chemical queries m Diego Stole College zn Diego, California
. . especially for high school teachers
Question Which is correct,, Celsius or centigrade? Answer
Temperature measurements are becoming more and more refined, with the result that earlier temperature standards are no longer reproducible within the more precise measurements now possible or soon to become possible. Within a few years, the number of degrees between the freezing and hoiling points of water may turn ont to he not precisely 100, the number implied in the designation "centigrade." Thus, t,his term is now misleading. Ant,icipating this development, the correct name for the temperature scale in the International System (the old "AIetric System") has been designated "Celsius," with the same abbreviation as before, "C." The triple point of water (O.Ol°C) can he reliably reproduced today in any good lahoratory with the proper equipment, but this is not true for the freezing and boiling points of water. Accordingly, the triple point has replaced the freezing point of water as the fixed point in the new Celsius scale. The value O.Ol°C was selected for the triple point because it does not change the familiar temperature scale mit,hin the accuracy now available. Other fixed points remain as before, e.g, the norn1a.l boiling point of water, 100°C; the freezing point of silver, 960.S°C; etc. The exact relation between this pract,ical scale and the true thermodynamic temperature scale is unknown; the two scales agree precisely a t only one point-the triple point of water. The other fixed points will he adjusted from time to time as their values with respect to the thermodynamic temperature scale become better known. Question I have heard that the waddle wheel effect in the Crookefi tube does not illustrate the property of mass of electrons. If not, haw is it explained? Answer
The rotation of the paddle wheel is caused by a thermal effect; a t low pressures this effect is more Eol~on'sNOTE: Chemical Queries is one part of a feature plsmred especially for high school teachers. This topic also will appear in the April, July, and October issues. I n the near fubwe we hope t,o have a column of particular interest t,a high school teachers in each issue. \leanwhile, we solicit qneslions for this column and reactions to the answers given. Co~~espoadence should be addressed to Dr. James G. hlalik a t San Diego State College, San Diego, Calif. 38
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Journal o f Chemical Education
pronounced. Electron impact on one side c ~ af blade of the paddle wheel heats that side, of course. and the molecules of gas present tend to move from the cooler side to this warmed side, thus producing a slight pressure difference, higher on the heated side. Hence the blade moves. The magnitude of the effect is proportional to the length of the periphery of the paddle blades around which the diffusion occurs, not to their J. A , area. (For a fuller description see CAMPBELL. J. CHEM.EDUC., 38, 480 (1961).) Question
In experiments with the Daniell electrochemical cell, the instructions call for adding NaB to the Cu-Cn2+ion side of the cell. As qualitatively predicted by Le Chabelier's pl.inriple, the cell's voltage drops. Ilowever, we have noticed that if a 1:wge amount of Na3Sis added, the polarity of the electrodes eviclenll?-becomes reversed became then we must change the connectiws on the voltmeter to properly register the reading. Has s. change in p H reversed the reaction or is s, new eleet,rode formed with s. new half reaction? Answer
I t is true that a change in pH can change the products formed in a reaction; hence changes in the emf for a redox reaction could he expected. Also. d i e n an electrode in a cell is altered a change in the enif also can be expected. However, there is yet a third possible explanation; this relates the polarity reversal to the decrease in the cupric ion concentration in solution, caused by the addition of sodium sulfide: and to the reaction of sulfide ion with metallic copper. \\'e will examine each of these explanations to determine their applicability to this problem. The emf of a Daniell cell is approximately 1 v when the zinc and copper ion concentrations are each approximately 1 m, and therefore a large change in pH would be required to change the polarity from +1 v to some value less than zero. However, when the change in p H is checked out in the laboratory with a p H meter or p H paper, it is found that the p H does not markedly change with addition of Sa2S. I t does not seem, therefore, that the results can be explained on the basis of n change in pH when Na2S is added to the copper containing half cell. Consider the possibility of a change in the copper electrode. It could become coated with CuS, thus altering its nature and affecting the emf of the cell. However, examination of the copper before and after addition of the N k S does not reveal an>- change. I t is unlikely that the copper could he coated d h even a
