition, a second connection that allows ions to flow between the compartments (shown here as a vertical dashed line to represent a porous barrier) is necessary to maintain electrical neutrality. As the reaction proceeds, the zinc rod dissolves, and a mass of metallic copper forms (Figure 19.2 "The Reaction of Metallic Zinc with Aqueous Copper(II) Ions in a Single Compartment"). The reaction occurs so rapidly that the copper is deposited as very fine particles that appear black, rather than the usual reddish color of copper.
To assemble the cell, a copper strip is inserted into a beaker that contains a 1 M solution of Cu2+ ions, and a zinc strip is inserted into a different beaker that contains a 1 M solution of Zn2+ ions. The two metal strips are connected by a wire that allows electricity to flow, and the beakers are connected by a salt bridge. The identity of the salt in a salt bridge is unimportant, as long as the component ions do not react or undergo a redox reaction under the operating conditions of the cell. Opening the switch that connects the wires to the anode and the cathode prevents a current from flowing, so no chemical reaction occurs.
The electrode can be made from an inert, highly conducting metal such as platinum to prevent it from reacting during a redox process, where it does not appear in the overall electrochemical reaction. An electrolytic cell consumes electrical energy from an external source to drive a nonspontaneous chemical reaction. One beaker contains a strip of tin immersed in aqueous sulfuric acid, and the other contains a platinum electrode immersed in aqueous nitric acid. The electric circuit is completed by the salt bridge, which permits the diffusion of cations toward the cathode and anions toward the anode.
In this line notation, called a cell diagram, the identity of the electrodes and the chemical contents of the compartments are indicated by their chemical formulas, with the anode written on the far left and the cathode on the far right. For example, the voltage produced by a redox reaction can be measured more accurately using two electrodes immersed in a single beaker containing an electrolyte that completes the circuit. Moreover, solution concentrations have not been specified, so they are not included in the cell diagram. Beginning on the left with the anode, we indicate the phase boundary between the electrode and the tin solution by a vertical bar. The oxidationa€“reduction reaction that occurs during an electrochemical process consists of two half-reactions, one representing the oxidation process and one the reduction process. What is the name of the apparatus in which two half-reactions are carried out simultaneously? If a piece of zinc metal is placed in a beaker of aqueous CuSO4 solution, the blue color fades with time, the zinc strip begins to erode, and a black solid forms around the zinc strip.
When an iron nail is placed in a gel that contains [Fe(CN)6]3a?’, the gel around the nail begins to turn pink. Much of this potential energy difference is because the valence electrons of metallic Zn are higher in energy than the valence electrons of metallic Cu. To develop a scale of relative potentials that will allow us to predict the direction of an electrochemical reaction and the magnitude of the driving force for the reaction, the potentials for oxidations and reductions of different substances must be measured under comparable conditions.
According to Equation 19.10, when we know the standard potential for any single half-reaction, we can obtain the value of the standard potential of many other half-reactions by measuring the standard potential of the corresponding cell. The standard hydrogen electrode (SHE)The electrode chosen as the reference for all other electrodes, which has been assigned a standard potential of 0 V and consists of a Pt wire in contact with an aqueous solution that contains 1 M HA +A  in equilibrium with H2 gas at a pressure of 1 atm at the Pt-solution interface.
In the molecular view, the Pt surface catalyzes the oxidation of hydrogen molecules to protons or the reduction of protons to hydrogen gas.
The potential of a half-reaction measured against the SHE under standard conditions is called the standard electrode potentialThe potential of a half-reaction measured against the SHE under standard conditions. Consequently, EA° values are independent of the stoichiometric coefficients for the half-reaction, and, most important, the coefficients used to produce a balanced overall reaction do not affect the value of the cell potential. When we close the circuit this time, the measured potential for the cell is negative (a?’0.34 V) rather than positive.
A mole growth might be level or elevated, with numerous moles building a elevated profile over time. Melanin is known as a pigment that happens normally within the skin, imbuing it featuring its exclusive coloration. Moles manifest due to irregular melanin distribution, whenever pigment cells heap collectively. In this particular straightforward, pain-free practice, your doctor may examine your epidermis from top to bottom, paying attention to virtually any skin moles which are visible on your body.
Between physician’s appointments, people need to keep a record of skin moles, paying particular awareness to every changes in measurement, contour, colouring, or size, in addition to all irritation, discomfort, blood loss, or oozing. Moles display at delivery, also referred to as congenital nevi, exist in about 1 % of the populace and therefore are prone to become melanoma.
