Electrochemistry half-cell

Oskaya, A.R. Conceptual difficulties experienced by prospective teachers in electrochemistry Half-cell potential, cell potential, and chemical and electrochemical equilibrium in Galvanic cells. Journal of Chemical Education 79 (2002), 735... 

BASIC CONCEPTS IN ELECTROCHEMISTRY 7.2.1 Half-Cell Reactions... 

Ozkaya (76) studied conceptual difficulties experienced by prospective teachers in a number of electrochemical concepts, namely half-cell potential, cell potential, and chemical and electrochemical equilibrium in galvanic cells. The study identified common misconceptions among student teachers from different countries and different levels of electrochemistry. Misconceptions were also identified in relation to chemical equilibrium, electrochemical equilibrium, and the instrumental requirements for die measurement of cell potentials. Learning difficulties were attributed mainly to failure of students to acquire adequate conceptual understanding, and the insufficient explanation of the relevant... 

Problems in this chapter include some brainbusters designed to bring together your knowledge of electrochemistry, chemical equilibrium, solubility, complex formation, and acid-base chemistry. They require you to find the equilibrium constant for a reaction that occurs in only one half-cell. The reaction of interest is not the net cell reaction and is not a redox reaction. Here is a good approach ... 

In the past, different sign conventions were used in electrochemistry, which led to difficulty in interpretation of experiments and results. Consequently the electrochemical literature requires an understanding of this problem to avoid confusion. The approach followed in this book is summarised in this section. As pointed out in the previous section, all electrochemical cells are regarded as a combination of two half cells, with each of the latter represented by a half reaction written as a reduction ... 

Electrodes or half-cells, used in electrochemistry can be diyided into following groups ... 

Each electrode reaction, anode and cathode, or half-cell reaction has an associated energy level or electrical potential (volts) associated with it. Values of the standard equilibrium electrode reduction potentials E° at unit activity and 25°C may be obtained from the literature (de Bethune and Swendeman Loud, Encyclopedia of Electrochemistry, Van Nostrand Reinhold, 1964). The overall electrochemical cell equilibrium potential either can be obtained from AG values or is equal to the cathode half-cell potential minus the anode half-cell potential, as shown above. 

Whereas the standard electrode potentials of many half-cell reactions have been known at ambient conditions and can be easily found in a number of reference books, almost none of them are documented for a region of high-temperature subcritical and supercritical conditions. Therefore, the creation of well-established approaches for developing a comprehensive list of the standard potentials measured over a wide range of temperatures remains a challenge for high-temperature experimental electrochemistry. The recently developed instruments for poten-tiometric studies at temperatures above 300 °C can be useful for developing such a database. 

Electrochemistry electrolytic and galvanic cells Faraday s laws standard half-cell potentials Nemst equation prediction of the direction of redox reactions... 

As already discussed, the standard hydrogen electrode (SHE) is the chosen reference half-cell upon which tables of standard electrode potentials are based. The potential of this system is zero by definition at all temperatures. Although this reference electrode was often used in early work in electrochemistry, it is almost never seen in chemical laboratories at the present time. It is simply too awkward to use because of the requirement for H2 gas at 1 bar pressure and safety considerations. 

The equilibrium electrochemistry of an element in aqueous solution can be represented graphically using coordinates of equilibrium half-cell potential, E, and pH. These graphical representations, known... 

Theoretical electrochemistry is concerned with developing models for these charge transfer processes and with deriving mathematical expressions based on these models from which values of the exchange current density may be calculated. It is sufficient for present purposes to examine one particularly simple model and derive, semiquantitatively, expressions for the exchange current density. Details of the model and the derivations are open to argument, but the result is of a mathematical form that is observed experimentally for a number of half-cell reactions. 

For most purposes in electrochemistry, it is sufficient to reference the potentials of electrodes (and half-cell emfs) arbitrarily to the NHE, but it is sometimes of interest to have an estimate of the absolute or single electrode potential (i.e., the potential of a free electron in vacuum). This interest arises, for example, if one would like to estimate relative potentials of metals or semiconductors based on their work functions. The absolute potential of the NHE can be estimated as 4.5 0.1 V, based on certain extrather-modynamic assumptions, such as about the energy involved in moving a proton from the gas phase into an aqueous solution (10, 29). Thus, the amount of energy needed to remove an electron from Pt/H2/H ( = 1) to vacuum is about 4.5 eV or 434 kJ. With this value, the standard potentials of other couples and reference electrodes can be expressed on the absolute scale (Figure 2.1.1). 

These equations (18.5,18.6) connect electrochemistry to the world of thermodynamics. It only remains to devise some way of determining the voltage associated with cells and half-cells, and we should then be able to determine free energies and perhaps other thermodynamic properties of cell reactions. 

The sign conventions of electrochemistry have caused students and researchers a great deal of difficulty and misunderstanding over the years. All electrochemical cells are considered as a combination of two half-cells—one for the reduction reaction, one for the oxidation reaction. To have current flow in any electrochemical system, both an oxidation and a reduction reaction must occur electrons must have someplace to go, they simply do not appear and disappear. 

