ELECTROCHEMISTRY, that branch of physical chem istry (q.v.) which is concerned with the relation between elec tricity and chemical change. Under ordinary conditions, the occurrence of a chemical reaction is accompanied by the libera tion or absorption of heat and not of any other form of energy, but there are many spontaneously occurring chemical reactions which, when allowed to proceed under special circumstances, lib erate electrical energy, an electric current being generated. Con versely, the energy of an electric current can be utilized to bring about many chemical reactions which do not occur spontane ously. A process of the former type involves the direct conversion of the chemical energy, which is causing a reaction, into electrical energy, and an apparatus whereby such a process is brought about constitutes a primary cell. On the other hand, in processes of the latter kind, electrical energy is directly converted into chemical energy, which is stored up in the products of the reaction : such a process is one of electrolysis—an electrolytic process. In virtue of their chemical energy, the products of an electrolytic process have a tendency to react spontaneously with one another, repro ducing the substances that were consumed during the electrolysis, so that, if this reaction is allowed to occur under such conditions that a primary cell is formed, a large proportion of the electrical energy used in the electrolysis may be regenerated. This possi bility is made use of in secondary cells—also known as accumu lators or storage batteries. The charging of an accumulator is a process of electrolysis, a definite chemical change being produced by the electric current passed through the cell. In discharging the cell, the reverse chemical change occurs spontaneously, the accumulator acting now as a primary cell furnishing an electric current. The "storage" of electrical energy in a secondary cell thus involves its conversion into chemical energy, which can, how ever, be reconverted into electrical energy when desired.
In addition to these purely electrochemical processes, it is cus tomary to include also under the heading of electrochemistry those processes in which the energy of an electric current is first converted into heat, which then serves to bring about a chemical reaction which would not occur spontaneously at ordinary tern peratures. These electrothermal chemical processes thus represent the indirect conversion of electrical energy into chemical energy through the medium of heat, and an apparatus in which they can occur is an electric furnace. Still more indirect processes, how ever, would not be regarded as electrochemical. Thus, in gener ating electricity on the large scale, the electrical energy is actually derived from the chemical energy of fuels such as coal, coke or oil. but the operation is not claimed as an electrochemical one, since the chemical energy is first converted into heat by com bustion of the fuel, heat is then converted into mechanical energy by some form of heat engine, and finally mechanical energy is transformed into electrical energy by means of the dynamo. Al though in this very indirect process only a fraction of the avail able chemical energy of the fuel can actually be obtained in the electrical form—mainly owing to the low efficiency of the heat engine stage—yet no known type of primary cell can be expected to compete with it, since in the operation of such cells relatively expensive material, usually zinc, is consumed continuously. At tempts to devise an efficient primary cell, capable of producing electrical energy directly from the chemical energy of ordinary fuels, have frequently been made, but although many difficulties have been overcome, none of the fuel cells yet designed have been successful on the commercial scale.
Finally, the passage of electricity through gases generally causes chemical changes, and this subject forms a separate branch of electrochemistry. When the current passes in the form of the so-called "silent" discharge, the chemical effects must be attrib uted to the direct action of the electrical energy supplied, but such a process is not an electrolysis, in the usual sense of the term. In cases where the discharge takes the form of an electric arc, much heat is liberated at very high temperatures and the chemical changes produced are the result of combined electrical and thermal conditions, i.e., such processes are partly electrothermal in character.
An electrolyte, prepared either by melting a suitable substance or dissolving it in water or other liquid, owes its characteristic properties to the presence in it of electrically charged atoms or groups of atoms produced by the spontaneous splitting up or dis sociation of the molecules of the substance when it is melted or dissolved. In the so-called strong electrolytes, most of the mole cules of the original substance, or probably all of them, have undergone this process of electrolytic dissociation into charged particles or ions (see SOLUTION). When a potential difference is established between electrodes dipping into an electrolyte, posi tively charged ions move towards the negative electrode and ions bearing a negative charge towards the positive electrode, the electric current being carried through the electrolyte by this migration of the ions (see CONDUCTION IN LIQUIDS). When an ion actually reaches the electrode of opposite polarity, the neu tralization of its charge converts it into an ordinary neutral atom or group of atoms and it is this discharge of ions which gives rise to the chemical changes at the electrodes. Thus, copper sulphate, when dissolved in water forms an electrolyte, since it dissociates under these conditions into positively charged copper ions, Cu++, and negatively charged sulphate ions, When an electric current is passed through the solution by way of platinum electrodes, the copper ions move towards the cathode and on reaching it are discharged, giving metallic copper which is deposited on this electrode. At the anode the (SO4) groups pro duced by the discharge of sulphate ions are unstable and react with water, producing oxygen which is evolved as bubbles of gas and sulphuric acid which accumulates in the solution around the anode. The total chemical change at the two electrodes may there fore be represented by the equation : A simpler case is that of an aqueous solution of copper chloride, in which the ions are Cu++ and the latter yielding only chlorine gas when discharged at the anode. The equation for the complete electrolysis is CuC12+electrical energy--* Cud-C12 (see ELECTROLYSIS) .
