TRAINING IN WRITING CHEMICAL. EQUATIONS I. OSER,SIMON GRATZ HIGHSCHOOL, PHILADELPHIA, PENNSYLVANIA JOSEPH Most chemistry classes, in spite of labor and pain on the part of conscientious teachers, contain students who apparently cannot understand the writing of chemical equations. To make up for this deficiency they often attempt to memorize all the chemical equations connected with the chemistry course. If anything tends as much as this regrettable procedure not only to deaden the student's interest hut also to instil a deadly dislike for chemistry I should like to know what it is. If students receive definite systematic training in equation writing, it should tend to improve their abilities along this line and eliminate "parroting" of equations. Di5culties in Writing Chemical Equations Having identified the reacting substances, one must know, or find out, or do, three things in order to write a chemical equation: 1. Ascertain the products of the reaction. 2. Write the formulas of the substances involved. 3. Balance the equation. The difficulties of equation writing, in the words I have heard many students use, are1. "I don't know what forms." 2. "I don't know the formula." 3. "I can't make i t balance." Most teachers will agree that the first is the biggest difficulty. The second is most easily overcome. The third can he conquered by special training which will he outlined below. Let us now discuss these difficulties, and see what can be done about them. "I Donit Know What Forms" This difficulty is, indeed, a real one. The student must he enough of a chemist to almost sense the products of a reaction. Aid can be given in developing this understanding by impressing a few (not too many) general rules. T h u s 1. Metal Acid (not nitric) +a Salt Hydrogen (Replacement) 2. Metal Oxide Acid -+ a Salt Water 3. Base Acid 4 a Salt Water (Neutralization) 4. Most reactions among acids, bases, and salts result in double decomposition. If one of the products so formed is unstable i t may further decompose (preparation of carbon dioxide, ammonia, sulfur dioxide, etc.). Knowledge of rules like these, plus much practice in writing equations, often causes marked improvement in the pupil's ability to write chemical equations.
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JOURNAL OF CHEMICAL EDUCATION
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"I Don't Know the Formula" Many students forget that chlorine, hydrogen, nitrogen, and oxygen contain two atoms to the molecule, and that their formulas are therefore written, respectively, Clz, Ha, Nz, and 0%.This should be driven home. I t goes almost without saying that thorough drill in valence and formula writing should precede the writing of chemical equations. Since sodium and calcium compounds are used so frequently in high-school chemistry work, special attention might be given these. How many times have we seen NaCla and NazCl instead of NaCl; Na(N03)%and NaaNOa instead of NaNOs; NaSOa instead of Na%SO4. The secret of getting the correct response lies in drill, drill, and still more drill. The pupil who does the correct thing often enough finally gets to know it. I t should be impressed on the student that if a formula is written incorrectly, it will often be impossible to balance the equation; even if balanced it will certainly be incorrect. As some one has said, "For want of valence a formula was lost; for want of a formula an equation was lost; for want of an equation a student was lost." However, it should be realized by both teacher and student that valence is merely an aid in writing formulas, and not an infallible scheme. Can't Make It Balance" Students are usually expected to acquire the knack of balancing equations on the wing, as it were. Many have the native intelligence to do this. That is, after they have watched the teacher balance two or three equations they can do it almost as well as he can. Where the knack of balancing is not so easily acquired, more specific instruction is needed. This should take the form of practice on carefully selected material of progressively increasing difficulty. (1) By the time the writing of equations is begun, the course will probably have included the study of oxygen, hydrogen, water, and hydrogen peroxide-perhaps, also, of chlorine and hydrogen chloride. The aspiring chemist who has seen the combustion in air or oxygen of carbon, hydrogen, phosphorus, sulfur, magnesium and iron, and the burning in chlorine of hydrogen, phosphorus, antimony, arsenic, iron, and sodium has at his command a fund of chemical experience which should be utilized. If he has also learned that almost all other elements combine with oxygen and chlorine, there is a t hand much material for the writing of simple equations. I suggest only a few examples (all requiring different procedures for balancing)-
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Magnesium Oxygen -+ Hydrogen Oxygen + Aluminum Oxygen +
