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HIGH-SCHOOL NOTES
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MODERN CHEMICAL CONCEPTS AND THE HIGH-SCHOOL CURRICULUM JOSEPH F. CASTKA B o p High School, New York City
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HE purpose of this article is to present, for high- of the concepts is attempted. With the increasing school use, some of the available theoretical and organization of honor schools throughout the country experimental material dealing with three of the this material may prove very valuable in the curriculum fundamental chemical concepts. The concepts that of such schools. will be considered are (1) the electronic theory of The concepts are discussed under the headings: matter, (2) the interionic attraction theory of strong (1) The present state of qur knowledge. electrolytes, and (3) the Br@nsted-Lowry acid-base (2) How much shall we present to high-school theory. pupils and why? It is characteristic of science to attempt to present Method of presentation to pupils--demonsea(3) intellectual concepts to pupils in what is considered tion experiments. the most correct form a t the present time. It is mainly on this basis that we should attempt the presentation I. THE ELECTRONIC THEORY OF MATTER to our pupils. Theoretical material of this sort is ( A ) The Present State of Our Knowledge stimulating to pupil imagination and furnishes splendid The neutron was discovered by Chadwick (1932) by material and opportunity for the teaching of scientific attitudes and methods. This teaching, howevei, must bombarding beryllium with alpha particles from polobe done on such a level that it helps in the groyvth of nium. The neutrons, emitted as a result of the disthe pupil's conceptual mass without imposing too drastic integration, were allowed to strike p a r a and caused an increase in the quantity of subject matter. There the emission of protons which were measured by a is no doubt that these concepts present difficulties in Geiger counter. Momentum and energy considerations presentation and comprehension. It is hoped that the caused Chadwick to conclnde'that only a particle of material herein presented will decrease these difficul- the approximate mass of 'a proton could cause the emission of protons. The particle that caused these ties. An analysis of the more recent high-school texts shows emissions had a great range and was not deflected in an that in many instances these newer concepts are treated electric or magnetic field. This particle, then, of mass somewhat extensively. The texts considered are listed approximately that of a proton, , a d of neutral charge, was christened the neutron. Thisparticle, as expected, in Table 1, and the extent of treatment indicated. showed no path in a Wilson cloud chamber but its TABLE 1 C presence was sometimes shown when i t struck someTrcnlnenr thing, and charged particles resulted with consequent Elcrlronic lhrory Inlnionic Brlnrlcd Tcxt Neubon end posllro(r lhcory thrmy tracks. Book 1 (1934) None Nme None The particle has also been produced as the result of Book 2 (1935) Extensive Moderate Moderate artificial disintegrations or transmutations. It is also Book 3 (1938) None None None Book 4 (1937) Moderate None None one of the projectiles used in producing such disinteBook 6 (1937) Extensive None None Book 0 (1939) grations. Fermi used it in this way and also in the Exten~ive Moderate Slight Book 7 (1938) Extensive Extensive Slight ~roductionof artificiallv radioactive materials. which. Book 8 (1939) Extensive None Moderate - apparently, were transuranic elements. More recently Book 3 is an historical introductory treatment of the neutron has been used in disintegrations that caused chemistry, and the electronic and ionization theories the release of large quantities of energy. are not even mentioned. Book 4 is primarily a book Closely allied with the neutron is the positron, which on consumer chemistry. Book 1is slightly out of date. Anderson discovered in the same year, 1932. The Consideration of the treatment in the other books study of a cloud track photograph disclosed a particle shows that these concepts are finding their way into of the same mass and quantity of charge as the electhe newer high-school texts. In none of these books is tron but opposite in sign. The presence of a magnetic any demonstration material suggested. This is par- field causes this particle to move in the opposite directicularly true of the interionic and Br#nsted theories. tion from an electron and, therefore, it must be opposite This same difficulty is present even in many college in charge. The positron, thus discovered, had been texts. Some simple demonstrations are described in driven out of the nucleus of lead by gamma radiations this article and may prove helpful where presentation (photons). 487
Positrons have since been produced in a variety of ways. The most interesting of these was the one performed by the Joliot-Curies in which energy was changed into matter (the materialization of energy). A gamma particle (photon) was shot into lead, and this particle was transformed into a positron and an electron. It is now possible to change matter into energy and energy into matter. From the study of artificial disintegrations and radioactivity and the above experiments a proton and neutron are defined as elementary particles in different quantum states. This definition is necessary since i t appears that
+
an electron (1) a neutron + a proton a neutron (2) a proton + a positron It is not possible, therefore, to say that a neutron is just a combined electron and proton. To further complicate matters new particles like neutrino and antineutrino seem necessary because of momentum and energy considerations. A newer particle, the mesotron, has also been announced recently. From the study of hypertine spectral lines and nuclear spin Pauli and Heisenberg have concluded that there are no free electrons in the nucleus of an atom. The nucleus consists of neutrons and protons and not of protons and electrons as was formerly supposed. The mass number of any 4 isotope is therefore the sum of the number of protons and neutrons.
