Another Method for Solving Problems Based on Hess's Law To the Editor: There appeared recently in THIS JOURNAL' a method for solving problems based on Hess's Law. I wish to reply and suggest an alternative which encourages students to place more emphasis on the understanding of the law than on a set of rules to be memorized. In order not to lose sight of the meaning of the law, we should, I believe, in any application of it try to show just how the law operates. This, I suggest, puts the emphasis on the value of the law and not on the difficulties which the concept mav mesent to students. In other words. I think we fail as there are difficulties, so what is to he done? The method I use, but for which I claim no originality,2.4 shows diagrammatically how the law generates a solution to a problem. Some t e x W 4 give diagrams of energy levels, but here one is never quite sure where to put the energy levels relative to one another until after the solution to the problem has been reached. Now let us reexamine Mr. Davik's problem.' The equation is 4NH4
+ 702 = 4NOa + 6H20 (all gases)
Beginning with the three elements nitrogen, hydrogen, and oxygen, surely all we need is a diagram to show that these elements can he reacted in two ways to give the substances (reactants and products) in the reaction we are discussing. I draw all these in "boxes" so nothing gets forgotten. The arrows tell us plainly what has to be added to what, and direct us to the required Hess's Law equation. Figure 1 shows my final OHP transparency built up in, say, four stages. The energy quantities are from tables. AH1 = 4 X AHi' ammonia = 4(-46.1) kJ AHz = 4 X AHi' nitrogen dioxide, and 6 X A Hio water = 4(33.9) 6(-241.8) kJ AH3 is to he calculated
+
+
Hess says, according to my arrows, AH1 AH3 = AH2
So that AH3 = A H 2 - AH, = 4(33.9) + 6(-241.8) - 4(-46.1) kJ = -1130.8 kJ
In this way, I believe the student sees how the available data can be related to the reaction being discussed and where his numerical solution originated.
' Davik, J., J. CHEM.EDUC.,57, 895 (1980).
Mahon, B. H., "College Chemistry," 1st Ed., Addison Wesley, Reading, Mass., 1966, pp. 276, 278. CBA, "Chemical Systems," 1st Ed., McGraw-Hill, St. Louis, 1964, pp. 356, 357. Quffieid Advanced Science, "Chemistry Student's Book 1," 1st Ed., Penguin-Educ.-Longmans. London. 1969, pp. 173-177. Robert Sutcliffe International School Holmbrook 20 2000 Hamburg 52
West Germany
To the Editor: Mr. Sutcliffe's alternative is an interesting one. I do not quarrel with placing emphasis on understanding a concept, but if a teacher employs a mnemonic device, a jingle, or some other device which assists the student with his difficulties, these devices do not interfere with learning. These devices can, in fact, lead to the desired understanding of the concept. In my classes I do not resort to aset of rules to "bypass" the law. In fact, the students do a lab activity, Experiment # 17, The Heat of Reacti0n.l In this lab activity they measure the heats of three reactions: Reaction 1
-
NaOHi,) Nat(,,) AH, = -21 kJ
+ OH-(,,)
+XI
kJ
Reaction 2 NaOHb)
+ Ht(,,]+ CI-I,,)
+
-Hz0 Nat(,,) AHz = - X * kJ
+ C1-l,,l + r2 kJ
Reaction 3 Na+ias) +
+ H+(Bq)+ C1-1~~i
-
H20
+ Nati,,) + CI-i,,) + xs kJ
AHa = - x s kJ
Through a series of questions the students determine that A H 2 = AHl M a . Class discussions of Hess's Law, assigning problems, and a lab activity can lead to an understanding of Hess's Law.
+
'
Dietz, Tellefsen,Parry, and Steiner, "Laboratory Manual Chemistry Experimental Foundations," 2nd Ed., Prentice-Hall, Inc., Engiewood Cliffs. NJ. 1975, pp. 46-47. John Davik Yolk Community High School 355 West St. Charles Rd. Elmhurst, IL 60126
362
Journal of Chemical Education