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WRITING CHEMICAL EQUATIONS
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O x y ~ e n4 Phosphorus Hydrogen Chlorine 4 Sudium Chlorine + Iran Chlorine 4
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Beginning with such simple combinations, the unskilful student not only gets practice in both formula writing and balancing but even acquires some little confidence in his ability to name the products of reactions. Such confidence is helpful to success. Considering the above as the most simple type of chemical change, other reactions useful for practice (in order of increasing difficulty) are: (2) The production of hydrogen by the solution of metals in acids (except nitric). Examples for practice:
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Zinc Hydrochloric Acid + Magnesium Hydrochloric Acid + Aluminum Hydrochloric Acid 4 Iron Sulfuric Acid + Zinc Sulfuric Acid +
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The solvent action of acids on oxides. Examples:
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Zinc Oxide Hydrochloric Acid + Ferric Oxide Hydrochloric Acid + Silver Oxide Nitric Acid + Aluminum Oxide Sulfuric Acid +
(4) Reactions showing double decomposition, especially neutralization. Examples:
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Sodium Hydroxide Hydrochloric Add + Barium Hydroxide Nitric Add + Aluminum Hydroxide Hydrochloric Add + Aluminum Hydroxide Sulfuric Acid 4 Silver Nitrate Calcium Chloride + Silver Nitrate Aluminum Chloride -+
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( 5 ) Reactions of double decomposition, with an unstable product (not within experience of and too difficult for beginners at equation writing). Examples:
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Ammonium Chloride Calcium Hydroxide + Ammonium Sulfate Sodium Hydroxide 4 Calcium Carbonate Hydrochloric Acid 4 Sodium Bicarbonate Sulfuric Acid + Sodium Sulfite Hydrochloric Add + Calcium Bisulfite Sulfuric Acid +
Two Other Points to Be Considered Two other points to be brought out are the following: (1) The fact that an equation balances does not necessarily.indicate that it is correct. Every chemical equation should represent an actual chemical change. Therefore, equations like the following are meaniugless; they purport to represent changes which really do not occur.
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JOURNAL OF CHEMICAL EDUCATION NaOH 2Au
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+ KOH +NaKO + HIO + 6HC1+ 2AuCh + 3H2
(2) After a formula is written correctly, according to the student's knowledge, it must not be changed to make the equation "easier to balance." Suppose a student begins an equation and has written CaCl.
+ Ag(NO3)
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AgCl
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I t frequently happens that he then adds the subscript "2" after the C1 in AgCl, and after the (NOa) in Ag(NOa)-"to make it balance." The teacher should emphasize the fact that equations are balanced by changing coeflcients and not subscripts.
A New Tendency-and
a Poor One
In order to "make a good showing" in standard tests, some teachers and some textbooks begin the "teaching" (if it can be called that) of chemical equations with the very first chemical change studied, or else at the very beginning of the chemistry course. That is, with no chemical experience to draw on, and without definite knowledge of atom or molecule, symbol or formula, the student is expected to write equations using formulas throughout. Since bright students can learn from any teacher, or by any method, they immediately do well with this scheme; and the class average jumps. Most of the class, in self-defense, adopts the memorizing of eqnations as the only alternative. They can then do more on the test paper; and not only the average, but also the median jumps. And thus is introduced something which should certainly be avoided. If anything tends to make the study of chemistry deadly dull, and abhorred, it is the mechanical memorizing of eqnations. There is no fun and no pride of achievement in doing equations in this way. (Of course, some who begin by memorizing equations later get rid of the habit.) The very early introduction of chemical equations is defended on the ground that boys and girls are especially interested in and are attracted by formulas and equations. So, as in other ultra-modem educational procedure, the stunt is to let them play around and think they are doing big things, even if they do not understand what it is all about. I t often is better to put the "psychological before the logical" but, I respectfully submit, the writing of equations is one division of chemistry teaching where the cart should not be placed before the horse. Let the student learn some chemistry, let him learn about atoms and molecules, symbols and formulas, and then he may be able to write chemical equations in an understanding way.