At the heart of electrochemistry is the electrochemical cell. We will consider the creation of an electrochemical cell from the joining of two half-cells. When an electrical conductor such as a metal strip is immersed in a suitable ionic solution, such as a solution of its own ions, a potential difference (voltage) is created between the conductor and the solution. This system constitutes a half-cell or electrode (Fig. 15.1). The metal strip in the solution is called an electrode and the ionic solution is called an electrolyte. We use the term electrode to mean both the solid electrical conductor in a half-cell (e.g., the metal strip) and the complete half-cell in many cases, for example, the standard hydrogen electrode, the calomel electrode. Each half-cell has its own characteristic potential difference or electrode potential. The electrode potential measures the ability of the half-cell to do work, or the driving force for the half-cell reaction. The reaction between the metal strip and the ionic solution can be represented as... 

Another common reference cell found in the chemical-processing industry and useful in organic electrochemistry is the silver/silver chloride half-cell. The cell consists of silver metal coated with a paste of silver chloride immersed in an aqueous solution sam-rated with KCl and AgCl. The half-cell reaction is... 

You ve heard electrochemistry of corrosion as a lecture I shouldn t spend much time on it but I d like to describe some electrochemical effects for film formers. First the general principles. If you put a good electronic conductor (a metal) in an aqueous solution, you will typically find that an electrical potential is developed between the piece of conductor and the solution. When ions of the metal enter the solution and leave extra electrons behind a negative potential is developed. All oxidation reactions occurring on the surface are expected to produce this result. Similarly, reduction reactions that use electrons from the metal are expected to produce a more positive potential in the metal. The solution potential of the metal influences the rate of an electrochemical half-cell reaction in accordance with Le Chatelier s Principle, so it is possible to predict through the use of the Nernst Equation the potential that will exist when the only significantly rapid reactions are the oxidation and reduction parts of a reversible reaction. When more than one potentially reversible process occurs, the rate of oxidation will be expected to exceed the rate of reduction for at least one and the converse for at least one. At... 

In conventional electrochemistry, a half cell is always combined with another (reference half cell), and its electrochemical equilibrium is investigated through the cell voltage (EMF). In contrast, the half cell of the present... 

The first two reactions we discussed in this chapter were the anodic and cathodic reactions for steel in concrete. The terms anode and cathode come from electrochemistry which is the study of the chemistry of electrical cells. Figure 2.5 is a basic Daniell cell which is used at high school to illustrate how chemical reactions produce electricity. The cell is composed of two half cells , copper in copper sulphate and zinc in zinc sulphate. The total voltage of the cell is determined by the metals used and by the nature and composition of the solutions. What is happening is that in each half cell the metal is dissolving and ions are precipitating, that is. 

This chapter has discussed the mechanism of what happens at the steel surface. The chemical reactions, formation of oxides, pitting, stray currents, bacterial corrosion, anodes, cathodes and reference electrode potentials (half cells) have been reviewed. A more detailed account of the electrochemistry of corrosion and corrosion of steel in concrete is given in Appendix B. Chapter 3 will discuss the processes that lead to the corrosion and the consequences in terms of damage to structures. We will then move on to the measurement of the problem and how to deal with it. 

The electrochemistry of corrosion, cells and half cells were discussed in Section 2.4. The standard reference electrode or half cell is a simple device. It is a piece of metal in a fixed concentration solution of its own ions (such as copper in saturated copper sulphate, silver in silver chloride, etc.). If we connect it to another metal in a solution of its own ions (such as iron in... 

With an external DC power supply connected to the electrolytic cell, the applied voltage that gives no DC current flow in the external circuit corresponds to the equilibrium potential of the half-cell (or actually the cell). It is the same voltage as read by a voltmeter with very high input resistance and virtually no current flow (pH meter). In electrochemistry, potentiometry is to measure the potential of an electrode at zero current flow, which is when the cell is not externally polarized. To understand the equilibrium potential with zero external current, we must introduce the concept of electrode reaction... 

From electrochemistry, our discipline has borrowed important terminology. One electrode with electrolyte is called a half-cell to underline that one electrode is not enough. Two electrodes, an electrode pair, are needed to close the electric circuit so that electric current can flow (CC electrodes). A whole cell is two electrodes both submerged in the same electrolyte (e.g., in a glass dish). 

The term electrode is widely used in electrochemistry. However, it designates objects that can significantly vary depending on the situation. For the purposes of this document, in examples chosen to illustrate simple electrochemical systems, the term will most often refer to the metal which constitutes one of the terminals in the system in question. For instance, a platinum electrode or a copper rotating disc electrode will be mentioned. When the system includes more than three materials, then the term electrode usually refers to the whole set of successive materials inserted between the metallic ending and the electrolyte material which makes up the core of the system. For instance, the term modified electrode will be used to refer to a metal whose surface has been covered with a film of conducting material or the term positive electrode in a battery will be used to refer to the composite material which is in contact with the electrolyte. In a third context, the term electrode will be used for an electrochemical half-cell this is the case with the electrode or reference electrode. In the final version of its meaning, the term electrode even stands for two half-cells combined to form the device, e.g., in the case of commercial systems for pH measurements by means of a combined electrode... 

Common experience reveals that iron easily oxidizes (rusts) but resists reversal back to the iron base metal. In electrochemistry, this process is known as electrode polarization. A result of polarization is higher electrode resistance to current flow in one direction versus the other. With polarization, the electrode half-cell potential value also tends to vary from table values and depends on the direction and magnitude of electrode cunent flow. It is desirable fw electrodes to be electrochemically reversible since this prevents the process Of polarizatimi. Electrode polarization is a problem in biopotential measurements because it is associated with electric instability. It gives rise to offset potentials between electrodes, electrode noise, and high resistance. 

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