+
( B ) What Materials Shall We Present to High-school Pupils and Why? At the high-school level it seems necessary to discard much of this available material. Neutrino, antineutrino, mesotron, and most energy and momentum considerations come under this classification. The positron might be mentioned as a result of nuclear disintegrations, but since there is no definite information that it actually exists in the ordinary nucleus * some complications might arise. It seems essential to present the following material.
. .
The first considered as of protons and electrons, then later, of protons and neutrons. as combined Neutrons to be and electrons. The atomic number is the number of protons in the nucleus. The atomic weight is numerically equal to the sum of the number of neutrons and protons. The neutron weight is about the same as that of a proton, or about the same as the sum of the weights of a proton and electron. The positron might be mentioned as a result of disintegrations. Matter-energy and energy-matter transformations, described on a simple basis experimentally, might prove valuable supplementary material.
These ideas should be presented because
(1) They more closely approximate the truth, as known, than our present materials. (2) They are expedient-too much subject matter cannot be added. (3) It should be easier to prevent the confusion that exists in the pupil's mind about atomic number and atomic weight. Atomic number is the numher of protons experimentally determined by the Moseley method. The difference between the atomic weight (experimentally determined) and the atomic number is equal to the number of neutrons. (4) Pupil reading in newspapers and scientificmagazines will be made easier and give greater reader comprehension. This is especially significant for those pupils whose formal education ends with the high school.
(C) Method of Presentation to Pupils This should present little difficulty for these ideas may be included in our regular scheme of presenting the electronic theory without the expenditure of much additional time. A possible presentation is the following. As a result of the use of the Crookes tube and of other demonstrations, such as a simple voltaic cell (zinc and copper strips in sulfuric acid) hookup with a small electric light bulb; or the fused salt experiment, described later, the pupil arrives a t the idea that matter is electrical in nature. By using the magnet with the Crookes tube it is possible for the pupil to see that the deflectedlight stream must consist of electricallycharged matter. Then since atoms are electrically neutral, the number of positively and negatively charged particles must be equal.. From the consideration of the fact that many of the atomic weights approximate whole numbers, the pupil arrives a t the idea that the atom must be made up of some fundamental building block of unit mass. The experimental evidence about the existence and comparative weights of the electron and proton is then introduced. Diagrams of the hydrogen atom, consisting of one proton and electron, and of the helium atom, consisting of four protons and electrons, can he drawn. Experimental evidence from the Rutherford nuclear study and the Moseley experiment described, and a second set of diagrams of is the hydrogen and helium atoms-drawn. The hydrogen atom.consists of a nucleus of one orotou and has one planetary electron. The helium atom has a nucleus of four protons and two electrons and two planetary electrons. The apparent discrepancy in the two diagrams of the helium atom should lead to pupil inquiry as to the cause of the difference. This speculation might be directed by citing once more that Moseley's experiments indicated only two protons in the nucleus. Presentation of Chadwick's experiment in simplified form, in which mention is made of a new experimental particle that is not deflected by passage between positively and negatively charged plates, should lead to a simplified drawing of the helium nucleus. The drawing might first contain two protons and two particles composed of a proton and
electron, and the final diagram would show the new particle, represented by n,taking the place of the combined electron-proton particles. The confusion resulting from the use of such terms as free proton, excess proton, and free and combined electron would then be eliminated. A statement to the effect that, experimentally, there are no electrons in the nucleus would clinch tlie matter. The terms atomic weight and atomic number now have a more significant difference. The use of the neutron also makes possible simpler diagrams of the nuclei of isotopes. The difference in nuclei is then seen to be due to difference in number of neutrons.
