Elementary Chemistry for Liberal Arts Students at St. John's College ANTHONY STANDEN S t . John's College, Annapolis, Maryland T WOULD probably be agreed upon as an axiom the analyses of carbon monoxide and carbon dioxide, that in elementary science courses the students the oxides of nitrogen, water and hydrogen peroxide. should themselves carry out a t least "token" experi- etc. But actually to analyze these substances would ments demonstrating the fundamental laws of the require troublesome methods which, even if done, would science concerned. However, for various reasons, in not give the students practice in any standard, generally all usual chemistry courses this is very far from being applicable methods of analysis. All chemical educators must have felt this difficulty. the case. The fundamental laws of chemistry may be cou- In our laboratory work a t St. John's College we have attempted to solve the problem by two radical desidered to be: partures from conventional practice: 1. Law of conservation of mass. 1. The students do volumetric analysis before being 2. Law of constant proportions. 3. Law of recipocal proportions. (Many modem introduced to the atomic theory, and consequently textbooks do not mention this as a separate without the help of formulas and equivalent weights. "law"; nevertheless it is a most important The students thus forgo the simplification of calculageneralization from chemical experiments. It tions which results from the system of normal and is not deducible from the law of constant decinormal solutions based on equivalent weights, but proportions, and it would, alone, be sufficient the computations are by no means too tedious, and the method forces the student to become quite clear about to lead to the atomic theory.) what happens in volumetric analysis. 4. Law of multiple proportions. 2. The laws of reciprocal and multiple proportions These laws are sufficient to support the atomic theory, are illustrated as referring to groups of atoms rather and to lead to the concept of equivalent weight. But in than to atoms. As an example, potash combines with order to determine atomic weights and valences there is oxalic acid to form four salts, in which the amounts of also necessary: oxalic acid in combination with the same amount of 5. Gay-Lussac's law of combining volumes of gases. potassium (or of potash) are in the ratio 1:2.4. This Of these five fundamental experiments surprisingly is just as valid an example of multiple proportions as few are ever carried out by students in any common course in elementary chemistry. Numbers 1 and 2 the analyses of binary compounds such as the oxides of nitrogen. I t was brought forward by Wollaston, are carried out in token form, but numbers 3 and 4 are never verified by students, as far as I am aware. And in 1808, in support of Dalton's theory.' As soon as the student has completed preliminary as for number 5, it is illustrated by one demonstration exercises, not differing much from the conventional -not a class experimentin which the instructor pattern, on combustion, composition of air, hydrogen, electrolyzes dilute sulfuric acid and says: "I have now acids, bases, and salts, etc., he starts volumetric analdecomposed water into oxygen and hydrogen; you can ysis. He makes solutions of sodium and potassium see that there is just twice as much hydrogen as oxygen. carbonates by weighing out the pure, dried substances. I can assure you that if the whole apparatus had been He makes a sulfuric acid solution by weighing out a subjacketed and kept a t a temperature above 100°C. stance which he is told is not pure, but contains perthe hydrogen and oxygen, on recombining, would give haps 5 per cent of water. And he makes a hydrosteam equal in volume t o the hydrogen." If he does chloric acid solution simply by diluting, to convenient not tell the class that the liquid used was dilute sulfuric strength, an acid which he knows is nowhere near pure. acid, he is cheating; if he does tell them, they may well He then titrates each acid against each alkali, making ask how he knows that it is not the sulfuric acid that is four titrations. Suppose he finds that 25 ml. of his decomposed. Any student who is absent on that parsodium carbonate solution neutralize 22.4 ml. of sulticular day never Sees even a demonstration of Gay furic acid, and 22.4 ml. of sulfuric acid neutralize 27 Lussac's law, and i t is the fundamental law upon which ml. of potassium carbonate, and 27 ml. of potassium depends the whole mass of atomic weights and valences carbonate neutralize 26.5 ml. of his hydrochloric acid. which,the student is required to memorize. The student then finds by experiment that 25 ml. of The reason for this extraordinary lack is that it is his sodium carbonate solution neutralize no more and very difficult to devise experiments, Suitable for class use "Foundations of the Atomic Theory, Papers, etc., by John with beginning students, which will illustrate the laws Dalton. William Hyde Wollaston, M.D., Thomas Thomson. of reciprocal and multiple proportions, l-.he law of M.D.," Alembic Club Reprints, No. 2, Gurney and Jackson, multiple proportions is usually illustrated by quoting ~ ~ 1923. ~ d ~ ~ , 555
