The Teaching of Equations CHARLES H. STONE Brookline, Massachusetts
T IS highly important for the student of chemistry to be able to write chemical equations readily and accurately. The process of learning to do this, however, is often regarded by the pupil as a dry and uninteresting task. Taught, as i t too often is, from the textbook without demonstrations, there is good reason for the pupil's lack of interest. But the process of learning to write equations may he made a pleasing and illuminating experience. Every chemical equation is the story of some chemical reaction. Why not, then, have the pupil perform the experiment before the class a t the teacher's desk, or in the laboratory, and then write the "story" of the observed reaction? It is assumed, of course, that already he has acquaintance with valences of the common elements and the formulas of their compounds. Given these, the writing of equations becomes more than intere&nrr .-.------...e. Simple reactions, such as that between hydrogen and chlorine or between sulfur and oxvzen need hardly detain us. The interest begins with equations somewhat more involved. We may begin with solutions, taking pains to arrange each pair so that a tangible product is formed and coefficients a t first are not needed. For example, the student comes to the desk and observes the result of combining solution A andsolution B as, for instance, solutions of silver nitrate and sodium chloride. I t is easy to show that the sodium nitrate formed in the re-
I
.-
action is soluble in water. The white product, therefore, cannot be sodium nitrate; i t must be silver chloride. The student is not to he told this but is required to find out for himself the truth of the above statement. The equation is written on the blackboard. The class observes the experiment and notes the equation. Some other pupil now comes to the desk and adds a solution of sodium carbonate to one of copper sulfate; the secondary product is sodium sulfate which the student may demonstrate is soluble in water. The green product, therefore, cannot be sodiuq sulfate; it must be copper carbonate. The written work on the hoard may take this form: Salution A AgN08
cuso4
--
Solution Secondary Primary NaCl NaNOI + AgCl + + N~.CO* N ~ ~ S +O cucoI ~ +
+
Color White Green
Certain oossihilities should be noted. If we are dealing with saturated or very strong solutions, the solubility of the secondary product may be less than that of one of the original substances, sd that we may have some precipitation of the secondary product in addition to the insoluble primary. This will not happen if the solutions used are dilute. The teacher will, of course, avoid such combinations as solutions of silver sulfate and barium chloride, from which both primary and secondary products are insoluble. It may add to the interest for the pupil to be given two solids that he is required to note in some detail ~
before dissolving in water. Thus, mercurous nitrate and potassium chloride are both white and soluble, hut the primary product is white and insoluble. So also, mercurous nitrate and potassium iodide are white and soluble, but the primary product is green and insoluble. After sufficient practice with equations of the type set forth above, we may introduce reactions in which i t is necessary to provide a coefficient for one of the reacting substances. Silver nitrate and sodium chromate, for example, react to form silver chromate. It is apparent that two formula weights of the silver salt are required for one of the chromate. So also with lead nitrate and potassium iodide, two formula weights of the latter being required to one of the former.
--
+
+ +
ZAgNO, NapCrO, 2NaNOa Ag,CrOr 2KI Pb(NO& 2KNOa PbIl 4
+
1
Red Yellow
Again the pupil may be required to explain how he knows which of the two products is the visible one. It adds much to the interest if the different products are of varying colors. Since the student has no way of knowing what color he is going to get in each succeeding experiment, the process of learning becomes a sort of game which arouses the expectant attention of the class. Following sufficient practice on the above types of equations, we may turn to those in which coefficients must be supplied to both of the reacting substances. Cobalt chloride and sodium phosphate may be cited as an example. 3CoCl.
-
+ 2Na3P0,
6NaCI
+ Coa(PO& 4
Purple
The preparation of the various phosphates may be followed by experiments involving the ferro- and ferricyanides. It has been no unusual thing for students in the writer's laboratory to remark, "This is the most interesting experiment we have had yet!" Reactions in the dry way may be introduced as occasion arises. The carbonates of the metals midway in the displacement series are decomposed into the oxide, with carbon dioxide escaping. The carbonates of copper, lead, and cadmium give, respectively, black, tan, and brown oxides. The nitrates of metals with valence two yield the oxide, with evolution of nitrogen oxides.
Oxalates and tartrates yield the pyrophoric iron and lead. Crystallized salts carrying water of crystallization yield the anhydrous compound if not heated too strongly. At very moderate temperatures green crystallized copper chloride forms the light-colored anhydrous product. Rose-colored cobalt chloride yields the blue anhydrous salt, and so on with other similar substances. An equation should be written, of course, in every case. Heated strongly, these same salts yield the oxide. Some explanation may be needed to show how water of hydration reacts with the salt a t the higher temperature.
-
CoCll 6 H ~ 0 COO
+ 2HCI + 5H10
The student may discover that certain compounds react with water. Unexpected results are observed. Bismuth trichloride and antimony trichloride produce the oxychlorides. BiCls
+ H1O
-
2HCI
+ BiOCl 4
Certain decomposition reactions come as a surprise to the student. To see a crucible of powdered ammonium dichromate suddenly blossom into a miniature volcano can hardly be without interest. Valence gain (and electron Loss) = 2 X 3
+
+
7 7
(N-8H~)ICr~'
NPO 4HaO C T ~ + ~ OGreen ~ -I Valence loss (and electron gain) = 2 X 3 L
+
The introduction of a little manganese dioxide powder into melted potassium chlorate which has ceased to give off oxygen results in a spectacular evolution of oxygen, and the splint blazes up with a fierce intensity. Valence gain (and electron loss) = 6 X 2 r 2KC1+603-'
+
1
ZKCI-I 302 -1 Valence loss (and electron gain) = 2 X 6
L-
Any teacher may devise many cases similar to those above. There seems to be no limit to the various ways in which the writing of equations with the accompanying process of learning may be made both pleasant and instructive.