forces increase with a subsequent lowering of equivalent conductivity and apparent degree of ionization. The experimental evidence favors the interionic attraction theory. Bragg's work shows that crystals of strong electrolytes are already completely ionized. All strong electrolytes of the same valence type show the same apparent degree of dissociation, and those of the higher valence type show a smaller apparent degree of dissociation on account of the greater strength of the ionic atmosphere. The formula, worked out by Debye and Hiickel for the ionic strength of solution, according to the interionic attraction theory, takes into account the greater charges on the higher valence types and ascribes greater attractive forces to these higher charges. 11. THE THEORY OF STRONG ELECTROLYTES (INTERIONICExperiment shows that the solubility product of difficultly soluble materials is increased by the presence of ATTRACTION) strong electrolytes and the higher valence type salts ( A ) The Present Stute of Our Knowledge have a greater effect in increasing the solubility. EfThe Arrhenius theory of ionization was propounded fects such as that of presence of potassium nitrate on long before the Bragg work on the ionic nature of crys- solubility of calcium sulfate are well known. tals. Working on the assumption that conductance Experiment also shows that the presence of the ionic in solution depends on three factors-(1) the number of atmosphere of a strong electrolyte will increase the ions, (2) the speed of the ions, and (3) the valence of ionization of a weak electrolyte. The following experithe ions-Arrhenins further assumed that the speed ment worked stepwise will show this effect and can be and valence of ions were constant and that changes in useful in a high-school demonstration. conductivity were due to changes in the number of ions. This was experimentally true for weak electrolytes, and by conductivity measurements it was possible to cal(1) Methyl Orange with water-orange-yellow culate the degree of ionization. (2) Methyl Orange with dilute hydrochloric add-red For strong electrolytes, however, grave discrepancies (3) Methyl Orange with dilute aeetie a c i d a l m m t a* red ar 2 Methyl orange with very dilvte aeetie aeid-a very alight d-orange arose. In calculating the per cent ionization for strong (4) I51 Same solution as (4). with much solid sodium chloride added-henges electrolytes by the same method such results as the from red-orange to a very definite pipr .. following appeared : This change iri the ionization of very dilute acetic acid TABLE 2 is due to the ionic atmosphere of sodium and chlorine 1 M SOLOTIDNI~ ions which prevent the reformation of molecular acetic acid and consequent increase in pH and indicator effects. This experiment shows,that there is such a thing as an ionic atmosphere and that it has significant and demonstrable results. These degrees of ionization were treafed as actualities even though solutions of potassium chloride showed ( B ) What Materials Shall We Present to High-school only properties of potassium and chloride ions, and no Pupils and Why properties of molecules of potassium chloride. ArrMost of this material does not lend itself to presentahenius ascribed these differences in ionization to the tion on the high-school level. - It is significant, however, formation of molecules. to show that strong electrolytes are completely ionized The more recent interionic attraction theory, acceptin solution just as they are in the crystalline lattice ing the work of Bragg and others, assumes complete structure and in the fused salts. It might also be imionization. It ascribes changes in conductance with dilution to the electrical forces between charged ions. portant to show ionization as a continuous phenomenon Each ion has about it an ionic atmosphere of oppositely in nature by demonstrating conductivity of salts in media other than water. An experiment, useful for charged ions which retard the speed of movement of ions through the solution. In other words, it assumes demonstrating this phenomenon, is described later. that the number of ions of strong electrolytes is always This conductivity in other media has no significance in one hundred per cent and that valence is constant. the interionic theory but is helpful in giving the pupil a complete picture of the phenomenon. This type of Changes in conductivity, therefore, are due to changes treatment will furnish the pupils with a truer conceptual in the speed of ions because of this ionic atmosphere. basis for the college work on ionization. As a solution becomes more concentrated the attractive Closelv allied with the subject of ionization is the valence and theories of valence. The text' CURTMAN,"Qualitative chemical analysis," 2nd ed., The subject book analysis, mentioned a t the beginning of the article, Macmillan Co., New York City, 1938.
