infight
DONNA BOGNER
Hutchinson Senior High School Hutchinson, KS 67501
/
Cr13+ KOH
A New Method to Balance Chemical Equations Universidad de Antioquia
Medellin, Colombia
There are several methods to balance oxidation-reduction chemical equations (13).One of them, based on changes in the oxidation number (4),can he applied to molecular and ionic reactions. The ion electron or half-reaction method is used only with ionic reactions (5). This paper presents a simple method, applicable to any kind of reaction that does not require the knowledge of oxidation numbers. Balancing Molecular Reactions
The chemical reaction is divided into two half-reactions andeach one is balanced independently. These two balanced half-reactions are added together to get the correct stoichiometry of the reaction. One half-reaction is formed with compounds that contain the same elements other than oxygen and hydrogen. The remaining compounds, and others if i t is necessary, constitute the other half-reaction. For example, for the reaction, FeS2+ HNOa
-
FedSO&
+ NO + HzSOI
the product chosen can he Fe2(S04)3, which can he formed from FeSz without considering the oxygen atoms, hut the sulfur also forms H2S04. These compounds correspond to one of the half-reactions: The other half-reaction must contain the remaining compounds, that is, HN03
-
NO
1. Elements other than oxygen and hydrogen. 2. Hydrogen atoms are balanced with water molecules 3. Oxygen atoms are balanced with oxygen "atoms". When these steps are applied to the above reactions, we get:
--
2FeSZ+ HzO + 150 Fer(S04)a + HBOI 2HN03 2N0 + H20 30
+
In order to cancel the oxygen "atoms", the second halfreaction is multiplied by five to get:
-
10HNOs
lONO + 5Hz0 + 150
Finally, adding the half-reactions, the balanced equation is obtained:
-
C4 + KOH
+
-
KC1
Applying the three steps to balance the half-reaction:
and The second half-reaction is multiplied by 21 (to cancel the oxygen "atoms"). A balanced equation is ohtained by adding the two half-reactions: This method is appropriate to balance any kind of reaction, even those that include complex ions or reactions of compounds with oxidation numbers difficult to determine.
Balancing lonlc Reactions
In a similar way, for ionic reactions the following steps are used: 1. Balance the elements other than oxygen and hydrogen. 2. - Balance charges in acid media with hvdroaen ions, H(+)and in h i r merlla with hvdroride mni. OH,-,. :I Hnlnnce hyhgen aromq with water molerulei. 1. Hnlnnre oxygen atoms wlth oxygfn "nlomr". ~
For example, the following reactions can he balanced in arid media:
lo3-+ Br-
Each half-reaction is balanced in the following sequence:
+ lOHNO3
KzCrO4 + KI03
In the overall reaction, chlorine is found as KCI, which requires a reaction with a potassium compound, so the second half-reaction is
Arceslo Garcla
2FeSs
-
-
+
10%- BIZ
Using the four steps, the balanced half-reactions are ohtained:
Adding these two half-reactions, the oxygen "atoms" cancel and the balanced reaction is ohtained: A second example is to balance in basic media the following ionic reaction: C10-
+ Pi
-
H2P04-+ CI-
Using the four steps mentioned, we get the balanced halfreactions:
Fez(S0d3 HzSOa + 4Hz0 + lONO
In a similar way, the following reaction can he balanced: Cd3 + CI2+ KOH
-
KzCrOl+ KI03 + KC1
The elements in Cr13 appear in the products as KsCrO4 and KI03. Therefore, acompound that contains potassium in the reagents, such as KOH, is added to the half-reaction:
T o cancel the oxygen "atoms", the second ldf-reaction is multiplied hy 10, and this result is added t o the first halfreaction:
Volume 64
Number 3
March 1987
247
Conclusion This method can be used to balance any type of reaction, including molecular ionic and more complex reactions in which it is difficult to find the oxidation number, as in the following reaction, (6):
Multiplying the second half-reaction by six, and adding them together, the balanced reaction is obtained:
+ Ca(CN)z + NaAIF4 + FeSOl + MgSi03 + KI + H3P04+ PbCrOl + BrCl + CFZCIZ+ SO2 PbBrz + CrCb + M&03 + KAI(OH)&+ Fe(SCNh + PI3 + NazSiOa + CaFz +
-
Hz
Hz0
This reaction can be separated in the following half-reactions, using chlorine as theelement to balance at the end:
+
+
+
-
+
+
64H2 15Ca(CN)2 6NaAIFr 10FeSOI 3MgSi08 6KI 2H3P04 3CFzClz 20S02 3MgC03 + 6KAI(OH14
248
+
+
Journal of Chemical Education
+
+
Literature Cited 1. Kolb. D. J . Chem. Educ. 1979.56,182. 2. Kolb, D. J. Chem Edue 1981,58,M2. 3. B1akev.G. R.J. Chem. Edue. 1979.55.182. 4. K " I ~ , D . J .Chem.Edue. 1978,15,326. 6. Petrucci. R. H.General Chemistry. MacMillan: New York, 1972 6. Swinehan.D.F J. Chem. Educ. 1985.62,55.