In the event the reports demonstrate that area is a skin cancer mole, the physician will certainly eliminate the complete mole plus an area of epidermis encompassing it. She is a top beauty expert as a result of sharing budget-friendly advice that helps all women, and even some men, look and feel their best. She has done over 500 radio interviews, and has appeared on myriad TV shows including: The Doctors, Extra, Style Network, Discovery Channel, Soap Talk, Los Angeles KTLA and KCAL Morning News, and many more. This information is not a substitution for diagnosis or treatment by a physician or health care provider. You may also download a PDF copy of this book (147 MB) or just this chapter (5 MB), suitable for printing or most e-readers, or a .zip file containing this book's HTML files (for use in a web browser offline).
The study of the relationship between electricity and chemical reactions is called electrochemistryThe study of the relationship between electricity and chemical reactions., an area of chemistry we introduced in Chapter 4 "Reactions in Aqueous Solution" and Chapter 5 "Energy Changes in Chemical Reactions". Finally, we examine two kinds of applications of electrochemical principles: (1) those in which a spontaneous reaction is used to provide electricity and (2) those in which electrical energy is used to drive a thermodynamically nonspontaneous reaction. The reductantA substance that is capable of donating electrons and in the process is oxidized. A redox reaction is balanced when the number of electrons lost by the reductant equals the number of electrons gained by the oxidant. With redox reactions, however, it is possible to physically separate the oxidation and reduction half-reactions in space, as long as there is a complete circuit, including an external electrical connection, such as a wire, between the two half-reactions. The potential difference between the electrodes (voltage) causes electrons to flow from the reductant to the oxidant through the external circuit, generating an electric current. These changes occur spontaneously, but all the energy released is in the form of heat rather than in a form that can be used to do work. The two metal strips, which serve as electrodes, are connected by a wire, and the compartments are connected by a salt bridgeA U-shaped tube inserted into both solutions of a galvanic cell that contains a concentrated liquid or gelled electrolyte and completes the circuit between the anode and the cathode., a U-shaped tube inserted into both solutions that contains a concentrated liquid or gelled electrolyte. When the switch is closed to complete the circuit, the zinc electrode (the anode) is spontaneously oxidized to Zn2+ ions in the left compartment, while Cu2+ ions are simultaneously reduced to copper metal at the copper electrode (the cathode). Without such a connection, the total positive charge in the Zn2+ solution would increase as the zinc metal dissolves, and the total positive charge in the Cu2+ solution would decrease. With the switch closed, however, the external circuit is closed, and an electric current can flow from the anode to the cathode. The two solutions are connected by a salt bridge, and the electrodes are connected by a wire.
The other beaker contains a solution of Sn2+ in dilute sulfuric acid, also with a Pt electrode. Phase boundaries are shown by single vertical lines, and the salt bridge, which has two phase boundaries, by a double vertical line.


This arrangement reduces errors caused by resistance to the flow of charge at a boundary, called the junction potential. Because the Zn(s) + Cu2+(aq) system is higher in energy by 1.10 V than the Cu(s) + Zn2+(aq) system, energy is released when electrons are transferred from Zn to Cu2+ to form Cu and Zn2+. Thus we can conclude that the difference in potential energy between the valence electrons of cobalt and zinc is less than the difference between the valence electrons of copper and zinc by 0.59 V.
To do this, chemists use the standard cell potentialThe potential of an electrochemical cell measured under standard conditions (1 M for solutions, 1 atm for gases, and pure solids or pure liquids for other substances) and at a fixed temperature (usually 298 K). Recall from Chapter 18 "Chemical Thermodynamics" that only differences in enthalpy and free energy can be measured.) We can, however, compare the standard cell potentials for two different galvanic cells that have one kind of electrode in common.
The zinc electrode begins to dissolve to form Zn2+, and H+ ions are reduced to H2 in the other compartment. Several moles could fade away with age, while others could adjust coloration or continue to be the same.
However, cancerous moles (malignant mole) in other people some may grow into a type of skin cancer referred to as mole melanoma (mole cancer).
This particular pigment is usually manufactured in particular cells referred to as melanocytes, situated underneath the skin surface. Sometimes, the doctor could advise that a skin mole get screened to guarantee it won’t present a danger. Huge skin moles, those that tend to be bigger than a pencil eraser,  having irregularly formed boundaries will also be prone to turn into cancer. She has made it her mission to help women solve their “head to toe” challenges with “look and feel good” tips that they can afford. In this chapter, we describe electrochemical reactions in more depth and explore some of their applications. Like any balanced chemical equation, the overall process is electrically neutral; that is, the net charge is the same on both sides of the equation. As the reaction progresses, the electrons flow from the reductant to the oxidant over this electrical connection, producing an electric current that can be used to do work. This type of electrochemical cell is often called a voltaic cell after its inventor, the Italian physicist Alessandro Volta (1745a€“1827).
In an electrolytic cell (right), an external source of electrical energy is used to generate a potential difference between the electrodes that forces electrons to flow, driving a nonspontaneous redox reaction; only a single compartment is employed in most applications. The ions in the salt bridge are selected so that they do not interfere with the electrochemical reaction by being oxidized or reduced themselves or by forming a precipitate or complex; commonly used cations and anions are Na+ or K+ and NO3a?’ or SO42a?’, respectively.
