Balancing Chemical Equations

Unit 6: Chemical Reactions and Stoichiometry                                                                                     Reading: Chapter 11
Working Mode: Pairs                                                                                                                                        GIA 6-1

Background: Matter is always conserved in a chemical reaction.  Therefore equations that represent chemical reactions must demonstrate the Law of Conservation of Matter.  To do this a chemical equation must be balanced.  This means that there are the same number and types of atoms on each side of the arrow in a balanced chemical equation.  Balancing chemical equations is more art than science.  You have to intuit which is the best method for you and develop your own style.  Enjoy.

Rules for Balancing Equations:

Rule 1: Be sure that you have correct chemical equations before you start.

Rule 2: Add whole numbers (coefficients) in front of compounds.

Rule 3: NEVER change subscripts once correct chemical formulas are written.

Rule 4: NEVER write a coefficient between atomic symbols in a chemical compound.

Rule 5: ALWAYS check your work.  (using “Tally Method”)  Your final balanced equation should have coefficients in the lowest whole number ratio.

Methods for Balancing Equations:

Method 1: “Tally Method” In the tally method you set up columns under the reactants and products and then tallying the number of each type of element.  You then add coefficients in front of the compounds in order to get the tally of the elements on each side of the arrow equal.

          Ex.     KClO3 à     KCl  +      O2

Method 2: “Tell the Story Method”  Sometimes by looking at how atoms are exchanged in a chemical reaction helps you to balance it.  You “tell the story” of the reaction.

            Ex.     Fe2O3  +      CO  à     Fe  +      CO2

Method 3: “Eye-ball Method”  If you have an innate sense on how to do this just go for it!

            Ex.      Ca   +     H2O   à       Ca(OH)2  +      H2

Method 4: “Trial and Error Method”  As I said earlier, balancing equations is more art than science so try your own ideas.

            Ex.     Mg(OH)2  +      HNO3  à      Mg(NO3)2  +      H2O

Hints for Balancing Equations:

Hint 1: Start by balancing the most complicated compound first.

Hint 2: Treat polyatomic ions as units, unless they break up.

Hint 3: In acid-base reactions think of H2O as H-OH.

Balance the following reactions (these fall into one of the five categories):

1.      KClO3  à    KCl  +    O2

2.      KBr +   F2 à   KF +   Br2

3.      S8 +   F2 à   SF6

4.      Na2SO4 +   Pb(NO3)2 à   NaNO3 +   PbSO4

5.      C3H8 +   O2 à   CO2 +   H2O

6.      Zn +   HCl à   ZnCl2 +   H2

7.      FeCl3 +   Na2CO3 à   Fe2(CO3)3 +   NaCl

8.      Na +   H2O à   NaOH +   H2

9.      C2H6 +   O2 à   CO2 +   H2O

10.    HgO à   Hg +   O2

11.    CaCl2 +   Na3PO4 à   NaCl +   Ca3(PO4)2

12.     N2O5 +   H2O  à   HNO3

13.     Mg +   CuCl2 à   MgCl2 +   Cu

14.     Al(OH)3 +   H2SO4 à   H2O +   Al2(SO4)3

 

More (and tougher) Balancing of Reactions  (These may not fit the five categories.)

1.      CO2 +   H2O à   C6H12O6 +   O2

2.      P4O10 +   H2O à   H3PO4 

3.      FeS3 +   O2 à   Fe2O3 +   SO2

4.      NaOH +   Cl2 à   NaCl +   NaClO +   H2O

5.      Na2O2 +   H2O à   NaOH +   O2

6.      Si2H3 +   O2 à   SiO2 +   H2O

7.      KClO3 à   KClO4 +   KCl

8.      SiCl4 +   H2O à   H4SiO4 +   HCl

9.      C7H6O2 +   O2 à   CO2 +   H2O

10.     H3BO3 à   H4B6O11 +   H2O