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showed that in some instances ideas of valence were not presented in their true light and that consequently wrong ideas might result. One author shows an electronic picture of hydrochloric acid in which it looks like an electrovalent compound. Another author shows an electronic diagram of magnesium oxide which makes i t look like a non-polar covalent compound. The diiculty is that we are too frequently satisfied with teaching the formation of a polar compound and its structure and with possibly a slight mention of nonpolar componnds. It would probably be better to teach compound formation as due to (1) real electrovalence as shown in sodium chloride, (2) polar covalence as shown in hydrochloric acid and acetic acid, and (3) non-polar covalence as shown in carbon tetrachloride. This treatment also presents a more accurate picture and shows the gradation in types of valence that is responsible for variations in chemical behavior.
(C) Method of Presentation to Puflils The groundwork for the correct approach to these concepts might consist of a review of the structure of such electrovalent componnds as sodium chloride with a redrawing of the electronic picture. (D) Demonstration (Conductivity of Fused Salts) A large crystal of potassium dichromate is placed between the electrodes in an electrode lamp hookup and the question asked, "If this crystal is made of ions why doesn't it conduct the current?" On heating the crystal in an evaporating dish with a Bunsen burner, fusion takes place very quickly, and the lamp lights. The pupils are asked to explain why the lamp lit after fusion. They should get the idea that heating provided additional energy which accounted for the motion of the ions and that the balanced electrostatic forces prevented such motion in the solid state. A brief report on Bragg's experiment would pe helpful a t this point. Potassium dichromate was selected because it fused very quickly. Other materials thSt also fuse easily are sodium hydroxide and silver nitrate. It would be ideal to use sodium chloride. This takes a little more time with an ordinary Buusen burner, and only a small quantity may be fused. If a small porcelain crucible and a modern Timll burner are used, the fusion takes place within a reasonable time. With sodium chloride it is advisable not to use platinum electrodes. Sodium hydroxide also has an effect on platinum and, besides, may destroy the crucible. Iron or graphite electrodes work best with these materials. The next question in the development should be, "How many ions are present in (a) the solid state, ( b ) the fused condition?" Then observation of the brightness of the lamp in the fused and dissolved condition (crystals in water) should lead to the idea that the material is practically all ionic in the solid, fused, and dissolved conditions.
(E) Demonstration on Valence The three types of valence can be shown by using
part of the above experiment to show that a fused salt (liquid) will conduct, while other liquids like acetic acid and carbon tetrachloride will not conduct. Addition of water to solid salt and to each of the above liquids will show that there are three diierent effects which can be explained on the basis of three diierent kinds of valence. The electrode lamp hookup is used in this experiment also. Originally, sulfuric acid was also tried as an example of a polar covalent compound which was much more ionized and therefore more polar than acetic acid. Ordinary laboratory sulfuric acid gave such a high conductivity even without the addition of water that it could not be used.
(F) Demonstration on Iaieation in Nan-aqueous Media With a galvanometer, electrode (platinum), electric bulb hookup test the conductivity of (1) acetic acid (glacial acetic was allowed to solidify, remaining liquid poured off and sufficient acetic anhydride added to remove any water that might be present), (2) sodium chloride dissolved in this prepared acetic acid, and (3) sodium acetate in the acetic acid. The results were (1) no conductivity, (2) slight conductivity by galvanometer, but bulb does not light, and (3) greater conductivity by galvanometer, but bulb still unlit. If solid potassium iodide is dissolved in acetone and the solution placed in the same setup, sufficientconductivity takes place to light the bulb. Iodine, as shown by the resultant brown coloration, is set free a t he anode. Potassium is liberated a t the cathode and a secondary action occurs in which bubbles of gas, prohably hydrogen, are liberated-flue to the action of the liberated pqtassium on the en01 form of the acetone. The resultant compound adheres to the electrode (cathode), and when the cathode is placed in a red litmus solution the litmus turns blue. That this litmus change was not due to the hydrolysis of any adhering potassium iodide was shown by placing a comparatively large quantity of the potassium iodide-acetone solution in red litmus with no effect. The potassium iodide is only slightly soluble, although much ionized. This experiment shows electrolysis in a non-aqueous medium and closely resembles the electrolysis of sodium chloride in water solution. 111.
