CHAPTER XXI.
CHEMICAL EFFECTS OF THE ELECTRIC CURRENT.
369. Chemical Action and Electricity. We have learned that the electric current is produced, in the cell, by chemical action. There is a definite relation between the chemical action and the current produced. We are now to study the changing of electrical energy back, again, to chemical energy.
370. Electrolysis is the name given to the process of decomposing chemical compounds by passing the electric current through them. The compound decomposed is the electrolyte. Fig. 112 shows a tumbler of liquid (electrolyte) through which the current is to pass in the direction of the arrow. Two carbon plates, A and C, are in the liquid, and are joined to the source of electricity. The current enters at A (anode) and leaves at C (cathode).
EXPERIMENT 147. To study the electrolysis of water.
Apparatus. The two simple cells (§ 275) joined in series (§ 364), although two Daniell or two dry cells will be better. A tumbler of water containing a few drops of sulphuric acid to make the water a conductor. Two pieces of sheet copper will serve as the electrodes. The galvanoscope may also be put into the circuit as in Fig. 113.
371. Directions. (A) Allow the current to pass, and note (1) whether gas is set free at both electrodes, A and C, and (2) at which the quantity of gas is the greater. If very little gas is produced use more cells.
(B) Remove A and C from the liquid, to remove the gas, then watch the action of the needle of G V as the water is again decomposed.
372. Composition of Water. The two gases liberated in Exp. 147 were hydrogen (H) and oxygen (O). The chemical formula for water is H2O, which means that it is composed of two parts, by volume, of H and one part of O. With proper apparatus these gases may be collected, tested, and the amounts measured.
373. Electromotive Force of Polarization. We know that H and O have a strong chemical attraction, or affinity, for each other. In order, then, for the current to decompose water, this attraction between the gases must be overcome; and as soon as the current ceases, these gases try to rush together again to form water. This sets up an electromotive force of almost 1.5 volts; in fact, a current is produced if the H and O be allowed to form water again (See Storage Cells). To decompose water the current must have an E. M. F. of over 1.5 volts to overcome this E. M. F. of polarization. It was seen in the study of simple cells that the current became rapidly weaker as hydrogen was deposited upon the copper plate, on account of this opposing electromotive force.
In decomposing other compounds, the anode is made of the metal which is to be deposited at the cathode. If copper is to be deposited from a solution of copper sulphate the anode should be a copper plate; this keeps the solution at same strength, and avoids the opposing E. M. F. of polarization; that is, a very weak current will do the work (See Exp. 149), because the electrodes are of the same metal.
EXPERIMENT 148. To coat iron with copper.
Apparatus. Iron nail, solution of copper sulphate (§ 283).
374. Directions. (A) Clean the nail with sandpaper, then hold it in the copper solution for a few seconds. Machinists often cover iron or steel tools with a thin coating of copper in this way.
EXPERIMENT 149. To study the electrolysis of a solution of copper sulphate.
Apparatus. Galvanoscope, G V; two-fluid cell, 2-F C; a tumbler, T, containing about an inch of copper sulphate solution (§ 283); a wooden cross-piece to which is fastened a copper strip; carbon rod, C; wire 2 is held to C by a rubber band. Arrange as in Fig. 114, so that Cu will be the anode (§ 370), the current passing as shown by arrow. A dry cell may be used for short experiments instead of the 2-F C.
375. Directions. (A) The carbon being clean, allow the current to pass, C and Cu being kept about ½ in. apart. Watch the surface of C, and note the beautiful color of the deposited copper. Save the coated rod for the next experiment. Has the Cu plate been acted upon?
376. Electroplating is the name given to the process of coating substances with metal with the aid of the electric current. The copper sulphate, CuSO4, is broken up into Cu and SO4 by the current. The Cu goes to the cathode, and the SO4 attacks the anode, gradually dissolving it if it be copper; that is, the metal part of CuSO4 is carried in the direction of the current.
Most metals are coated with copper before they are silver or gold plated. A solution of silver is used for silver plating, silver being used as the anode.
EXPERIMENT 150. To study the chemistry of electroplating.
Apparatus. Same as in last experiment, but use two carbon rods for the electrodes. Arrange as in Fig. 114, with the Cu replaced by another carbon. Two simple cells (§ 275) are also needed.
377. Directions. (A) Allow the current to pass as before. Is copper still deposited? Does anything occur now at the surface of the anode? Is the copper deposited as rapidly as before?
