Chemistry
Electrochemistry
Electrochemistry is the study of the relationship between chemical reactions and electricity. It involves the movement of electrons in chemical reactions, and the conversion of chemical energy into electrical energy, and vice versa. Key concepts in electrochemistry include redox reactions, electrolysis, and electrochemical cells, which have applications in batteries, corrosion prevention, and industrial processes.
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eBook - ePubElectrical Engineering
Fundamentals
- Viktor Hacker, Christof Sumereder(Authors)
- 2020(Publication Date)
- De Gruyter Oldenbourg(Publisher)
6 Electrochemistry6.1 Basic electrochemical concepts
With special regard to electrical engineering, this chapter covers the branch of Electrochemistry that deals with the generation and storage of electric current. The electrochemical oxidation and reduction reactions take place at the phase boundaries of the electrode and the electrolyte.Galvanic cellChemical energy is transformed into electrical energy, current is produced, and electrochemical reactions take place spontaneously (negative free enthalpy). Galvanic cells are categorised into three subgroups:- Primary cells
- Secondary cells
- Fuel cells
Electrolytic cellElectric energy is transformed into chemical energy. Two electrodes made of electron-conducting material, and the electrolytes with ion conductivity are conductively connected62 to each other. At the two spatially separated electrodes electrochemical reactions take place.Half-cellA half-cell consists of one single electrode and an electrolyte into which the electrode is submerged (e.g. copper in a copper sulphate solution). If a (metal) electrode is submerged into a metal salt solution (same metal), the surface of the electrode becomes charged. With base metals (e.g. zinc) some metal atoms enter the solution and the released electrons stay on the surface of the electrode, which is now negatively charged. The positively charged metal ions remain bound to the negatively charged metal surface. Thereby an electrical double layer is formed where the negative and the positive charges balance each other out. When two half-cells are combined, a galvanic cell (connected through ionic conductor and electron conductor) is formed.Anode
eBook - ePubExtractive Metallurgy 1
Basic Thermodynamics and Kinetics
- Alain Vignes(Author)
- 2013(Publication Date)
- Wiley-ISTE(Publisher)
Chapter 8Electrochemical Reactions
8.1. Overview of electrochemical processes
A chemical reaction is a reaction where only chemical species (neutral molecules and positively or negatively charged ions) are involved. An electrochemical reaction is a reaction where chemical species and free electrons are involved. The two elementary electrochemical reactions are:– oxidation : liberation of electrons. For a metal (in the solid state) immersed in an electrolytic solution, the reaction is a dissolution, which gives a metallic ion in solution (corrosion):[8.1.1]– reduction : absorption of electrons. For a metallic ion in an aqueous solution (heterogeneous precipitation, see Chapter 5 , section 5.6.1 ):[8.1.2]Various overall reactions are the result of elementary electrochemical reactions:– redox chemical reactions resulting from two simultaneous elementary electrochemical reactions where the electrons are directly transferred between the reactants. They are homogeneous chemical reactions (see Chapter 5 , section 5.2.1.1
- Kevin Reel, Derrick C. Wood, Scott A. Best(Authors)
- 2014(Publication Date)
- Research & Education Association(Publisher)
13Electrochemistry
Application of Redox
Electrochemistry is the application of oxidation and reduction half reactions. There are numerous oxidation–reduction (redox) reactions that occur spontaneously, such as the displacement of silver metal when a copper wire is submerged in a silver nitrate solution.Thermochemistry is closely associated with Electrochemistry. The Gibbs free energy, ΔG, not only indicates whether a reaction is spontaneous or nonspontaneous, it also is a measure of the amount of work that can be done by a system on its surroundings. Although the preceding reaction is spontaneous, the free energy of this reaction cannot be harnessed.In Electrochemistry, there are two different types of electrochemical cells:1. A voltaic cell (galvanic cell) is commonly called a battery. This type of cell contains a spontaneous chemical reaction that produces electricity and supplies it to an external circuit.2. Nonspontaneous reactions give rise to an electrolytic cell . This cell uses electrical energy from an external source to force a nonspontaneous redox reaction to occur.DID YOUKNOW?The coating of metals onto a second surface is accomplished through a process called electroplating . Chrome—which is found on car bumpers, motorcycle frames, and exhausts—is a thin layer of chromium plated onto steel. The chromium not only adds an attractive silvery shine, but also inhibits corrosion.Voltaic Cells
Electrical conduction in chemistry occurs through the motion of electrons through a metallic medium or by the movement of ions in an aqueous solution or molten salt. In order to harness the free energy of a spontaneous redox reaction, the two half-reactions of the redox reaction are separated into two half-cells. These half-cells consist of an electrode and a solution of the ion involved in either the oxidation or reduction process. Electrodes are surfaces upon which a reduction or oxidation half-reaction occurs. They may participate in the reaction or simply serve as a medium through which electrons travel (inert electrode). The mnemonic “Red Cat and An Ox” is useful to help remember that reduction occurs at the cathode (positive electrode) and oxidation at the anode (negative electrode). See Figure 13.1
eBook - ePub- John Kenkel(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
HAPTER 11ELECTROANALYTICAL METHODS11.1 INTRODUCTION
The subject of electroanalytical chemistry encompasses all analytical techniques which are based on electrode potential and current measurements at the surfaces of electrodes immersed in the solution tested. Either an electrical current flowing between a pair of immersed electrodes or an electrical potential developed between a pair of immersed electrodes is measured and related to the concentration of some dissolved species.Electroanalytical techniques are an extension of classical oxidation-reduction chemistry, and indeed oxidation and reduction processes occur at or within the two electrodes, oxidation at one and reduction at the other. Electrons are consumed by the reduction process at one electrode and generated by the oxidation process at the other. The complete system is often called a “cell,” the individual electrodes “half-cells,” and the individual oxidation and reduction reactions are the “half-reactions.” Electrons flow on a conductor between the half-cells, and this flow constitutes the electrical current that is often measured. A “galvanic” cell is one in which this current flows spontaneously because of the strong tendency for the chemical species involved to give and take electrons. A battery that has its positive and negative poles externally connected is an example of a such a cell. An “electrolytic” cell is one in which the current is not a spontaneous current, but rather is the result of connecting an external power source, such as a battery, to the system. A rechargeable battery, when it is positioned in the recharging unit, would be an example of such a cell (see Figure 11.1 ). Electroanalytical techniques utilize both general types of cells.FIGURE 11.1
