Electricity from a chemical reaction

Electron-transfer reactions occur all around us. Objects made of iron become coated with mst when they are exposed to moist air. Animals obtain energy from the reaction of carbohydrates with oxygen to form carbon dioxide and water. Turning on a flashlight generates a current of electricity from a chemical reaction in the batteries. In an aluminum refinery, huge quantities of electricity drive the conversion of aluminum oxide into aluminum metal. These different chemical processes share one common feature Each is an oxidation-reduction reaction, commonly called a redox reaction, in which electrons are transferred from one chemical species to another. 

Batteries produce electricity from a chemical reaction. So to recharge a battery, we would have to be able to reverse the chemical reaction by applying an electric current. In truth most batteries use chemical reactions that could be reversed, but there are efficiency considerations that come into play. Rechargeable batteries make use of reactions that can be reversed easily, many times, without too much degradation of the battery materials. One-time-use batteries typically use reactions that could only be reversed and reused a couple of times before they would not work very well anymore. 

For an electrical current to flow and light up the bulb shown in Figure 8.5, there must be a voltage difference (difference in electrical potential) between the iron and copper bar. These bars are used to transfer electrons between a wire and solution—and in this case, they actually participate in the oxidation-reduction reaction that occurs. The metal bars are called electrodes. Because this cell is used to generate a voltage and to extract electricity from a chemical reaction, it is called a voltaic cell. 

A primary battery produces electricity from a chemical reaction that caimot be reversed. As a result the battery cannot be recharged. 

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). 

The problem was solved by Francis Bacon, a British scientist and engineer, who developed an idea proposed by Sir William Grove in 18.39. A fuel cell generates electricity directly from a chemical reaction, as in a battery, but uses reactants that are supplied continuously, as in an engine. A fuel cell that runs on hydrogen and oxygen is currently installed on the space shuttle (see Fig. L.l). An advantage of this fuel cell is that the only product of the cell reaction, water, can be used for life support. 

The amount of heat that is absorbed or liberated in a physical or chemical change can be measured in a well-insulated vessel called a calonmeter (Figure 14-1) Calorimetry is based on the principle that the observed temperature change resulting from a chemical reaction can be simulated with an electrical heater The electrical measurements of current (/), heater resistance (R), and duration ( ) of heating make it possible to calculate how much heat is equivalent... 

When electricity passes through a circuit consisting of both types of electrical conductors, a chemical reaction always occurs at their interface. These reactions are electrochemical. When electrons flow from the electrolytic conductor, oxidation occur at the interface while reduction occurs if electrons flow in the opposite direction. These electronic-electrolytic interfaces are referred to us electrodes, interfaces where oxidation occurs are known as anodes and those ai which reduction occurs, as cathodes. An anode is also defined as that electrode by which "conventional" current enters an electrolytic solution, a cathode as that electrode by which "conventional" current leaves. Positive ions, for example, ions of hydrogen and the metals, are called cations while negative ions, for example, acid radicals and ions of nonmctals. are called anions. 

Information technology has revolutionized daily life in the last decades and the continuously increasing amount of data to be stored and manipulated strongly stimulated the search for switching and memory elements as tiny as a single molecule. Molecular switches can be converted from one state to another by an external stimulus such as light, electricity or a chemical reaction. Like with their macroscopic counterparts, one is able to control numerous functions and properties of materials and devices. 

It is in this sense it is said that in an electrochemical energy converter, the ideal maximum efficiency is 100% for, as in the above idealized situation, if one could carry out reactions in such a way that the electrode potentials were infinitely near the equilibrium values, the electrical energy one could draw2 from the reaction would be nFVe and this is all of the free-energy change AG, which is the maximum amount of useful work one can obtain from a chemical reaction. 

Fig. 1.6. Grove was the firstto obtain electric power directly from a chemical reaction.

To what temperature would the water in Example 5 be warmed if 55.7 J of electrical energy or 55.7 J of energy from a chemical reaction had been added It doesn t matter to the water what form of energy has been used just the quantity of energy is important. (See Supplementary Problems 13, 21, and 22.)... 