The salt bridge allows charges to be neutralized by a flow of anions into the Zn2+ solution and a flow of cations into the Cu2+ solution.
The potential (Ecell)Related to the energy needed to move a charged particle in an electric field, it is the difference in electrical potential beween two half-reactions.
In contrast, electrons flow toward the Pt electrode, so that electrode must be electrically positive. We could include H2SO4(aq) with the contents of the anode compartment, but the sulfate ion (as HSO4a?’) does not participate in the overall reaction, so it does not need to be specifically indicated. The overall redox reaction is balanced when the number of electrons lost by the reductant equals the number of electrons gained by the oxidant. Just like water flowing spontaneously downhill, which can be made to do work by forcing a waterwheel, the flow of electrons from a higher potential energy to a lower one can also be harnessed to perform work. It consists of a strip of platinum wire in contact with an aqueous solution containing 1 M H+. Whenever melanin is created, it goes towards the skin surface, usually scattering equally over the surface area.
To avoid the creation of further skin moles, it’s best to reduce the exposure to the sun in addition to utilize sun block all the time. An apparatus that is used to generate electricity from a spontaneous redox reaction or, conversely, that uses electricity to drive a nonspontaneous redox reaction is called an electrochemical cellAn apparatus that generates electricity from a spontaneous oxidationa€“reduction (redox) reaction or, conversely, uses electricity to drive a nonspontaneous redox reaction..
In contrast, an electrolytic cellAn electrochemical cell that consumes electrical energy from an external source to drive a nonspontaneous (I”G>0) oxidationa€“reduction (redox) reaction. In both kinds of electrochemical cells, the anode is the electrode at which the oxidation half-reaction occurs, and the cathode is the electrode at which the reduction half-reaction occurs. In the absence of a salt bridge or some other similar connection, the reaction would rapidly cease because electrical neutrality could not be maintained. The cathode compartment contains aqueous nitric acid, which does participate in the overall reaction, together with the product of the reaction (NO) and the Pt electrode.
An electric current is produced from the flow of electrons from the reductant to the oxidant.
What will happen if a piece of copper metal is placed in a colorless aqueous solution of ZnCl2? Corrections for nonideal behavior are important for precise quantitative work but not for the more qualitative approach that we are taking here. The [H+] in solution is in equilibrium with H2 gas at a pressure of 1 atm at the Pt-solution interface (Figure 19.6 "The Standard Hydrogen Electrode"). In some instances however, such cellular material heap collectively, developing skin moles. People with greater than 50 dysplastic nevi have an increased danger for progression of melanoma.
The associated potential energy is determined by the potential difference between the valence electrons in atoms of different elements. These are written as HNO3(aq)a??NO(g)a??Pt(s), with single vertical bars indicating the phase boundaries.
An electrochemical cell can either generate electricity from a spontaneous redox reaction or consume electricity to drive a nonspontaneous reaction.
1 atm for gases, pure solids or pure liquids for other substances) and at a fixed temperature, usually 25A°C.
Specific mole types could darken during the period of their lifetime, specifically throughout puberty and pregnancy, and may even furthermore darken due to exposure to the sun. Furthermore, moles that initially emerge following age twenty, or that transform contour, coloration, measurement, or height, ought to be evaluated with a doctor to make sure that they aren’t malignant. Thus we have carried out the same reaction as we did using a single beaker, but this time the oxidative and reductive half-reactions are physically separated from each other. Because electrons from the oxidation half-reaction are released at the anode, the anode in a galvanic cell is negatively charged. In a galvanic (voltaic) cell, the energy from a spontaneous reaction generates electricity, whereas in an electrolytic cell, electrical energy is consumed to drive a nonspontaneous redox reaction. Any skin mole that oozes, bleeds, itches, or even gets inflamed ought to be examined by the specialist. The electrons that are released at the anode flow through the wire, producing an electric current.
Both types of cells use two electrodes that provide an electrical connection between systems that are separated in space. Galvanic cells therefore transform chemical energy into electrical energy that can then be used to do work. The oxidative half-reaction occurs at the anode, and the reductive half-reaction occurs at the cathode.
The electrodes are also connected by an electrolyte, an ionic substance or solution that allows ions to transfer between the electrode compartments, thereby maintaining the systema€™s electrical neutrality. A salt bridge connects the separated solutions, allowing ions to migrate to either solution to ensure the systema€™s electrical neutrality. A voltmeter is a device that measures the flow of electric current between two half-reactions. The potential of a cell, measured in volts, is the energy needed to move a charged particle in an electric field. An electrochemical cell can be described using line notation called a cell diagram, in which vertical lines indicate phase boundaries and the location of the salt bridge.



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