THE ENERGY FACTOR IN CHEMICAL CHANGES This paper sets forth the importance of the energy factor i n chemical reactions. Certain familiar experiments are cited to show that the tendency of one substance to react with another i s measured by the amount of energy set free. It .is shown that one factor i n the conductiwity of solutions i s directly dependent upon the amount of the energy change which accompanies the electrolysis of materials. The use of the term polarity i s somewhat extended to express the degree of activity which one substance has for another. A table of the heats of formation of some metal halides i s given, and the connection of these henu with the degree of activity of the component elements is noted. Of the three outstanding factors in chemical changes, viz., energy level, contact of materials, and catalyzers, the first merits more attention than is given to i t in current texts on general chemistry. The mathematical treatment accorded i t in works on thermodynamics gives it a reputation of being too difficult to inspire students of chemistry to make an early attempt to understand it. It is hoped that the following presentation may be read with interest even by students in the first year of their course. When a piece of zinc is put into a solution of copper sulfate and allowed to remain for some time, two important changes will be observed. First the surface will become coated with copper, and second the blue color of the solution will slowly disappear. We explain this by saying that zinc has displaced copper and that zinc sulfate has been formed. The student of physics is taught to observe that this change is accompanied by a flow of electricity. When a plate of copper and a plate of zinc are connected with a voltmeter and then dipped into a solution of copper sulfate, a pressure of about 1.1 volts is shown. In order t o show the maximum voltage, the solution should have zinc sulfate in one section of the cell and copper sulfate in the other. The zinc plate is dipped into the former and the copper plate into the latter. By this arrangement copper ions will discharge on the copper plate instead of on the surface of the zinc, and the zinc plate will dissolve, forming more zinc sulfate. I n this experiment the production of electricity is of no less interest than the changes in the substances. How can it be accounted for? By making use of the ionic theory, we frame the reply about as follows: when zinc as a free metal is transformed into zinc ions more energy is released than is required t o transform copper ions into free copper, and the surplus flows through the wire as electricity. Some confirmation for this view may he found by examining heats of formation as found in the international critical tables. It is there shown that the heat of formation of zinc sulfate is 960 kilojoules, while that of copper sulfate is 748. Since the group SO1 is common to these two sub683
684
JOURNAL OF CHEMICAL EDUCATION
APRIL,1931
stances, we may properly infer that the difference in these heats of formation is due to the difference in energy necessary to transform Zn t o Zn++ and that required t o change Cu++ t o Cu. Let us note the behavior of other metals in solution of salts under parallel conditions. Both zinc and copper will displace silver from AgN03. What would happen if we place both of these metals in a silver nitrate solution a t the same time? Let us examine the heats of formation and from them predict what should occur. The heat of formation of Zn(NOs)~(two norms) is 569 kj.; that of Cu(N0a)a (two norms) is 303 kj.; and that of 2AgN03 (two norms) is 206 kj. The drop in energy level from Zn to 2Ag is 363 kj.; while that from Cu t o 2Ag is 97 kj. These figures show how much more energy is given off in changing Zn to Zn++ than is shown when Cu changes to Cu++. This should lead us t o anticipate that Zn(NO& rather than Cu(NO& would be formed when silver ions are transformed into silver metal. Now when we try the experiment we obtain a clear solution. If Cu(N03)~had been formed, the solution would have been blue. The evidence of the experiment confirms these data, and we infer a general principle: Among seweral chemical units i n contact with each other, that reaction mill take ?lace which will yield the greatest amount of free energy. Nernst ( I ) was the first t o state the fundamentals of this principle. He observed: That every metal has a certain solution tension or pressure tending to drive it into solution (i. e., to become ions). The value of this pressure becomes rapidly less as we pass through the series from magnesium to gold.
Van't Hoff (2) expressed it as follows: The maximum work is a direct, exact measure of the tendency of substances to Ract with each other.
Heats of formation are very significant data when we wish to anticipate the behavior of substances in chemical reactions. The figures given in the International Critical Tables were obtained partly by experiment and partly by calculation. I n most cases the heat of formation is the amount of free energy given off when elements come together t o produce a compound. As a rule the energy content of elements is greater than that of the compounds formed by their union. The change is accompanied by the production of heat, light, or electricity. I n a comparatively small number of compounds, heat is absorbed during their formation. For example, when acetylene is formed from carbon and hydrogen it absorbs 215.5 kj. That is, its heatofformation is -215.5 kj. or -55.1 cal. When a formula weight of i t is burned, just that much more heat is produced than would be given off in burning the same amount of carbon and hydrogen.