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no less than 26.5 ml. of his hydrochloric acid. This principally by volumetric analysis, in which by this example of the law of reciprocal proportions is of great time he is proficient. The steps are asfollows: intellectual interest to him, and he is, by this time, a A weighed amountof pure silver is dissolved in fair manipulator in volumetric analysis. nitric acid, boiled and carefully neutralized, and made The law of multiple ~ ~ o ~ is ~represented ~ ~ o by n sup to a volumetric solution. This is titrated against a experiments on the carbonate of sodium. On passing volumetric solution of potassium chloride, giving the carbon dioxide for a considerable time into a saturated ,tio : KCI. interesting sideline here is a few solution of sodium carbonate a precipitate forms. This test-tube experiments on silver chloride and silver substan% formed by the action of excess carbon di- chromate, from which i t can be deduced that potassium oxide on sodium carbonate, is called, provisionally, chromate will be an indicator for the reaction, and "sodium extracarbonate." The student then finds from which the action of this indicator can be underfor himself that sodium "extracarbonate" contains stood. ~~~t indicators are "black to the stu. twice as much carbon dioxide, per unit of sodium, as dent who is notyetdeep in organic chemistry. sodium carbonate does, and he is then entitled to call it Silver oxide is then weighed dissolved in nitric "sodium bicarbonate." This important analysis is acid, neutralized, and titrated with the same potassium carried out in two quite independent ways, described chloride solution. This titration, together with the helow. first, gives Ag :Agio, and thus the equivalent weight of 1. A tenth of a gram of sodium carbonate is in- silver. traduced into a mercury-filled eudiometer tube, a small The student then carries Out a gravimetric deteramount of acid is introduced, and the volume of gas produced is measured, and comected for temperature, mination of silver as silver chloride, +ing ~g : A ~ C I , pressure, and watervapor. ~h~ same experiment is and from this and the previous results the equivalent then carried out with sodium ~ ~ The weights~of potassium and~chlorine can be obtained. ~ ~ Having seen how equivalent weights can be deteramount of carbon dioxide produced is greater, but not twice as great, because equal weights of the carbonate mined, the student can then be introduced to the and the "extracarbonate" contain unequal amountscomplicated chain of reasoning by which the system of sodium. B U then ~ the ratio of the amountof sodium of atomic weights and valences is set up. The starting in one gram of carbonate to the amount of sodium in one point in this is the law of Gay-Lussac and the hypogram of "extracarbonate" is easily obtained by weighing thesis of Avogadro. In order to verify Gay-Lussac's law, out volumetric solutions of the two solids and titrating the students carry out gas analysis, with an Orsat aPParatus. Hydrogen and oxygen are measured, and their them against the same acid solution. This method is fairly clear, because the carbon dioxide combiningvolumesdetermined, and by exploding carbon obtained from solid is actually seen, as a gas, and monoxide with oxygen and absorbing the carbon dioxide its volume is measured. Unfortunately it is not a very fomed it can be sbo.wn that the volumes of the reaccurate method, and it is therefore by actants and of the product are in a ratio of small whole the next method described. numbers. 2. Titrations have hitherto been made with methyl In this system the student himself carries out token orange as indicator; phenolphthalein is now produced experiments in all the fundamentals of the atomic and the student can find by experiment that sodium theory. The method is unusual, and admittedly not "extracarbonate" is a neutral substance, according suitable if the object of the course is the rapid acquisito phenolphthalein, although it is alkaline according tion of information. But, surely, this is not the ohto methyl orange. He can then understand the double ject of liberal education. What liberal education titration of sodium carbonate with the two indicators, should be, i t is not the place to discuss here, but it and when the result gives him exactly twice as much certainly must include training in the scientific method, acid required when methyl orange is used as when learning scientific truth from experiments and not phenolphthalein is used, the student has again demon- from books. These notes are submitted as suggestions strated the law of multiple proportions. for teachers who may be looking for ways in which the The student then proceeds to the determination of fundamentals of chemistry may be deduced from exthe equivalent weights of silver, potassium, and chlorine, periment, rather than accepted on authority.