- (Author)
- 2013(Publication Date)
- Elsevier(Publisher)
If two in each other immiscible liquids (one of them ionic solution) contact, then segregations at the interface occur and electric potential between both phases is generated. This is the case, e.g., when liquid metal contacts with liquid slag. Using electrochemical measurements the reaction course, reaction kinetics and adsorption mechanisms can be investigated. In chemical practice, current–potential dependencies of interfacial tension are frequently recorded. Their course and characteristic points deliver the information about the adsorption phenomena at the interface. Measurements using a.c. current and using nonstationary methods as, e.g., galvano or potentiostatic impulse, are applied to get more information about the nature of the adsorbed layers and the migration of ions near the interface. Electrochemical potential and electrochemical double layer are shortly discussed as well as the effect of adsorption on the electric potential and the interfacial tension. Finally some practical aspects of electrocapillarity phenomena are presented.1.6.1.1 Electrochemical Equilibria
An electrochemical system contains usually two phases, which are separated by the interface. In the metallurgical systems these are the liquid metal α , which is treated as the electrode, and liquid slag β , which is treated as the electrolyte. In the electrode, the atoms and electrons are transported and in the electrolyte the ions, some neutral molecules and under certain conditions also electrons.The interfaces can be polarized and nonpolarized. The charge exchange at the full polarized interfaces is not possible and they behave as condenser. The capacity depends among other things on the degree of polarization. In real systems, the interfaces are partially not polarized. In this case, the electrons and ions can partially move throughout the interface. Even at small electric potentials high currents can be measured [1 ]. The resulting difference of the Galvani potentials Δφ αβ of both phases is therefore characteristic for the analyzed system and depends on the chemical composition of phases and the temperature [2 ,3 ].When both phases come into contact the electrochemical reactions at the interface starts. For example, the cations of the electrode will be transferred to the electrolyte with parallel negative charge of the electrode. This negative charge of the electrode prevents the generation of new cations and their transfer to the electrolyte. On the side of the electrolyte the cations or anions generate adsorption layers at the interface (see Section 1.6.1.2 ). This process is running until no more current is transferred through the interface [3 ]. At this stage, the electrochemical equilibrium is reached, the double layer is developed and the Galvani potentials Δφ αβ (or contact potential) becomes a stable value. The electrochemical equilibrium is therefore then reached, when besides the chemical potential equilibrium resulting from reaction or mass transport also the electrical potentials of reacting species are taken into account for the estimation of equilibrium. Therefore the energy equation (1.1.10 –1.1.12 ) in the Section 1.1.2 and the chemical potentials μ i of the species i
eBook - ePubCorrosion Engineering
Principles and Solved Problems
- Branko N. Popov(Author)
- 2015(Publication Date)
- Elsevier(Publisher)
The electrochemical corrosion process consists of two partial electrochemical reactions: the anodic partial reaction, consisting of oxidation/dissolution of the metal, and the cathodic partial reaction, consisting of the reduction of water, hydrogen, or oxygen gas. The energy change of the partial corrosion reactions provides a driving force for the process and controls its direction. Electrochemical corrosion reactions have different thermodynamic and kinetic properties than chemical reactions. For example, if a redox reaction proceeds as a chemical reaction, it is necessary for the reacting particles to come into contact with each other so that electrons can be transferred from one reactant to the other. Thermodynamically, the reaction is controlled by the ratio of the internal energy of the reactants to their activation energy. The collisions of the particles are not limited in the reaction space and may occur in any direction. As a consequence, the electrons also move in any direction in the reaction space. The only requirement is that the path of the charge transfer must be very small. With an electrochemical reaction, the activation energy of corrosion reactions and their kinetic properties depend not only on activity, chemical potential, and temperature, but also on the electrocatalytic properties of the materials.The thermodynamics of corrosion processes provides a tool to determine the theoretical tendency of metals to corrode. Thus, the role of corrosion thermodynamics is to determine the conditions under which the corrosion occurs and how to prevent corrosion at the metal/environment interface. Thermodynamics, however, cannot be used to predict the rate at which the corrosion reaction will proceed [1 –6 ]. The corrosion rate must be estimated by Faraday’s law and is controlled by the kinetics of the electrochemical reaction.Depending on the nature of the metal, the oxidation reaction may occur uniformly (as in carbon steel) or may be localized (as in hard alloys such as Inconel or Monel). In localized corrosion (pitting), the corrosion proceeds through the formation of narrow cracks after penetrating the grains of the metal. Corrosion occurs along the grain boundaries of the metal, known as intercrystalline corrosion. Besides the hydrogen evolution reaction, there are other cathodic depolarization reactions such as:Oxygen reduction in alkaline or neutral solutions :O 2+ 2H 2O + 4e −→ 4OH −(2.1)Oxygen reduction in acidic solutions :O 2+ 4H ++ 4e −→ 2H 2O(2.2)Metal deposition in galvanic corrosion :M ++e −→ M(2.3)Reduction of metal ions :M+3 +e −→M2 +The overall anodic reaction for the corrosion of iron in neutral or alkaline solutions is described as:(2.4)