The quantity of electricity required to produce, say, I gram of copper by the electrolysis of either of the above solutions is 96,500/31.78=3,036 coulombs, since 31.78 is the chemical equiv alent of copper in salts of this type. But the quantity of electrical energy needed for this purpose is given by the product of the quantity of electricity and the voltage which was applied across the . electrodes during the electrolysis. If the current strength through a given electrolyte is to be I amperes, then the applied voltage, is given by: = -I R+7r, where IR is the voltage used in overcoming the resistance, R ohms, of the column of elec trolyte between the electrodes ; and 7r denotes the of the cell, a quantity which depends on the nature and concentra tion of the electrolyte, the nature of the electrodes, and the cur rent density, i.e., the ratio of I to the surface area of the elec trodes in contact with the electrolyte. If the current density is large, 7r may be considerable, but as I is decreased it approaches zero. The minimum value of which is approached as I is diminished is the decomposition voltage of the given electro lyte with the given electrodes. This represents the voltage which must be exceeded if electrolysis is to occur at all. For an aque ous solution of copper chloride of normal concentration, volt. The electrical energy needed to liberate I gram of copper from this solution under given conditions is 3,036XE joules, and the minimum value of this quantity under any conditions is joules. This value represents also the decrease of chemical energy when I gram of copper and the equivalent amount of chlorine ( I.115 grams) are converted into copper chloride solution. In practice rather more electrical en ergy is always needed, since is necessarily somewhat greater than This additional energy is used in overcoming resistance and polarization eff':cts and is thereby converted into heat.
In virtue of their chemical energy, the products of an electroly sis tend to react with one another if allowed to come into con tact. Thus, copper and chlorine interact readily, forming copper chloride and liberating a considerable quantity of heat—the heat of reaction. By setting up a primary cell, however, in which a copper electrode and a chlorine electrode dip into a solution of copper chloride, this same reaction can be caused to occur so as to yield electrical energy. For this purpose the copper electrode may consist of a plate of the metal, but, in order that chlorine may function as an electrode, a conductor of some material that is not attacked by chlorine must be used to act as an inter mediary between the gas and the solution. A plate of platinized platinum or a rod of porous carbon kept saturated with the gas will act as a chlorine electrode. If the copper chloride solution is of normal concentration, the chlorine electrode will be found to assume a potential which is I.06 volt more positive than the copper electrode, i.e., the electromotive force of the cell=1.06= the decomposition voltage of the electrolyte. If now the two electrodes be connected by means of a wire outside the cell, a current will pass through the circuit formed by the electrolyte, the electrodes and the external wire. Whenever the cell is allowed to deliver a current through an external circuit, chemical changes occur at the electrodes, viz., copper dissolves from one electrode forming Cu++ ions and chlorine goes into solution as ions at the other, so that, although the copper and chlorine do not actually come into contact, the total chemical reaction is Cu-{-C12 energy, which is exactly the reverse of that occurring in the electrolysis of copper chloride solution.
For every gram of copper which dissolves during the action of the cell, 3,036 coulombs of electricity are obtained. The amount of electrical energy obtained, however, is 3,036XEI, where EI is the actual voltage or potential difference between the electrodes of the cell, when it is furnishing a current of I amperes. For a primary cell, where these symbols have the same significance as above, 7r' being the polarization of the cell under the given conditions. If I is made very small by using an external circuit of very high resistance, 7r' approaches zero, EI approaches and the electrical energy obtainable approximates to joules per gram of copper dis solved. This quantity represents the maximum amount of elec trical energy obtainable from the given chemical change and is a measure of the chemical energy which is causing the reaction to proceed. It should be noticed that the heat evolved when a reaction occurs under ordinary conditions is not generally equal to the chemical energy. Hence, in general, some heat is evolved or absorbed even when a primary cell is operated so that it is furnishing the maximum amount of electrical energy correspond ing to the chemical change. Thus, a lead accumulator absorbs heat from its surroundings when it is being discharged.
Returning for a moment to the question of electrolysis, it must be pointed out that, if a copper chloride solution is electrolyzed using copper electrodes, copper is deposited on the cathode, but under these conditions copper is also dissolved from the anode at an equal rate, so that no chemical change occurs in the cell as a whole. Copper is merely transferred from the anode to the cathode and the decomposition voltage is zero. Processes of this kind are largely employed in refining metals. Using anodes of the crude metal, conditions are arranged so that impurities in these anodes do not deposit with the metal on the cathode during elec trolysis. The small voltage required is merely that needed to overcome resistance and polarisation effects.
Apart from electrolytic conductivity, which has been considered above, and electronic conductivity, which is ascribed to the move ments of free electrons through a metallic conductor, the conduc tivity of gases should also be mentioned. Under ordinary condi tions, gases are practically insulators, the very small electrical conductivity which they do possess being attributed to the acci dental presence of a few gaseous ions (charged atoms or mole cules). When a high potential difference is established between electrodes in a gas, these ions move towards the electrode of opposite polarity with considerable velocities, so that if they col lide with neutral molecules of the gas, they may be able to split up the latter into ions. With a sufficiently high voltage, this ioniza tion by collision becomes important, and, as more and more ions are formed, the conductivity of the gas increases. But ions carry ing charges of opposite sign may also collide with one another and form neutral molecules again. With a given voltage the cur rent through the gas will attain a steady value when the rate of formation of ions is equal to their rate of recombination. With oxygen gas, some oxygen molecules, may be ionized into O+ and whereas others may yield and a free electron. If now the ions and collide, a molecule of ozone, may be formed. Chemical changes due to the passage of electricity through other gases arise in a similar way.
The heating effects, which are produced when large electric cur rents are driven by means of high voltages through metallic con ductors or gases, are utilized in electrothermal chemical processes. Alternating currents can generally be used for this purpose.