chemical
J. A. YOUNG Auburn University Auburn, Ahbarno
J. G. MILK
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. . . especially
Son Diego Slole College Son Diego. Colifornio
Question What evidence, understandable by and acceptable to students, do most teachers cite to describe the transfer of charge from one electrode to another in the direct current electrolysis of an electrolyte solution?
Answer
I t is impractical to conduct a poll in order to answer this question reliably, but based on numerous informal conversations and reading of the pertinent literature, most teachers of beginning chemistry courses probably refer to factual evidence involving electrode processes. The facts are these: We can indirectly count electrons which enter the solution and those which leave; in any given period of time, the numbers are equal. Further, in all direct current electrolyses, an observable change occurs at each electrode-solution interface. The change which is observed a t the anode is always an oxidation, or another reaction which is the result of an oxidation. Similarly, the observed changes at the cathode-solution interface clearly suggest that a reduction reaction takes place in that region. Taken together, these facts imply the existence of a charge transfer process, through the medium of charged ions in solution. One way this can occur would involve chemical reactions near each electrode. At the cathode, for example, positive or negative ions are reduced by the entering electrons: CuZ+to Cn+, or to Cu; Mn04to MnOz (or to other oxidation states lower than +7, in the presence of hydronium ion). Or, the solvent might be reduced: HZO to Hz and OH-. Often, especially a t high concentrations of solute and high current density, more than one reduction process might occur. At the anode electrons are generated by oxidation: Common examples of such oxidation processes include the oxidation of the solvent: HzO to 0% and H30+; the oxidation of ions: CI- to CI2; Fez+ to Fea+; the oxidation of the electrode itself; Ni to Ni2+. Again, more than one process may take place. Additional evidence for the existence of ionic species in solution is often presented. The commonly cited examples for this include comparisons of differences in the lowering of the freezing point of water by equally concentrated solutions of sugar and of salt, or other electrolytic solutes. Or, an examination of the conductivities of dilute and concentrated solutions of a weak electrolyte, such as acetic acid. Or, an examination of the colligative properties of an electrolyte
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introductory chemistry teachers dissolved in a solvent of low dielectric constant (HC1 in toluene, for example) compared to the colligative properties of a similar solution, but using a solvent of higher dielectric constant (HCI in water). This evidence, indeed, says nothing about electrolytic processes, but i t does support the postulate that ions are present only in those solutions which do "conduct" an electric current, and thus buttresses the details suggested above. Introductory discussions often stop at this point. However, further exposition could point out that other events can be expected: For example, if Cu2+ is reduced at the cathode, the region near the cathode becomes negatively charged, due to the presence of spectator anions. These will tend to leave the cathode region a t some average rate, and other positively charged cations will tend to enter the region, at some other average rate. The two rates will not ordinarily be the same. The anions, or cations, are surrounded by an environment of oppositely charged ions, and by molecules of solvent as well. These "drag" the moving ion, reducing its otherwise greater rate away from, or toward, the cathode as the case may be. We conclude that the effective movement of an ion might depend upon several factors: the magnitude of its charge, its size, the degree of solvation, and the concentration of counter-charged ions are the most important factors. I n less simple electrolyte solutions, the possibility of complex ions should be included, such as CuCla2- for the example mentioned; such ions will not only move in a different direction than solvated Cu2+ ions, they will move a t a different rate. To summarize, charge is transferred externally by electron flow, and internally by a variety of ion movements at different rates and directions, which depend upon the characteristics of each different ion that is present. Question What is a. compound? In most beginning texts the lrtw of definite proportions (composition) is usually stated in a way to imply that to be a compound, s. material must have a. fixed, definite, composition; yet there are other substrtnces, I understand, which do not have fixed compositions but are called "nonstoichiometric compounds!' Is or is not a. definite composition a necessary characteristic of a. compound?