BR~~NSTED-LOWRY THEORY
OF ACIDS AND BASES
( A ) The Present State of Our Knowledge It is impossible in this brief treatment to present the many complicated phases of the theory and the information given here is more or less on an elementary level. The classical concept of an acid is that of a substance which produces hydrogen ion in water solution. Experimental evidence now shows that (1) ionization takes place in many other media besides water; (2) indicator reactions, acidic action on carbonates, and catalytic activity by acids take place in other media besides water; and (3) (on a more complex level) indicator reactions, catalysis, action on carbonates, and measurement of hydrogen electrode potentials
take place and are measurable even in non-ionizing solvents like benzene. Although item (3) (above) will not be considered a t length in the discussion i t is well established that the fundamental concept of an acid, according to Brgnsted, amply covers and explains the experimental material. The Brglnsted definition is, an acid is a substance that can dissociate into a proton and a corresponding base, e. g., Acid --t proton base C1HC1-+ proton (H+) HAc --t proton (H+) Ac-
+
+
+
This generalized definition is one of the most fundamental concepts in chemistry. Together with the other basic generalization Reductant -*oxidant
+ electron
it serves to explain the two most fundamental chemical processes in terms of the two most fundamental units of matter, the electron and proton. Another fundamental idea is that, in ionizable solvents, a double acid-base reaction occurs in which the solvent may serve as either an acid or a base depending on what is added. The typical reaction is
Hydrolysis need no longer be considered a separate phenomenon in water because there is an easy explanation. Sodium acetate reacts basically because it yields the acetate ion which is a base. Ammonium chloride reacts acidicallybecause i t yields theammonium ion which is an acid. This presents a simpler explanation in many cases, but in all cases it is still necessary to realize that water is an ampholyte, a substance capable of associating or dissociating a proton. Strength of acids depends on such factors as the dielectric constant of the solvent and the ability of the solvent to accept protons. Acids show marked differences in strength in other solvents when compared with their strength in water. Experiments in nonionizable media like benzene show particularly significant results in the acid strength of so-called strong acids in water.
(B) What Material Shall We Present to High-school
Pupils and Why? The idea is that, because of such similar phenomenon as conductivity and indicator reactions in other ionizable solvents besides water, we can extend the definition of an acid to a more fundamental concept-an acid is a substance that can donate a proton a n d a base can accept the proton. This gives a more nearly true base 1 Acid 1 base 2 + acid 2 picture of one of the most fundamental concepts. We HCl Hz0 + H30+ C1teach the oxidation-reduction concept in terms of the C1HC1 NHs + NH4+ electron, why not teach this other fundamental conHCl HAc + H.HAcf C1cept in terms of the proton? OHHzO NHI --t NH4+ Practically all of the other ideas must be rejected. The concept is in a state of flux so that even in modern Exurnfiles are (1) HCl in water; (2) HC1 in liquid college texts the ideas are not too clearly presented. ammonia; (3) HC1 in acetic acid; (4) NH3 in water. In line with the more fundamental concept that acid Any further attempt to exten8 the ideas would lead to yields proton plus base, it is evident that acid 1 has a confusionin the minds of the pupils that would destroy conjugate base 1, just as base 2 has a conjugateacid any conceptual basis for college teaching, There is called acid 2. Examples 1 and 4 show the solvent little material that can be demonstrated to the pupil on a acting as either an acid or a base. A further conclusion simple basis. The indicator reactions that wodd be demonstrated is that hydrogen ions in water solution are really hydrated protons, H30+,called the hydronfum or oxonium to the pupib would naturally cause an extension of the ion. This ion is similar to the solviited proton in idea of neutralization as discussed above, although ammonia, NH4+, and the solvated proton in acetic this need not he emphasized. (C) Method of Presentation to Pupils acid, H.HAc+. A comparatively new idea is that the negative ion, C1-, is now a base just as OH- is the ~ ionic b t theory i o the ~ pupils In 0" ~ ~ ~ ~of the ordinary base in the present water system. Subarrive a t the definition of an acid as a subtance that stanceslike sodium hydroxide and potassium hydroxide, ftn~fshes hydrogen ion in water solution. A base is which are commonlv called bases. are salts and not - - - ~ ----- -, , ~ ~ - bases in this newer concept.