VOL.2. NO. 5
THE'VALUE
OP
TEACHING VALENCE
383
THE VALUE OF TEACHING VALENCE PRIOR TO BALANCING CHEMICAL EQUATIONS J. A. ERNEST ZIMMERMANN,HIGHSCHOOL, M I L L V ~ LN. E , J. It is not the author's intention to upset or overthrow the method of teaching valence but to present a workable method by which the chemical equation may be made more interesting and more definite to our high-school students. It is not to be disputed that valence was a development and
384
JOURNAL oa CREMICAL EDUCATION
MAY,1925
outcome of chemical theories and practices. It was developed after chemists used the method of representing reactions by means of symbols, but order was only restored after men had a definite idea of valence, which has been elaborated and built into its present workable form, especially the structural formulas that convey to the mind in a clear and definite manner the combining powers of the elements. But why should the old traditional, historical idea be followed and valence be taught after the student has mastered a few practical equations? Pedagogically, there is an advantage in teaching valence first and making i t the basis of all equational work that follows. The value of teaching valence prior t o the balancing of chemical equations has been satisfactorily demonstrated in the classes conducted by the author during the last six years. The method employed can best he shown by a practical illustration. The two gases which are studied a t the beginning of any elementary course are oxygen and hydrogen and these combine to form water; hydrogen has a valence of one and oxygen has a valence of two. Water is used as the basis for all work that follows. I n all valence work the author employs the hand as a simile a t the beginning, a device which is dropped after the idea has been carried over. Thus, hydrogen is considered as onehanded and oxygen as two-handed; hydrogen can grasp hands with any other one-handed element or any many-handed element, i. e., i t can only grasp one hand of the many-handed element with its one hand. Oxygen is two-handed and can grasp two hands and only two hands, so water shows this condition in this manner, H-GH, oxygen is reaching out with two hands grasping in each a hydrogen atom and thus the condition is very harmoniously satisfied. The second step is to write water as a hydroxide of hydrogen (HOH or H-(OH)) ; in this case it is well t o affix the ionic signs H +(OH)- showing that the OH radicle has a valence of one since i t acts as a unit and grasps the one-handed hydrogen. Using this idea the students are led to determine the valences of several elements by giving them the hydroxide formulas. They experience very little difficulty by this method since they have been shown that the OH radicle has a valence of one and the reasoning shown t o them by bivalent and polyvalent elements leads them t o tell a t sight the combining powers. It may be well t o take a bivalent hydroxide to illustrate their powers; Ca(OH)%,OH has a valence of one, there are two OH groups linked with one atom of Ca, therefore Ca must have a valence of two. To this the student must affix the ionic signs, first the negative t o the known OH group and then the positive t o the Ca. The third step is to have every one acquainted with the formulas of the common acids employed in their laboratory work, HCl, HzSOa and HN08. Since they have learned that hydrogen has a valence of one they can be very readily taught the valence of the negative radicles Hz(S04), and
H(N03). Using this as a basis, they are able to determine the valences of the elements in combination with the radicles to form the salts. The above method shows the adolescent mind that valence is not a matter of memory training but of reasoning. A drill of this kind a t the same time fixes the most important valences in the mind of the student without any apparent strain. It will no doubt be well to cite instances occurring in the classroom with a trivalent element and a bivalent radicle, Alr(SO&. Knowing that this is a compound of sulfuric acid, (Sod) has a valence of two, A12(SOa)3--, but there are three SO1 radicles in combination with two of Al, therefore, there are six valence bonds in that radicle. Now since there are two A1 atoms combined and held by six valence bonds, the valence of Al must be six divided by two, or three. The foregoing knowledge and training has been found very valuable to students learning to balance equations, since it is a mathematical basis for reasoning that can readily be seen. To illustrate-and the students have always enjoyed this-use the equation representing the reaction for the preparation of hydrochloric acid with sodium chloride and sulfuric acid. It is well to teach that the first element of the first compound usually unites with the second element or radicle of the other compound. Na
IClfHII SO,
combining NaCl
=
HCl and
+
H2S01
=
HCI
+
NaSO4.
Since the knowledge of valence has been made a prerequisite it is very easy to balance, first the formula and then the equation as follows: in HCl, chlorine has a valence of one since i t is linked to one hydrogen, therefore the valence of Na is one. The SO4 radicle has a valence of two since two hydrogens with a valence of one are linked with it, and since Na has a valence of one (from above) it requires two atoms of Na to link with SO1 to balance the formula, However, this only balances our formulas. There are two atoms of H in the left member of the equation and only one in the right member. There are also two atoms of Na in the right member which are required to balance the formula but only one on the left. This means that the molecule of NaCl has to be doubled which also gives us two atoms of C1 which combine with the two atoms of H to form HCI. The equation is then
Checking and taking account of stock, two atoms of Na in both members,
two atoms of C1 in both members, two atoms of H in both members and one radicle of So1in both members. The author has employed the foregoing method during the last six years with excellent results and the students have learned to equate more quickly and more efficiently than by the other method. The evolution of this method was due to his inability to grasp the process of equating during his high-school course in chemistry, nor did he grasp the idea until he had reasoned himself through the problem of valence by shaking off the old historical development and conservatism and employing the pedagogically, logical procedure as outlined above. It was the mastery of valence that unlocked the door to the enjoyment of balancing equations. This mastery bas been one of his greatest aids in teaching that part of chemistry to secondary students.
