Electrochemical cell electron movement

A characteristic feature of an electrochemical cell is that the electronic current, which is the movement of electrons in the external circuit, is generated by the electrochemical processes at the electrodes. In contrast to the electronic current, the charge is transported between the positive and the negative electrode in the electrolyte by ions. Generally the current in the electrolyte consists of the movement of negative and positive ions. 

A simple electrochemical cell can be made from two test tubes connected with a third tube (the crossbar of the H ), as shown in Figure 12-1. The hollow apparatus is filled by simultaneously pouring different solutions into the two test tubes, an aqueous solution (aq) of zinc sulfate into the left tube and a copper sulfate solution into the one on the right. Then a strip of zinc metal is dipped into the ZnS04 solution, a piece of copper is inserted into the CUSO4 solution, and the two ends of the metal strips are connected by wires to an voltmeter. The lateral connecting tube allows ionic migration necessary for a closed electrical circuit. The voltmeter will show the electrical potential of 1.10 volts, which leads to the movement of electrons in the wire from the zinc electrode toward the copper electrode. 

An electrochemical cell generally consists of two half-cells, each containing an electrode in contact with an electrolyte. The electrode is an electronic conductor (such as a metal or carbon) or a semiconductor. Current flows through the electrodes via the movement of electrons. An electrolyte is a phase in which charge is carried by ions. For example, a solution of table salt (sodium chloride, NaCl) in water is an electrolyte containing sodium cations (Na+) and chloride anions (CE). When an electric field is applied across this solution, the ions move Na+ toward the negative side of the field and CE toward the positive side. 

So far the spontaneous functioning of an electrochemical cell has been described, which corresponds to the transformation of energy obtained in a chemical reaction into electron movement, that is electrical energy. This type of cell is a galvanic cell. 

In cyclic voltammetry, both the oxidation and reduction of the metal complex (called the analyte from now on) will take place in one electrochemical cell. This cell houses the analyte solution as well as three electrodes, the working electrode, the auxiliary electrode and the reference electrode. Electron transfer to and from the metal complex takes place at the working electrode surface (Fig. A.2.2) and does so in response to an applied potential, /iapp, at the electrode surface. During the experiment, current develops at the surface as a result of the movement of analyte to and from the electrode as the system strives to maintain the appropriate concentration ratio (0, through electron transfer, as specified by the Nemst equation. 

A well-known property of metals is their ability to carry an electrical current by means of the movement of the electronic cloud associated with the metal atoms. Most metals are solids at room temperature and only a few, such as mercury, are liquid. Because of their importance in the design of electrochemical cells, their electronic properties are considered here in some detail. 

Oxidation-reduction (redox) reactions Involve the movement of electrons. The half-reaction method of balancing a redox reaction separates the overall reaction into two half-reactions. This reflects the actual separation of the two half-cells in an electrochemical cell... 

Whether an electrochemical process releases or absorbs free energy, it always involves the movement of electrons from one chemical species to another in an oxidation-reduction (redox) reaction. In this section, we review the redox process and describe the half-reaction method of balancing redox reactions. Then we see how such reactions are used in electrochemical cells. 

Electrolyte - A nonmetallic (liquid or solid) conductor that carries current by the movement of ions (instead of electrons) with the liberation of matter at the electrodes of an electrochemical cell. 

A dc electrochemical cell consists of two electrical conductors called electrodes, each immersed in a suitable electrolyte solution. For a current to develop in a cell, it is necessary (1) that the electrodes be connected externally with a metal conductor, (2) that the two electrolyte solutions be in contact to permit movement of ions from one to the other, and (3) that an electron-transfer reaction can occur at each of the two electrodes. Figure 22-la shows an example of a simple electrochemical cell. It consists of a silver electrode... 

Oxidation-reduction (redox) reactions involve the movement of electrons from one reactant to another. The half-reaction method of balancing redox reactions separates the overall reaction into two half-reactions, which mimics the actual separation of an electrochemical cell into two half-cells. Two types of electrochemical cells are distinguished by whether they generate electrical energy (voltaic) or use it (electrolytic). In both types of cell, electrodes dip into an electrolyte solution, the oxidation half-reaction occurs at the anode, and the reduction half-reaction occurs at the cathode. (Section 21.1)... 

Electrochemistry is concerned with the interconversion of electrical and chemical energy. Electrical effects occur as a result of the movement of electrical charge, either as mobile ions in an aqueous solution or melted liquid or as delocalized electrons in a conductor. Electrolytic and voltaic cells are the two types of electrochemical cells. 

In an electrochemical cell the coupling between electric current and a chemical reaction occurs in a boundary layer between electrolyte and electrode. Assume an electrochemical cell through which a constant electric current passes. In the electrolyte the electric current is carried by ions, the movement of which involves transport of matter at the same time. In the electrodes the electric current is carried by electrons. Therefore, when the current passes through the boundary layer between electrolyte and electrode, a change occurs in the nature of the chargecarrying particles as shall be seen, this change involves oxidation or reduction of compounds. 

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