Oxidation-reduction reactions electrochemical cells

In an electrochemical cell, electrical work is obtained from an oxidation-reduction reaction. For example, consider the process that occurs during the discharge of the lead storage battery (cell). Figure 9.3 shows a schematic drawing of this cell. One of the electrodes (anode)q is Pb metal and the other (cathode) is Pb02 coated on a conducting metal (Pb is usually used). The two electrodes are immersed in an aqueous sulfuric acid solution. 

Analytical methods based upon oxidation/reduction reactions include oxidation/reduction titrimetry, potentiometry, coulometry, electrogravimetry and voltammetry. Faradaic oxidation/reduction equilibria are conveniently studied by measuring the potentials of electrochemical cells in which the two half-reactions making up the equilibrium are participants. Electrochemical cells, which are galvanic or electrolytic, reversible or irreversible, consist of two conductors called electrodes, each of which is immersed in an electrolyte solution. In most of the cells, the two electrodes are different and must be separated (by a salt bridge) to avoid direct reaction between the reactants. 

Oxidation—reduction reactions, commonly called redox reactions, are an extremely important category of reaction. Redox reactions include combustion, corrosion, respiration, photosynthesis, and the reactions involved in electrochemical cells (batteries). The driving force involved in redox reactions is the exchange of electrons from a more active species to a less active one. You can predict the relative activities from a table of activities or a halfreaction table. Chapter 16 goes into depth about electrochemistry and redox reactions. 

The thermodynamic criterion for spontaneity (feasibility) of a chemical and electrochemical reaction is that the change in free energy, AG have a negative value. Free-energy change in an oxidation-reduction reaction can be calculated from knowledge of the cell voltage ... 

Virtually all energy transductions in cells can be traced to this flow of electrons from one molecule to another, in a downhill flow from higher to lower electrochemical potential as such, this is formally analogous to the flow of electrons in a battery-driven electric circuit. All these reactions involving electron flow are oxidation-reduction reactions one reactant is oxidized (loses electrons) as another is reduced (gains electrons). 

Batteries are electrochemical cells. Where would we be without batteries A battery is needed to start a car. Batteries power flashlights, move toys, and make watches work. Jewelry with lightbulb designs can use tiny batteries. A battery provides an electric current through oxidation-reduction reactions in which the flow of electrons is directed through a wire. The force of the electrons through the wire is measured in volts. 

In an electrochemical cell, the oxidation reduction reactions initially proceed at a constant rate. Usually the reaction rate is appropriate for uniform conversion of metal ions to metal atoms at the cathode and even metal coating of an object to be plated. However, sometimes it is necessary to change the rate of metal atom deposit. This can only be accomplished in an electrolytic cell where an external voltage source controls metal atom deposit. 

An electrochemical cell reaction, like any oxidation-reduction reaction, can be written as the sum of an oxidation half-reaction and a reduction half-reaction. In the case of a cell, these half-reactions correspond to the reactions at the two electrodes. Since the cell reaction is the sum of the half-cell reactions, it is convenient to think of dividing the cell potential into half-cell potentials. Unfortunately, there is no way of measuring a half-cell potential—we always need two half-cells to make a cell, the potential of which is measurable. By convention, the half-cell reaction,... 

A battery is a complex device that delivers electrical energy by transforming chemical energy. The electrical energy is provided by electrochemical reactions (oxidation-reduction reactions) that take place at the anode and the cathode of the battery. While the term battery is often used, the basic electrochemical unit being referred to is the cell [1]. A battery is composed of several cell units that are connected in series or in parallel... 

Many oxidation/reduction reactions can be carried out in either of two ways that are physically quite different. In one, the reaction is performed by bringing the oxidant and the reductant into direct contact in a suitable container. In the second, the reaction is carried out in an electrochemical cell in which the reactants do not come in direct contact with one another. A marvelous example of direct contact is the famous silver tree experiment, in which a piece of copper is immersed in a silver nitrate. solution (Figure 18-1). Silver ions migrate to the metal and are reduced ... 

We learn much about chemical reactions from the study of electrochemistry. The amount of electrical energy consumed or produced can be measured quite accurately. All electrochemical reactions involve the transfer of electrons and are therefore oxidation-reduction reactions. The sites of oxidation and reduction are separated physically so that oxidation occurs at one location, and reduction occurs at the other. Electrochemical processes require some method of introducing a stream of electrons into a reacting chemical system and some means of withdrawing electrons. In most applications the reacting system is contained in a cell, and an electric current enters or exits by electrodes. 

The electrochemical approach uses a sterilizable stainless steel probe with a cell face constructed of a material which will enable oxygen to permeate across it and enter the electrochemical chamber which contains two electrodes of dissimilar reactants (forming the anode and cathode) immersed in a basic aqueous solution (Fig. 2). The entering oxygen initiates an oxidation reduction reaction which in turn produces an EMF which is amplified into a signal representing the concentration of oxygen in the solution. 