Answer
by Laurence E. Strong, Earlham College, Richmond, Indiana The first question can be answered in two rather different ways. One way is in theoretical terms. A Volume 47, Number 7, July 1970
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compound is a material for which the appropriate mental model consists of a collection of electrons and nuclei exhibiting a definite structure with two or more kinds of nuclei present. For any sample the structure is described in terms of a repeated unit which is either a molecule or, for some solids, a unit cell. Another way to define a compound is through the use of experimental terms. In this definition a set of experimental operations are described which, if performed on a material that is a compound, would give results that would be characteristically different from the results on all materials that are not compounds. There are two general classes of materials that are not compounds: solutions and heterogeneous mixtures. (Another class of non-compounds, the 100 or so elements, is not considered further here.) The experimental operations can be regarded as a scheme for classifying diverse materials, on the basis of experimental observations, as compounds, solutions, or other mixtures. Two of these experimental operations, or definitions, will illustrate the means for classification. For example, when a homogeneous material is prepared by "assembling" two or more elements, we ask: is it a solution or a compound? To find out, the assembled material is subjected to an operation in which its phase is changed (solid to liquid, liquid to gas, etc.) by altering the temperature or pressure or by adding some other substance. The material is a compound if, during the phase change, the portion of the initial material that has not yet transferred to the new phase continues to have the same characteristic properties as did the entire mass of initial material. (Example: a liquid assembly of salt and water is not a compound since, upon partial freezing, the residual liquid has properties different from the original liquid; a higher boiling point, and other changed properties, in this case.) Usually, more than one type of phase change is necessary to eliminate ambiguities. (A eutectic mixture of salt and water would pass the freezing test with unchanged residual liquid, so one would perhaps also wish to vaporize this particular mixture and examine the residual liquid after partial vaporization.) In a few words, a compound is a material which remains invariant during changes in phase and is produced from two or more elements. A somewhat different experimental definition of a compound can be based upon observations made during the synthesis of a supposed compouud from its elements or from other materials. When a material is prepared by mixing together two or more elements, or materials, the product is a compound if, during its preparation, the following observations are made: For different mass ratios of the original reactants, the same characteristic properties are found for the prepared material. (Example: For several different mixtures of H1 and Oz, each with different proportions of H2 and 02,only one invariant material is produced; and except for one unique case, one or the other of the original reagents remains, in excess.) In a few words, a compound is a material that is invariant when prepared from varying mass ratios of two or more elements. In general, the two experimental definitions lead to the same conclusions about any given material. Which definition is used will ordinarily depend upon the circumstances. Ideally, both criteria would be applied. 524
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Among materials that are gases or liquid there is no problem in sharply distinguishing between compounds and solutions. The same is true of so-called molecular solids, such as solid carbon dioxide, or urea. However, there are other kinds of solids, in which the distinctions are less clear. That is, there are some solid materials often referred to as nonstoichiometric compounds. An example is provided by copper sulfide. A crystalline copper sulfide can be prepared with a composition corresponding accurately to CuzS and which behaves in phase changes about as a compound does. However, it is also possible to increase the relative amount of sulfur in this sulfide in a continuous fashion over a small range.' Some chemists prefer to call this copper sulfide a nonstoichiometric compound while others prefer to designate it as a solution of sulfur and CuzS or possibly a solution of copper and sulfur. (By contrast, another copper sulfide, CuS, behaves in a straightforward way as a compound, though it is not readily prepared by beginners in laboratory chemistry by heating a mixture of copper and ~ulfur.~) Another type of solid material is exemplified by titanium dioxide. Here the parent compound is Ti02, but it is possible to reduce the relative amount of oxygen and prepare a continuous series of materials. Hence, titanium dioxide has sometimes been called a nonstoichiometric compound. However, recent precision X-ray studies reveal that there are a number of well-defined and discrete crystals with such compositions as TiOz, Til0O1o,Tipon,and so on to TiaOi. (This exemplifies a third experimental definition: the existence of discrete, well-defined crystal structures, as disclosed by X-ray diffraction; to identify the existence of a compound.) The variable composition of titanium dioxide is really a varying mixture of several compounds, each of fixed composition. What appear to some to be compounds of variable composition and to others as solid solutions has been discussed by Phillips and Williams3who state The distinction between solid solutions, phases stable over wide composition ranges (with or without some st,ructural feature and naturally without stoichiometry) and compounds stable over narrow composition limits with well-defined stractures and possible stoichiametric formulas, depends upon the changes of A (Helmholz free energy) with composition.
This point of view leads Phillips and Williams to conclude that two elements, X and Y, form a compound when the interaction of X and Y is strongly cooperative. That is, when not only the binding for X-Y is greater than the mean of X-X and Y-Y, but the binding of X-Y becomes stronger as more and more units interact with one another. Put succinctly, strongly cooperative bonds lead to an ordered structure, even when the composition is somewhat variable (this raises questions about the validity of the third experimental definition, above). Or, one could say that solid solutions of X and Y differ in degree but not in kind from compounds of X and Y. I i o s ~ n o o a l?. , H., Economic Geology, 61, 641 (1966). DINGLEDY, D., AND BARNARD, W. M., J. Cnlihc. EDUC., 44, 242 (1967). PHILLIPS, C. S. G., AND WILLI.AMS, R. J. P., "Inorganic Chemistry," Oxford Univ. Press, Oxford, 1965, Val. 1, pp. 283306. 1
There is no generally agreed upon answer to the second question asked. My personal opinion is that it is best to regard a compound as a material with jixed composition during transfer between phases and with a composition independent of the mass ratio of the elements used to assemble the system in which the material is synthesized. If this is accepted, the term "nonstoichiometric compound" would refer to a material which behaves somewhat like a compound as its phase is changed, but also behaves like a solution in that its composition is determined by the mass ratio of the components of the system from which it is prepared. Question What is a molecule?
