Chemical reactions from electricity

Etectrotytic celts Getting chemical reactions from electricity... 

Fuel cells have attracted considerable interest because of their potential for efficient conversion of the energy (AG) from a chemical reaction to electrical energy (AE). This efficiency is achieved by directly converting chemical energy to electricity. Conventional systems burn fuel in an engine and convert the resulting mechanical output to electrical power. Potential applications include stationary multi-megawatt power plants, battery replacements for personal electronics, and even fuel-cell-powered unmanned autonomous vehicles (UAVs). 

A pacemaker obtains electrical energy from a tiny battery that lasts for about seven years before it must be replaced. But how do batteries supply electrical energy The answer lies in a branch of chemistry known as electrochemistry. In this unit, you will learn about the connection between chemical reactions and electricity. You will also learn about the chemical reactions that take place inside batteries. 

Electrochemistry — Electrochemistry, as the name suggests, is a branch of chemical science that deals with the interrelation of electrical and chemical phenomena [i]. From the very beginning electrochemistry covers two main areas the conversion of the energy of chemical reactions into electricity (electrochemical power sources) and the transformations of chemical compounds by the passage of an electric current (-> electrolysis). 

Electrical work is produced by an electrochemical cell only if AG < 0, or when > 0, which amounts to the same thing. This relationship provides a direct way to determine free energy changes for chemical reactions from measurements of cell voltage. 

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. 

Since very little is known about the true nature of chemical reactions involving ions, electrons, radicals, photons, etc., it is not possible to predict a priori what would be expected from one type of device or another under specified conditions. For that matter, in most studies of chemical reactions in electrical discharges, very little is known or reported... 

There are two types of electrochemical cells voltaic (galvanic), which produce energy from a chemical reaction, and electrolytic (voltammetric), which require or use up energy. In voltaic cells, a spontaneous chemical reaction produces electricity. These cells are important in potentiometry. In electrolytic cells, electrical energy is used to force a chemical reaction to take place such as in voltammetry. In summary ... 

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. 

Fig. 3 Conversion of the energy from chemical reactions to electrical...

Fuel cell electrochemical reactions convert free energy change associated with the chemical reaction into electrical energy directly. The Gibbs free energy change in a chemical reaction is a measure of the maximmn net work obtainable from a chemical reaction [24]. 

Fuel cell is a device to convert Gibbs free energy in chemical reaction into electricity through electrochemical cell reactions. In an H2-O2 fuel cell, electricity is obtained through formation of water from O2 and H2. When an acidic electrolyte is used, electrochemical oxidation of H2 to e and H" occurs at an anode and reduction of O2 with e and to H2O occurs at a cathode. The net reaction is formation of water from H2 and O2. In other words, catalytic reaction of water formation can be decomposed to two electrochemical reactions at an anode and cathode. This principle indicates that catalytic oxidation and reduction in chemical synthesis can convert fuel cell reactions at an anode and cathode. For example, the Wacker oxidation of ethylene to acetaldehyde with O2 would be able to perform using fuel cell reactions. 

Thermal energy (high or low temperature) resulting from mechanical work, radiation, chemical reaction, or electrical resistance... 

The PACVD process (piasma assisted chemicai vapour deposition) is a refinement of the CVD process, with the advantage of much iower process temperatures below 200 °C. This process is therefore becoming more popular in plastic mold making, since there is aimost no risk of distortion, uniike with CVD. The chemical reaction from the gas phase at low temperature is enabied by a strong electric field between the mold and the backplate electrode. 

According to the definition, electrochemical cells are the devices transferring electrical energy from chemical reactions into electricity, or helping chemical processes through the introduction of electrical energy or electrical field. A common example in this category is battery, which has evolved into a big family and is currently used in all kinds of applications. 

Heat and temperature — Overexposure to heat and temperature extremes may result in a range of injuries from bums to frostbite. Temperature indicates the level of heat present. The second law of thermodynamics states that heat will flow from an area of higher temperature to one of lower temperature. Heat is produced as a result of chemical reaction, combustion, electrical current, mechanical motion, and metabolism. Heat is transferred by ... 

A secondary battery produces electricity from a reversible chemical reaction. When electricity is passed through the battery in the reverse direction the battery is recharged. 

Modelling plasma chemical systems is a complex task, because these system are far from thennodynamical equilibrium. A complete model includes the external electric circuit, the various physical volume and surface reactions, the space charges and the internal electric fields, the electron kinetics, the homogeneous chemical reactions in the plasma volume as well as the heterogeneous reactions at the walls or electrodes. These reactions are initiated primarily by the electrons. In most cases, plasma chemical reactors work with a flowing gas so that the flow conditions, laminar or turbulent, must be taken into account. As discussed before, the electron gas is not in thennodynamic equilibrium... 

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