Many oxidation-reduction reactions may be carried out in such a way as to generate electricity. Such an arrangement for the production of an electric current is called a galvanic or electrochemical cell. In principle, this may always be done for spontaneous, aqueous, oxidation-reduction reactions for which the following experimental requirements are met. 

We saw earlier that one can predict the thermodynamic open circuit (zero current) potential of an electrochemical cell by combining two half-cell reactions, one for the anode and the second for the cathode. The half-cell reaction with the lower (i.e., more negative) equilibrium potential will proceed spontaneously in the anodic direction, where the electrode acts as an electron sink for the anodic de-electronation (oxidation) reaction and the higher equilibrium potential reaction will occur spontaneously at the cathode, where the electrode acts as an electron source for the electronation (reduction) reaction. A cell with spontaneous reactions at the anode and cathode is called a self-... 

Solid materials, in general, are more or less subject to corrosion in the environments where they stand, and materials corrosion is one of the most troublesome problems we have been frequently confronted with in the current industrialized world. In the past decades, corrosion science has steadily contributed to the understanding of materials corrosion and its prevention. Modem corrosion science of materials is rooted in the local cell model of metallic corrosion proposed by Evans [1] and in the mixed electrode potential concept of metallic corrosion proved by Wagner and Traud [2]. These two magnificent achievements have combined into what we call the electrochemical theory of metallic corrosion. It describes metallic corrosion as a coupled reaction of anodic metal dissolution and cathodic oxidant reduction. The electrochemical theory of corrosion can be applied not only to metals but also to other solid materials. 

It is useful to note that there are other methods for supplying electrons to complexes that will lead to a change in oxidation state. Whereas oxidation-reduction reactions are partnerships between two compounds, an alternative is to offer a direct source of or sink for electrons this is achieved by electrodes in an electrochemical cell. At the appropriate potential, a... 

Review of Oxidation-Reduction Concepts Half-Reaction Method for Balancing Redox Reactions Electrochemical Cells... 

A battery is a collection of one or more electrochemical cells that convert chemical energy into electrical energy via electrochemical reactions (oxidation-reduction reactions). These reactions take place at the battery s anode and cathode. The electrochemical cells are connected in series or in parallel depending on the desired voltage and capacity. Series connections provide a higher voltage, whereas parallel connections provide a higher capacity, compared with one cell. 

Electrochemistry is ranked by teachers and students as one of the most difficult curriculum domains taught and learnt in secondary school chemistry (cf. Davies, 1991 Griffiths, 1994). For that reason, in this chapter, we primarily discuss this domain at the secondary level but also make connections to the tertiary level. In many chemistry curricula and textbooks, it is common to divide electrochemistry into two topics redox reactions (oxidation and reduction) and electrochemical cells (galvanic and electrolytic). The usual rationale for this distinction is that students need an understanding of oxidation-reduction to apply it to electrochemical cells. This analytical distinction, based on differences in the location of the half reactions, is used throughout the chapter. 

This is a reduction reaction because the positively charged metal ions have gained electrons, lost their charge, and become neutral atoms. The neutral atoms deposit on the electrode, a process called electrodeposition. This electrode is termed a cathode. At the cathode, reduction of an electroactive species takes place. An electroactive species is one that is oxidized or reduced during reaction. Electrochemical cells also contain nonelectroactive (or inert) species such as counterions to balance the charge, or electrically conductive electrodes that do not take part in the reaction. Often these inert electrodes are made of Pt or graphite, and serve only to conduct electrons into or out of the half-cell. 

Many chemical oxidation-reduction reactions can be performed in an electrochemical cell in which electrons liberated by oxidation of a species at one electrode (anode) flow through an external conductor to a second electrode (cathode) where they are consumed by a reduction reaction. A corresponding ionic current flows through the electrolyte that separates the electrodes in the electrochemical cell. In such a device. 

An electrochemical cell that produces a current from an oxidation-reduction reaction is often called a(n) cell. 

A DMFC (direct methanol fuel cell) converts the chemical energy stored in liquid methanol to usable electrical energy by a direct electrochemical oxidation-reduction reaction. 

The following oxidation-reduction reactions are used in electrochemical cells. Write them using cell notation. 

This reaction leaves a surplus of negatively charged electrons on the zinc electrode, so that it has a negative charge. It is an oxidation half-reaction. The electrode at which oxidation occurs in an electrochemical cell is always called the anode. If the two electrodes are coimected, electrons will flow from the zinc anode to the carbon cathode. Such a flow of electrons is an electrical current from which useful work can be extracted. Every time that 2Mn02 molecules are reduced, as shown in the first half-reaction, IZn is oxidized. Therefore, the overall oxidation-reduction reaction is obtained by multiplying everything in the reduction half-reaction at the... 

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