A Balancing Act Kenneth Schug
l ~ t e p Products l
Reoctontr
Illinois Institute of Technology Chicago, Illinois 60616 The principle mistakes made by many beginning students of chemistry in learning to balance chemical equations are 1) changing the numerical values of the subscripts within the formulas provided rather than adjusting only the coefficientspreceding the formulas;and 2) inventing species of their own ehwsing to obtain an atom balance.
For example, the skeleton equation for the formation of water from molecular hydrogen and oxygen
-
Ha + Oz
Hz0
02
Hz02
H2 0~ Hz0
(2)
(type A mistake)
+0
(3) (type B mistake)
An approach which helps avoid these errors is described below; when supplemented with the use of simple molecular models, i t not only leads to correctly balanced equations but also emphasizes the principles involved in obtaining a balanced equation. Step 1. Write the "unbalanced" equation containing all reactant and product molecules. Hz
+ 0%-Hz0
Explain that this equation is a chemist's way of reporting that in the reaction of interest the only observed reactants are Hz and O2molecules and the only observedproducts are Hz0 molecules. Stress that "balancing" the equation involves varying the relative numher of these molecules until the total numher of atoms of each element is the same on each side of the equation (conservation of atoms). Step 2. Do a preliminary atom count. element left side right side H 2 2 0
2
1
Step 3. Since there are fewer oxygen atoms on the right, "add" another HzO molecule by writing another formula (not by changing the coefficient!) original: Hz + OZ + H20 add: to give:
Hz
+
0%
Step 4. Do another atom count. element left side
H
a hydrogen molecule to the left side to give
H,
+
2 2
-.
H,O
right side 4
0 2 Step 5. Now the number of hydrogen atoms differ, so add 370 / Journal of Chemical Education
0,
+
H,O
Step 6. Another atom count gives element left side
right side
4
H
-
+
m e l s lo represent a balanced equation. Arrows lndlcate transfer of molecules from supply boxes to reaction site.
(1)
can he "balanced" in either of the following two ways: HZt
Use of mlecular
4
4 4 0 Equation is balanced. Step 7. Now combine similar molecules by using coefficients to indicate the number of molecules of each type (2 Hz = HZ
+ Hz)
2H~tOz-2HzO (balanced equation in conventional form)
The same procedure applied to a more complicated reaction leads to expressions such as C,& + o3 CO, H,O 0, C0, H20 02 CO, H,O 02 Ha0
-
0"
+
-
+
or W, 502 3C0, + 4 H a Although the sequence of adding molecules is immaterial, the process is more direct if those elements occurring in fewer different compounds are balanced first (e.g., C and H before 0 in the example above). In addition, if there is a choice of molecules which can he added in some step, use the one which does not "unbalance" an existing balance whenever possible. If the coefficients in the final eauation contain a common factor, they should he reduced tb the smallest set of integers. A parallel activity which helps to reinforce the logic of the halancing act is to represent the same equation by means of simple molecular models. Each added molecule is removed from a supply of identical molecules with compositions decided in aduance (See Figure). The models can usually he dispensed with after 2 or 3 demonstrations. I wish to thank the high school teachers in my present NSF Implementation Project for encouraging me to publish this suggestion.