~h~~ may be treated as defined as a substance that furnishes hydroxyl ions in bases, however, because they furnish a large supply of water solution. The demonstration showing ionization the strongest base or proton acceptor in aqueous solu- 1" acetic acid and acetone could be used a t this point to bring out the idea of ionization in other media. tion, the OH- ion. A salt is now simply defined as an ionic compoun~ The definition is then extended to emphasize the acid as a proton donor. Comparison of the behavior of the and its definition need no lonper be tied u~ with " hydrated proton in water with the ordinary properties tralization. of the electrical particle would lead to the idea that ionizable media neutralization now ,becomes a generalized definition in which a base and an acid there is a difference in these two particles and that an acid has a hydrated proton. The term oxoninm or hyunite to form molecules of solvent. dronium ion might be introduced as well as the symbol, In water HsO+ OH- -+ 2Ha0 q5 H30+. The simple ionic equation, now used for I n ammonia NH,+ NHn- ---t 2NH3 neutralization, might be used to bring out the idea of
+
+
+ + + +
~
~
+ +
+
+
~
~
~
~~
+
+
~~~
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the base as a proton acceptor. The new equation nsing the oxonium ion would then cause a somewhat enlarged view of neutralization as resulting in the formation of solvent. H+ + O H - + H*O HaO+ OH- + 2Hz0
+
These might serve as the mairi outgrowths of one of the experiments described which show indicator effects and neutralization in media other than water.
( D ) Demonstrations on Indicator Reactions and Neutralization (1) A one per cent solution of basic fnchsin in (a) water and (b) refined glacial acetic acid is the indicator used, respectively, in (a) water and (b) refined acetic acid media. Solid oxalic acid is used as the acid and solid sodium acetate as the base. Incidentally, TABLE 4 F~uchrinin Wafer RI1u11nnt color Reogcn1
B&C
),,(
O ~ a l i ea d d in water
Purple'
( b ) sodium aeetate in Pink water (c) ~ d d i t i o nof ereuo d i d NaAe to purple solution (4, from the excess solid add is removed causes the pwple color to disappear and the pink color sppeare.
Soma Ixdirotor i n Acrlic Acid Rp.ge"i
Rerullnnl color
(a) Oralic a d d in acetic Pvrple add ( b ) sodium acetate in Pink aeetie acid ( 6 ) Duplication in water of this procedure cause* the~sameresult(. Changw in bath media seem to indicate neutralization.
the fact that NaAc acts as a base with litmus in water and acts as a base in HAc should prove stimulating to pupil inquiry. (2) The same experiment will also work with methyl orange indicator in methyl alcohol. One or two drops of this indicator are added to either the water or methyl alcohol medium. The same acid and base may be used. TABLE 5 Mdhyl Oronge in Wolar Rcngrnl (a) Oxalie acid in water
Resull
Red
( b ) Sodium acetate in wafer Yellow ( r ) Add excern oxalic acid as in Experimcnt 1and the yellow disappears and red rerults. The reverse procedure doer not give as good results near yellow as
Soma Indicator in Methyl Alcohol fingcnl RErllll ( 0 ) Oxalie acid in methyl Red alcohol ( b ) Sodium acethte in Yellow methyl slcohol (c) Duplicate (r)-similar results. The reverse in which cxcsodium acetate ir added to the acid gives orange.
er~ected.
(3) If these materials are not available the common indicator, phenolphthalein, will give good results with oxalic acid and solid sodium hydroxide used in water and methyl alcohol solvents. The experiment with basic fuchsin is probably most legitimate and the last one employing phenolphthalein probably the least legitimate. However, the pupils are already familiar with sodium hydroxide and probably with oxalic acid. Since saturated solutions are being dealt with in all cases, we do not get as marked effects as might be possible with dilute solutions.
