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The problem of balancing redox equations has been under considerable discussion in the Journal lately (1-5). However, as was shown by Hart (3), chemical knowledge is not necessary for properly balancing chemical equations. Therefore, in the process of teaching redox reactions, the importance of proper balancing should not be overemphasized. Instead, it is worth concentrating on the properties of the reagents. Students should be able to predict the products for given substrates and next to balance the equation. Some examples, which represent a middle level of difficulty, are listed below (students know only the left-hand side of an equation).
a.
KMnO4+KI+H2SO4->
[MnSO4+K2SO4+I
2+H2O]
b.
KMnO4+Na2SO3+H
2O->[MnO2+Na2SO
4+KOH]
c. KMnO4+NaNO2+KOH->[K
2MnO4+NaNO3+
H2O]
d.
CuS+HNO3(concd)->[CuSO4+NO
2+H2O]
e. Cu +
HNO3(dil)->[Cu(NO3)
2 + NO + H2O]
f. Na2S2O3 +
I2->[Na2S4O
6 +NaI]
g. NaBr
+NaBrO3+H2SO4->
[Na2SO4 +Br2
+H2O]
h. K2Cr2O7 + FeSO4+H2SO4->[Cr2(SO4)3 + Fe2(SO4)3 + K2SO4 + H2O]
i. SnCl2+K2Cr2O7+HCl->[SnCl4+CrCl3+KCl+H2O]
j. KMnO4 + HCl->[MnCl2 + Cl2 + KCl + H2O]
k. KMnO4+H2O2+H2SO4->[MnSO4+K2SO4+O2+H2O]
l. H2O2 +KI->[I2 +KOH]
m. KI +KClO3 +H2SO4->[I2 +KCl +K2SO4 +H2O]
n. KNO2 +KI +H2SO4->[NO +I2 +K2SO4 +H2O]
o. Cl2 +KOH(hot)->[KClO3 +KCl +H2O]
The solution of such formulated problems gives the chemical sense of the reactions and enables students to learn more about redox properties of chemical compounds. Thus, the first three reactions show the oxidative properties of permanganate in acidic, neutral, and basic media, respectively. Reactions d and e indicate the properties of nitric acid according to its concentration. Reaction f is frequently used in analytical chemistry, whereas reaction g is an example when an oxidant and a reductant give the same product. The oxidative property of dichromate is shown in reaction h. Reactions i and j indicate that in contrast to dichromate, permanganate can oxidize Cl- to Cl2 and therefore, in the latter case, HCl cannot be used as a reaction medium. Oxidative and reductive properties of hydrogen peroxide are visualized by reactions k and l, respectively. Reaction m shows the oxidative properties of chlorates and reaction n shows the oxidative properties of nitrate, which are not always recognized by students. Finally, reaction o is an example of the dismutation reaction.
Literature Cited
- Stout, R. J. Chem. Educ. 1995, 72, 1125.
- Ludwig, O. D. J. Chem. Educ. 1996, 73, 507.
- Hart, D. M. J. Chem. Educ. 1996, 73, A226.
- Zaugg, N. S. MJ. Chem. Educ. 1996, 73, A226.
- Stout, R. J. Chem. Educ. 1996, 73, A227.
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