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 electrochemical cells were developed during the 1800s. Electrochemical cells are arrangements of electrodes and sources of ions that either generate electric current from a chemical reaction, or alternatively, use electricity to drive a chemical reaction. Today these cells find applications in daily life, such as in the batteries that power your car or cell phone. Today electrochemistry still constitutes an important field of research and is one that will likely continue to lead to the development of new products and technologies. 

Electrochemistry is best defined as the study of the interchange of chemical and electrical energy. It is primarily concerned with two processes that involve oxidation-reduction reactions the generation of an electric current from a chemical reaction and the opposite process, the use of a current to produce chemical change. 

A device that produces an electric current from a chemical reaction is called an electrochemical cell. Stricdy speaking, a battery is a series of electrochemical cells. We will, however, stick with the everyday use of battery to refer to any device that converts chemical enei to electrical energy. 

A fuel cell that uses the chemical combination of Hj and Oj to produce electricity directly from a chemical reaction is shown in Figure 8.12. This device consists of two porous graphite (carbon) electrodes dipping into a solution of potassium hydroxide, KOH. At the anode, Hj is oxidized, giving up electrons, and at the cathode, Oj is reduced, taking up electrons. The two half-reactions and the overall chemical reaction are... 

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]. 

Towards the end of the nineteenth century Walther Nernst had used the powerful system of energy relations known as thermodynamics to show how one could calculate the maximum amount of electrical energy that could be obtained from a chemical reaction occurring in an electrochemical cell. These predictions can often be achieved in practice but only if the current flow is very small, that is the process is carried out very slowly. This observation shows that electrochemical reactions like all chemical reactions occur at a finite rate. The study of the rates of reaction is called kinetics and the understanding of the kinetics of electrochemical reactions has been the central preoccupation of electrochemists this century. 

Luminescence includes phenomena such as fluorescence and phosphorescence. It comes from the radiative deactivation of excited matter following an excitation (the mechanism of the excitation, as well as fluorescence and phosphorescence is explained below). The excitation can come from light (photoliuninescence), electricity (electroluminescence), a chemical reaction (chemoluminescence or bioluminescence, if the reaction takes place in a biological system), or a mechanical stress (triboluminescence). We focus on photoluminescence, because most of the other excitation sources require special conditions and are, with the exception of electroluminescence, quite rare, especially when dealing with the luminescence of the lanthanides. 

However, if we assemble the same components into a battery at 15° and 1 atmosphere so that it functions perfectly reversibly, the electrical energy obtained is only equivalent to 66,000 calories. The term free energy AF) is given to the maximum work which is obtainable from a chemical reaction taking place imder completely reversible conditions at constant temperature and pressure. In the case of the reaction between zinc, mercurous sulphate and water, AH — AF = (—82,000) — (-66,000) = -16,000 cal/mole. 

We saw in Chapter 12 that an electric current can cause chemical reactions. The process can be reversed, and electricity obtained from a chemical reaction. This discovery, by Alessandro Volta in 1796, provided for the first time a source of continuous electric current, and thus made possible the great electrical and electrochemical discoveries of the early nineteenth century, Today, the electrochemical cell, or battery of cells, has been generally superseded by the generator as a source of electricity. However, cells still have a place as portable sources of small amounts of power. They may be making a comeback in the form of fuel cells (page 352), What is more important for chemistry is that the electrical work that can be obtained from a chemical reaction provides a measure of the driving force of the reaction and of its equilibrium constant. 

Why do we want to model molecules and chemical reactions Chemists are interested in the distribution of electrons around the nuclei, and how these electrons rearrange in a chemical reaction this is what chemistry is all about. Thomson tried to develop an electronic theory of valence in 1897. He was quickly followed by Lewis, Langmuir and Kossel, but their models all suffered from the same defect in that they tried to treat the electrons as classical point electric charges at rest. 

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