/f the law of conservation of energy holds, there must necessarily be a direct relation between electrical energy and chemi cal energy on the one hand and heat energy.on the other. This brings us first to the discussion of Faraday's laws (q.v.), two of the most fundamental statements in natural sci ence. Faraday's first law specifies that the amount of chemical action produced by an elec tric current in a circuit is directly proportional to the quantity of electricity which passes through the circuit. The second law specifies that the quantities of different substances which are produced by the same amount of elec tricity passing.are directly proportional to the chemically equivalent weights of the substances concerned. These effects are entirely independ ent of the concentration or temperature of the solution, the size or distance apart of the elec trodes, and all other conditions. These laws hold with great exactness not only for ordinary aqueous solutions, but also for non-aqueous solutions and for fused salts. The quantity of electricity that is necessary to deposit the chemically equivalent weight of any substance is 96,500 coulombs (ampere seconds) and this unit quantity is known as a Faraday, after its discoverer. Another, and possibly better, way of stating this is to say that one Faraday of electricity, 96,500 coulombs, is required to make a unit change in valence of any element or rad ical. One Faraday then will deposit as metal 56/3 grams of iron from a solution of ferric iron (a change of three valences) or it will reduce 56 grams of iron from ferric to ferrous, a change of one valence. This holds .equally well whether it is a decrease of valence accom panying a chemical reduction, or whether it is an inerease of valence accompanying an oxida tion. The fact that 96.500 coulombs will de posit one chemical equivalent of an element makes it possible to calculate from this rela bon the amount of any element that would be deposited by any given amount of current. cording to this, one coulomb should deposit grams of hydrogen or 63.57 2x96,500 grams of copper. These values are known as the electrochemical equiv alents (q.v.) and can readily be calculated for any material. The ampere-second values are usually used for scientific work, but for practi cal work in the plant, larger units for the ampere hour or ampere day may be used. Faraday's laws refer only to quantities of elec tricity involved in bnnging about certain changes, but say nothing about the quantities of electrical energy necessary for the change. To arrive at values for the energy involved, we must consider not only the quantity factor of the current used, but also the intensity factor. In other words, Faraday's laws deal with am pere changes, while the energy involved is con cerned with amperes x volts, or watts.
All chetnical reactions can be compared from an energy standpoint on the basis of the thermochemical changes accompanying the re action.
1 calorie=4- .186 watt seconds 1 watt second=1 coulomb X 1 volt or 1 volt-coulomb=/.2389 calorie.
Then 1 volt-Faraday=02389 X 96,500.= 23,054 calories. Any given reaction involving one Faraday will then require as many volts as the heat balance of the reaction will contain 23,054. For example, the heat of formation of water is 69,000 calories, and to decompose it an equivalent amount of energy must be sup plied. A molecule of water, MO, includes two chemical equivalents, so per chemical equivalent, there must be supplied 34,500 calones. The voltage required for the decomposition will 34,500 then be —=1.495 volts. The decom 23,054 position of one molecular weight (18 grams) of water then by electrical energy would require 2 x 96,54=193,080 ampere seconds of electricity 193,080 X 1.49
at a voltage of 1.49 volts, or 3600 X 1000 11.08 kilowatt hours of electrical eneru.
This same principle can be applied to the calculation of the electromotive force of pri mary or secondary batteries (q.v.) when applied to the thermochemical balance of the chemical reaction that takes place in the cell. The chemical reaction in the Daniell cell is Zn+CuSO4=ZnSO4-f-Cu.
The heat of formation in dilute solution of CuSO4 is 197,500 cal. and of ZnSO. is 248,000 cal.. leaving an excess of 50,500 cal. for two Faradays, or 25,250 cal. for 23.250 one Faraday. 23,054 — 1.094 volts supplied by the cell.
The reaction on charging a lead storage battery is 2 PbSO4-1-2H.O=Pb0.+Pb-1-2 H-SO4 215,700 2(69,000) 63,400 2(210,200) This reaction shows a deficit of 569,400-473,- 800=95,600 cal. for 1 PbOs (two Faradays) or 47,800 cal. for one Faraday. It will then re .
quire 23 ,800 ,054 = 2.073 volts to charge the cell, and since the reaction is reversible, when once charged, it will be capable of generating the same voltage.
Of the phenomena accompanying electrol ysis with unattackable electrodes, two of the most interesting are polarization and over-vol tage. With electrolyses that are more or less reversible, it may be noted that after the pas sage of the current has caused some decompo sition, there is a tendency for recombination of the materials present at the electrodes. If the current is stopped, it will be noted that for a short time there will be generated a small cur rent in the opposite direction from that of the current originally imposed. This is known as the polarization current and the voltage gener ating it is known as the polarization voltage. This polarization voltage, being in the reverse direction from the voltage causing the original electrolysis, will reduce the electromotive force on the cell, and the current passing. In an electrolysis involving the separation of a free gas on an unattacked electrode, it is well known that the voltage required for decomposition is greater than that calculated from the heat of formation. This excess of voltage required over the theoretical is called over-voltage, or more recently, gas voltage. These voltages vary widely for various metals and an explana tion of the differences has long been sought. Recent investigations seem to indicate that the differences are mechanical rather than chem ical. Calorimetric measurements show that the amount of electrical energy disappearing as chemical work is the equivalent of the nor mal decomposition voltage for the reaction tak ing place, and that the over-voltage appears in the solution as heat. This would indicate that the nature of the over-voltage was mechanical, and the probable explanation is that it repre sents the amount of energy necessary to over come the resistance of the film of gas on the electrode. The gas as first formed on the electrode is a thin film over the entire surface, and then as the amount of gas increases sur face tension begins to act to form the film into bubbles of gas which detach themselves from the electrode and escape from the solution. The amount of energy necessary to force the current through this gas film over the surface of the electrode will of course increase with the thickness of the film, and in turn the thick ness of the film will be dependent on the ease with which the gas mechanically separates itself from the surface of the electrode. This will naturally vary with the material of the electrode and with the condition of its surface.