(B) Try the effect of the two simple cells joined in series, Instead of the two-fluid cell.
(C) After a fair coating of copper has been deposited upon the carbon cathode, reverse the direction of the current through the copper solution; that is, use the coated rod for the anode. Allow the current to pass until a change takes place in the anode.
378. Discussion. Ions are the names given to the parts into which an electrolyte is decomposed by the electric current. In the case of CuSO4, the ions are Cu and SO4, which is called an acid radical. This SO4 can not dissolve carbon or platinum, so these are used when water is to be electrolyzed. Where copper is used as the anode for copper plating, the SO4 attacks it, forming CuSO4 again, and this keeps the solution strong. If carbon were used instead, the SO4 would take H2 from the water around the anode and H2SO4 (sulphuric acid) would be formed, the oxygen of the water being set free at the anode. The amount of Cu dissolved from the copper anode equals nearly the amount deposited upon the cathode. Exp. 150 shows that the metal is carried in the direction of the current. As hydrogen is produced at the cathode it is chemically considered a metal.
379. Electrotyping consists in making a copy in metal, of a woodcut, page of type, etc. A mould or impression of the type is first made in wax, or other suitable material (the pages of this book, for example, as set up by the printer). These moulds are, of course, the reverse of the type. They are coated with graphite to[155] make them conduct electricity, and hung as the cathode, in a bath of copper sulphate. After a thin coat of copper has been deposited by an electric current, the wax is removed and the thin copper backed with soft metal. The metal surface next to the wax will be just like the type, only made of copper. These plates or electrotypes can be printed from, the original type being used to set up another page. (See "Things a Boy Should Know About Electricity.")
380. Voltameters are cells used to measure the strength of an electric current. In the Water Voltameter the hydrogen and oxygen produced are measured. The H acts like a metal and goes to the cathode, two parts of H being formed to one of O.
Copper Voltameter. This cell measures the amount of copper deposited in a given time by a current. The copper cathode is weighed before and after the current flows. The weight of Cu deposited is then divided by the number of seconds during which the current passed, and this result, in turn, by .000328, which will give the average strength of the current in amperes. (See § 351.) Other forms of voltameters are also used.
In all voltameters the quantity of metal deposited is proportional to the time that the current flows, and to its strength.
EXPERIMENT 151. To study the construction and action of a simple "storage" cell.
Apparatus. Two lead plates, L P, (Nos. 77, 78) fastened to a wooden cross-piece (§ 275). The spring-connectors should not be forced upon the thick lead. Fasten one end of the wire under the screw-head. A tumbler two-thirds full of dilute sulphuric acid (§ 258); the astatic galvanoscope, A G; wires to form connections; the two simple cells joined in series. Arrange as in Fig. 115. One L P is joined to binding-post, L, of A G by the wire marked 1; wire 2 connects the other L P to the copper Cu. Wire 3 joins the zinc to any thin metal plate, M P, which is used for convenience, so that the spring connectors can be quickly slipped on or off. Wire 4 joins M P with binding-post R of A G.
381. Directions. (A) Get clearly in mind the direction in which the right-hand end of the astatic needle is deflected when the current passes, remembering that it passes into A G at L and leaves at R. Allow the current to flow for 10 or 15 minutes through the circuit, at the same time watching the needle to see whether the strength of the current remains constant.
(B) Remove the connector from Cu, swing it over into the position of the dotted line (Fig. 115), slip the connector upon M P and watch the needle. This cuts the cells out of the circuit; but, if you desire, also remove wire 3 from M P. Does the storage cell, S C, produce any current? Does it pass through A G in the same direction as that which came directly from the two cells?
(C) Try the dry cell in place of the two simple cells. Try 2 other cells in series if you have them.
382. Secondary or Storage Cells must be charged by a current before they can give out a current. Electricity is not really stored. Chemical changes are produced in the storage cell by the charging current, as in the voltameter or electroplating bath; and it is, then, potential chemical energy that is stored. When the new compounds are allowed to go back to their original condition by joining the electrodes of the charged cell a current is produced. In other words, an electric current[157] produces chemical changes in the cell by electrolysis, and these new compounds have an E. M. F. of polarization because they are constantly willing and anxious to get back to their old state. The plates are lead and are usually coated with compounds of lead. Hydrogen and oxygen are given out at the electrodes. The current from a dynamo is used to charge secondary batteries. (See "Things a Boy Should Know About Electricity.")