VOL. 8, NO. 4
ENERGY FACTOR IN CHEMICAL CHANGES
685
The general rule is that free elements are on a higher energy level than their compounds. Like a stored-up liquid energy spills over when the equilibrium is disturbed. This takes place when the temperature of a system containing diverse chemical units is raised to the kindling point. If the quantities are large, a contlagration ensues, and a vast amount of energy is dispersed in the form of heat. Now when we wish to extract elements from their compounds a quantity of energy equal to the heat of formation mnst he supplied usually as heat or electricity. We note the influence of this principle in a study of the conductivity of solutions. Conductivity i s a measure of the work done by the current in getting through the solution. One of the most important factors is the consumption of energy in the transformation of materials. When active metals are set free, the quantity is large, though it is mostly radiated as heat when they react with water. When hydrogen ion is transformed into hydrogen, very little energy is required (3) ; for this reason acids are among the very best conductors. One cannot make a direct comparison between tables of conductivity and heats of formation, because the movements of the ions are hindered more or less by mechanical influences. For instance the more they are hydrated, the larger the mass which must be moved by a unit of attraction. Again the force of diffusion (osmotic pressure) may be directly opposed to the direction required for the best conductivity. Also (4),the electrical restraint to which attention has been called by Debye and Hiickel would also influence their motion to some extent. It has been shown (5) that conductivity cannot be satisfactorily interpreted in terms of the single principle of ionic motion as attempted by Kohlrausch. When the electric current is passed through a fused dry salt such as sodium chloride and the product is kept away from the air, the energy consumed is largely stored up in the free elements sodium and chlorine; but when the current is passed through an aqueous solution of sodium chloride, the storage of energy is hut momentary, for the elements react with water as soon as formed, giving out much heat and yielding compounds on a lower energy level. It is something like pumping water up into a basket to store it. All changes in energy are capable of doing work; hence work performed is a measure of energy absorbed. Energy changes accompany many phenomena which are not ordinarily classed as chemical. Such are heats of solution, expansion of gases, and osmotic pressure. Any attempt to interpret heats of formation as a measure of chemical activity must take into account the possibility of other causes affecting the amount of heat transformed. It is highly desirable to have a direct method of expressing the difference in compounds depending upon their heats of formation. Fortunately,
686
JOURNAL OF CHEMICAL EDUCATION
APRIL,1931
the term polarity is coming into use for that purpose. For many years the terms polar and non-polar have been used to express two classes of compounds as if they were wholly distinct and independent of each other. It is now understood that the differences are by no means absolute but wholly relative. Prof. John Warren Williams of the University of Wis-
TABLEOF HEATSOP FORMATION These data are taken from International Critical Tables. The unit is the kilojoule instead of the usual calorie. When the number of kilojoules is multiplied by 0.239 the product is kilo-calories.
consin in an address before the WashingtonSection of the American Chemical Society showed that even the so-called non-polar compounds (organic) manifest polarity over a wide range. It seems logical t o link heats of formation with polarity. The electromotive series of elements is a ladder by which the energy levels of;'the different elements may be judged a t a glance. Polarity may be defined
VOL.8, No. 4
ENERGY FACTOR I N CHEMICAL CHANGES
687
as the degree of difference i n the energy levels of elenents which combine to form a compound. The most polar compound it is possible to make must be composed of cesium a t the top of the ladder with fluorine a t the bottom. It should have the greatest energy of formation, and would also require the maximum of energy to restore the elements to the free state. Combinations of other elements which come in between will make compounds of less polarity which would require less energy to break them up into the free elements. The table (page686)which accompanies this paper gives a remarkable confirmation of the theories expressed herein. If we look at the column headed Fa and go to the right t o Is, we see that the heats of formation diminish progressively. If we begin with Ag near the top, the heats of formation increase with few exceptions. The heats of formation of CdF2, RbF, and several lithium compounds appear t o be too large, while several copper compounds appear t o be too small. Such discrepancies are to be expected and do not disturb the usefulness of the general principle. Indeed they may point the way to new truth in some unexplored field. I t may be properly pointed out that what has been called chemical affinity can be measured and definitely determined in terms of heats of formation. Chemical energy is nothing more than the energy which is inherent in substances. If the energy content of a substance changes in any degree, as when the valence of an ion changes, the effectis immediately seen in its properties. Literature Cited ( I ) SMITH'S"Inorganic Chemistry," Revised edition. The Century Co., New York City, 1910, p. 670. ( 2 ) EUCKEN,JETTE, and LAMER. "Physical Chemistry," McGraw-Hill Book Co., Inc., New York City, 1925, p. 64 (3) See table on page 686. (4) Ref. (Z), p. 317. (5) CLINTON,'Further Light on the Conductivity of Solutions." The Chemical Publishing Ca., Easton, Penna.
