Electrochemistry

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Electrochemistry, that part of the science of chemistry that deals with the interrelationship of electrical currents, or voltages, and chemical reactions, and with the mutual conversion of chemical and electrical energy. In the broadest sense, electrochemistry is the study of chemical reactions that produce electrical effects and of the chemical phenomena that are caused by the action of currents or voltages.

ELECTRIC CURRENT AND ION MOVEMENT

Most inorganic and some organic chemical compounds, when in a molten state or when dissolved in water or other liquids, become ionized; that is, their molecules become dissociated into positively and negatively charged components, which have the property of conducting an electric current (see Ion; Ionization). If a pair of electrodes is placed in a solution of an electrolyte, or an ionizable compound, and a source of direct current is connected between them, the positive ions in the solution move toward the negative electrode and the negative ions toward the positive. On reaching the electrodes, the ions may gain or lose electrons and be transformed into neutral atoms or molecules, the nature of the electrode reactions depending on the potential difference, or voltage, applied.

The action of a current on an electrolyte can be understood from a simple example. If the salt copper sulfate is dissolved in water, it dissociates into positive copper ions and negative sulfate ions. When a potential difference is applied to the electrodes, the copper ions move to the negative electrode, are discharged, and are deposited on the electrode as metallic copper. The sulfate ions, when discharged at the positive electrode, are unstable and combine with the water of the solution to form sulfuric acid and oxygen. Such decomposition caused by an electric current is called electrolysis.

In all cases, the quantity of material evolved at each electrode when current is passed through an electrolyte follows a law discovered by the British chemist and physicist Michael Faraday. This law states that the quantity of material transformed at each electrode is proportional to the quantity of electricity passed through the electrolyte; and that the weight of the elements transformed is proportional to the equivalent weights of the elements, that is, to the atomic weights of the elements divided by their valences. (See Chemical Reaction; Valence).

All chemical changes involve a regrouping or readjustment of the electrons in the reacting substances; hence all such changes may be said to be electrical in character. To produce an electrical current from a chemical reaction, it is necessary to have a reducible substance, that is, a substance that can gain electrons easily; and an oxidizable substance, one that can give up electrons easily. A reaction of this kind can be understood from the operation of a simple type of electrochemical cell, or battery. If a zinc rod is placed in a dilute solution of sulfuric acid, the zinc, which oxidizes readily, will lose electrons, and positive zinc ions will be liberated into the solution. The free electrons stay in the zinc rod. If the rod is connected through a conductor to an inert-metal electrode placed in the sulfuric acid solution, the electrons will flow around this circuit into the solution, where they will be taken up by the positive hydrogen ions of the dilute acid. The combination of the electrons and the ions produces hydrogen gas, which appears as bubbles on the surface of the electrode. The reaction of the zinc rod and sulfuric acid thus produces a current in the external circuit. An electrochemical cell of this kind is known as a primary cell, or voltaic cell.

In the storage battery or accumulator, commonly known as a secondary cell, electrical energy is fed to the cell from an outside source and stored within in the form of chemical energy. The chemical reaction of a secondary cell is reversible, proceeding in one direction when the cell is being charged, and in the opposite direction when it is discharging. Because the reaction is of this type, a secondary cell can be discharged again and again.

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