fine film of CuS which would he unobservable, therefore, we can dismiss this explanation, at least tentatively. We have, finally, the lowering of the concentration of the copper ion, as evidenced by the disappearance of the blue color, and the formation of CuS in the presence of excess sulfide ion. Ordinarily, the reaction which takes place in the Daniel1 cell is: From emf tables, Go = +0.76 - (-0.34) = +1.10 volts. From the Nernst equation, one can calculate what the voltage would be for any set of conceutrat2ons (activities, more precisely speaking) of the ions. Using brackets to signify activities, a t 2 5 T we have:
Answer
The concentrations are not critical; the same results are observed over a wide range of concentrations of these reagents. The critical aspect is the relative amounts of reagents. When tin(I1) ion is not present in excess, mercury(I1) is chiefly reduced to mercury(I), and the relatively insoluble, white mercury(1) chloride forms. With an excess of tin(I1) ion, some of the mercury(I1) ion is reduced to metallic mercury, which appears black. However, normally, some mercury(1) chloride is also formed, which, with the black, finely divided metallic mercury, forms the mixed precipitate which is dark gray. Question Which is it, Avogadro's Law, or Theory, or Hypothesis? Answer
Notice that if the activities of the Cu2+and Zn2+ions are equal, or approximately equal, the second term on the right of the equals sign has a numericalvalue much less than 1.10. Since this numericalvalue is smaller than the value of Go (which is positive) the voltage of the cell is positive even though this numerical value might be negative. Hence, the cell reaction proceeds in the direction indicated in eqn. (1). I n the presence of sulfide ion, however, it is reasonable to postulate this half reaction in the copper containing half cell: For the other half cell containing the zinc the reaction is:
If these two half reactions take place, the polarity of the cell would be reversed, since the zinc electrode would be the cathode instead of the anode. The Go values for eqns. (3) and (4) are +0.80 and -0.76 v, respectively. Combining eqns. (3) and (4), the postulated cell reaction is: For activities near unity, at 25"C, application of the Nernst equation to eqn. (5) predicts a positive emf for eqn. ( 5 ) .
Since eqn. (5) is consistent with the observed reversed polarity, it probably describes the cause of this observed fact. Note: The value of Go for eqn. (3) is not well established, it ranges from +0.74 to +1.01 v. Thevalue +0.80 is consistent with K,, = 1 X for cus. Question When I add a few drops of 0.1 M tin(I1) chloride solution to several milliliters oi 0.1 A1 mmereury(I1) chloride solution, a white precipitate appeals; but when I do the opposite, and add a few drops oi 0.1 M mercury(I1) chloride solution t o several milliliters of 0.1 molar tin(I1) chloride d u t i o n , u white precipitate forms, which rapidly turns dark gray. What is the explanation?
Avogadro's Hypothesis, or Theory, is preferable. A law, in science, is a summary of a set of observed facts. Thus, we have the Law of Multiple Proportions, Raoult's Law, the Ideal Gas Law. Each of these laws (and others) should be thought of as expressing a set of facts which are observed. However, observations are always subject to error or uncertainties. For example, the Law of Multiple Proportions infers that from the observed data we will obtain a ratio of two integers; only exceptionally, however, do the data yield exact integers. This same characteristic is especially notable in the case of the Ideal Gas Law where it is found that gases, as observed and measured, "obey" this law only to about two or three significant figures, and even then only over a limited range of pressures and temperatures. Nevertheless, these laws do summarize a large set of facts in a satisfactory manner. A hypothesis is an attempt to explain a set of facts in such a way that the explanation can be tested by further experimental investigation. That is, a hypothesis is a useful statement; it implies that certain experimental tests might be made and predicts the results (not always correctly). I n this sense, the word hypothesis might better be qualified by the phrase, "tentative hypothesis." When the experimental work, which is often implicitly described by the hypothesis, is carried out, and the predictions of the hypothesis found wrong factually, it is then desirable to modify the explanation to obtain a new, less tentative hypothesis. After sufficient testing, and consequent modifications, the explanation will conform rather closely to the criterion of correct prediction; when this happens, the explanation is usually called a theory. Therefore, a theory is a hypothesis which has been modified since it was first proposed, but which has not yet in its modified form been found to predict erroneously. Avogadro's explanation, as it happens, has never been modified since it was first proposed as a hypothesis; therefore, it is still properly called Avogadro's hypothesis. On the other hand, it has been extensively tested and not yet found factually wrong; so, it could be called Avogadro's Theory. But it is not a law, nor will it be until we can obsewe equal numbers of molecules in equal volumes of gases under similar conditions.
Volume 45, Number 1, h n u u r y 1968
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