Answer
by Laurence E. Strong, Earlham College, Richmmd, Indiana
I n a few words, a molecule is the dynamic unit in the mental model of a gas. For a gas that is a single substance all molecules are identical and the number of molecules per gram is an extremely large number. A few more words may be in order to make clear what is meant by a dynamic unit. A gaseous molecule is assumed to be in continuous random translational motion. If that molecule has more than one nucleus, it will also possess rotational and vibrational motions. These various motions determine the kinetic energy of each molecule. A molecule is also assumed to have a structure provided by an arrangement of nuclei and electrons. To a good approximation the mass of a molecule is the mass of the nuclei present while the volume is contributed by the electrons. The configuration of nuclei and electrons with their associated charges provides an electrostatic potential energy. Since most liquids are rather easily converted to gases it is reasonable to assume that the mental model of a liquid could also use a molecule as a dynamic unit. Other things being equal, a liquid will possess a substantially lower potential energy than its corresponding gas. Or, put differently, the units which were considered to be molecular, and separate, units in the gaseous state are bonded together, loosely or strongly, in the liquid. Hydrogen bonding of H 2 0units is a familiar example; some suggest that the unit in liquid water is (HzO),, where n is a varying number, ranging from perhaps 3 or 4 upward (depending upon the temperature). The question is whether this concept of a unit is useful when one attempts to account for the chemical and physical properties of liquid water; and, on this,
opinions differ. (Indeed, it is useful as a concept for this purpose, but it is not therefore necessary to consider (H,O),, itself, as a unit, some aver.) On the other hand, it is generally agreed that some solids are most usefully (in the above sense) thought of in terms of a mental model with the (gaseous) molecule as a dynamic unit; solid carbon dioxide is an example of this. But other solids are not usually interpreted as molecular (even though they may he thought of as capable of existing as gases in molecular units); cesium chloride serves as an example in this instance. Such solids are thought of as ionic, and the "unit" is only mentally separable; in solid cesium chloride, again, a "unit cell" can be thought of as consisting of one ion surrounded by eight one-eighth-spheres of oppositely charged ions, cubically disposed. Clearly, this is not a molecule in the sense suggested in the first paragraph. Some chemists choose to regard a solid crystal such as diamond, or a metal, as a molecule which is coincident with the entire crystal. Such a view does not seem to have any particular logical consequences different from simply regarding the crystal as an ordered set of atoms. Therefore, it does not seem useful to apply the molecule idea to such crystalline solids.
Editors' Comments The above discussions were generated by questions we have received and by stimulating private, and vigorous, discussions with Professor Strong. I n particular, we also wish to acknowledge the remarks of Professor Harvey P. Stein of Trenton State College, Trent,on, N. J., who has properly argued against the statement that "a molecule is the smdlest particle of a subAs he stance which exhibits the properties of that substance!' says, even casual reflection indicates that this definition is defective. It fails to take into account that we observe properties of bulk matter, hence of t~ggregatesof molecules. No single molecule of CClc for example, has a boiling point of 76.75"C, nor 8. refractive index for sodium D line of 1.46305, nor any other similar type of property. Further, single molecules, could they be observed, would often be much more reactive than when in an environment of many similar molecules. But, though we agree with the position taken by Professor Strong, and endorse the caveat suggested by Professor Stein, we feel that the question remains: What does one tell beginning students? Certainly, they should not be told all that is here discussed when they are first introduced to the subject. Certainly, they should not receive an inaeeurrute definition of a molecule or a compound. We hope that the presentation here will help each teacher to reach his own decisions. We suggest, for further reeding, two other disoossions: A Provocative Opinion, by Professor Strong, in THIS JOUIINAL, 45, 51 (1968), and one which presents a slightly different point of view, on p q e s 67-69 of the book "Chemical Bonding Clsrified through Quantum Mechanics," by G. C. Pimentel and R. D. Sprately, [HoldenDay, Inc., San Francisco, 19691; that discussion is not laced with obscurely described quantum mechanics. JGM and JAY
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