Electrochemistry

Galvanic Cells

Galvanic cells harness the electrical energy available from the electron transfer in a redox reaction to perform useful electrical work. The key to gathering the electron flow is to separate the oxidation and reduction half-reactions, connecting them by a wire, so that the electrons must flow through that wire. That electron flow, called a current, can be sent through a circuit which could be part of any number of electrical devices such as radios, televisions, watches, etc.

The figure below shows two typical setups for galvanic cells. The left hand cell diagram shows and oxidation and a reduction half-reaction joined by both a wire and a porous disk, while the right hand cell diagram shows the same cell substituting a salt bridge for the porous disk.

Figure 2.1: Diagram of a Galvanic Cell

The salt bridge or porous disk is necessary to maintain the charge neutrality of each half-cell by allowing the flow of ions with minimal mixing of the half-cell solutions. As electrons are transferred from the oxidation half-cell to the reduction half-cell, a negative charge builds in the reduction half-cell and a positive charge in the oxidation half-cell. That charge buildup would serve to oppose the current from anode to cathode-- effectively stopping the electron flow--if the cell lacked a path for ions to flow between the two solutions.

The above figure points out that the electrode in the oxidation half-cell is called the anode and the electrode in the reduction half-cell is called the cathode. A good mnemonic to help remember that is "The Red Cat ate An Ox" meaning reduction takes place at the cathode and oxidation takes place at the anode.

The anode, as the source of the negatively charged electrons is usually marked with a minus sign (-) and the cathode is marked with a plus sign (+). Physicists define the direction of current flow as the flow of positive charge based on an 18th century understanding of electricity. As we now know, negatively charged electrons flow in a wire. Therefore, chemists indicate the direction of electron flow on cell diagrams and not the direction of current. To make that point clear, the direction of electron flow is indicated on figure 2.2 with a arrow and the symbol for an electron, e^- .

Figure 2.2: Diagram of a Galvanic Cell Showing Direction of Electron Flow

Cell potential, electrical work, and free energy

Batteries

A battery is a device that converts chemical energy into electrical energy. A dry-cell flashlight battery consists of an electric cell, but larger batteries are made up of a group of cells that are connected to act as a source of direct electric current at a given voltage. A cell consists of two dissimilar substances, a positive electrode and a negative electrode, that conduct electricity, and a third substance, an electrolyte, that acts chemically on the electrodes. A group of several such cells connected together is called a battery. The two electrodes are connected by an external circuit (e.g., a piece of copper wire); the electrolyte functions as an ionic conductor for the transfer of the electrons between the electrodes. The voltage, or electromotive force, depends on the chemical properties of the substances used, but is not affected by the size of the electrodes or the amount of electrolyte. Batteries consisting of carbon-zinc dry cells connected in various ways (as well as batteries consisting of other types of dry cells) are used to power such devices as flashlights, portable computers, and pocket-sized CD players.  A battery called the storage battery is generally of the wet-cell type; i.e., it uses a liquid electrolyte and can be recharged many times, unlike the ordinary dry-cell battery, which uses a paste electrolyte and can be recharged few times, if at all. The storage battery consists of several cells connected in series. Each cell contains a number of alternately positive and negative plates separated by the liquid electrolyte. The positive plates of the cell are connected to form the positive electrode; similarly, the negative plates form the negative electrode. In the process of charging, the cell is made to operate in reverse of its discharging operation; i.e., current is forced through the cell in the opposite direction, causing the reverse of the chemical reaction that ordinarily takes place during discharge, so that electrical energy is converted into stored chemical energy. The storage battery's greatest use has been in the automobile where it was used to start the internal-combustion engine. Improvements in battery technology have resulted in vehicles, some in commercial use, in which the battery system supplies power to electric drive motors instead.

In the United States the lead storage battery is commonly used; the nickel-cadmium battery, although far more costly, is also in wide use. The cell of the lead storage battery consists of alternate plates of lead (negative electrode) and lead coated with lead dioxide (positive electrode) immersed in an electrolyte of sulfuric acid solution; when fully charged, it produces a voltage of between 2.0 and 2.5 volts. In the discharging process lead sulfate is deposited on both the negative and the positive electrodes, while the sulfuric acid electrolyte becomes weaker. Another type of storage cell, called the Edison cell, has a nickel oxide positive plate and an iron negative plate suspended in a solution of potassium and lithium hydroxides.

Corrosion

Corrosion is an atmospheric oxidation of metals (see oxidation and reduction ). By far the most important form of corrosion is the rusting of iron . Rusting is essentially a process of oxidation in which iron combines with water and oxygen to form rust, the reddish-brown crust that forms on the surface of the iron. Rust, a chemical compound, is a hydrated ferric oxide Fe ₂ O ₃ · n H ₂ O, where n is usually 1 12 . The chemical mechanism of rusting is not fully known, but is thought to involve oxidation of metallic iron to ferrous ion (Fe ⁺⁺ ) and reaction of the ferrous ion with oxygen and water to form rust. The reaction is catalyzed by water, acids, and metals (e.g., copper and tin) below iron in the electromotive series . Because iron is so widely used, e.g., in building construction and in tools, its protection against rusting is important. Although metals (e.g., aluminum, chromium, and zinc) above iron in the electromotive series corrode more readily than iron, their oxides form a tenuous coating that protects the metal from further attack. Rust is brittle and flakes off the surface of the iron, continually exposing a fresh surface. Rusting can be prevented by excluding air and water from the iron surface, e.g., by painting, oiling, or greasing, or by plating the iron with a protective coating of another metal. Metals used for plating include chromium, nickel, tin, and zinc. Zinc plating is called galvanizing. Many alloys of iron are resistant to corrosion. Stainless steels are alloys of iron with such metals as chromium and nickel; they do not corrode because the added metals help form a hard, adherent oxide coating that resists further attack. The iron hulls of ships can be protected against rusting by attaching magnesium strips to the underside of the vessel. An electric current is generated, with the magnesium and iron acting as electrodes and seawater acting as the electrolyte. Because magnesium is above iron in the electromotive series, it serves as a “sacrificial anode” and is oxidized in preference to the iron. This is called cathodic protection, since the iron serves as the cathode and thus escapes oxidation. This method is also used to protect the pipes of electric generating plants where saltwater is used as a coolant.

Commercial electrolytic processes:  batteries of the future

The electrolytic process requires that an electrolyte , an ionized solution or molten metallic salt, complete an electric circuit between two electrodes. When the electrodes are connected to a source of direct current one, called the cathode, becomes negatively () charged while the other, called the anode, becomes positively (+) charged. The positive ions in the electrolyte will move toward the cathode and the negatively charged ions toward the anode. This migration of ions through the electrolyte constitutes the electric current in that part of the circuit. The migration of electrons into the anode, through the wiring and an electric generator, and then back to the cathode constitutes the current in the external circuit.

For example, when electrodes are dipped into a solution of hydrogen chloride (a compound of hydrogen and chlorine) and a current is passed through it, hydrogen gas bubbles off at the cathode and chlorine at the anode. This occurs because hydrogen chloride dissociates (see dissociation ) into hydrogen ions (hydrogen atoms that have lost an electron) and chloride ions (chlorine atoms that have gained an electron) when dissolved in water. When the electrodes are connected to a source of direct current, the hydrogen ions are attracted to the cathode, where they each gain an electron, becoming hydrogen atoms again. Hydrogen atoms pair off into hydrogen molecules that bubble off as hydrogen gas. Similarly, chlorine ions are attracted to the anode, where they each give up an electron, become chlorine atoms, join in pairs, and bubble off as chlorine gas.