Why Balance?
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Law of Conservation of Mass – mass of all substances before a reaction
must equal the mass of all substances after the reaction.
– Mass is neither created nor destroyed during ordinary chemical
reactions of physical changes.
– The mass of the reactants = mass of the products; therefore, the
number of atoms of each element must be the same on each
side of the arrow.
Suggestions for Balancing Equations:
1) Look at one element (or polyatomic ion) at a time and compare how
many are on the left side to how many are on the right side.
2) Check for Diatomics
3) Change coefficients ONLY - do not change subscripts
4) Start with the left most element and work towards the right
5) Leave H and O for last - it usually helps
6) Trial & Error - Use a pencil!
7) Recheck everything when you think you are done.
8) Coefficients must be WHOLE numbers
9) Coefficients must be expressed in their simplest relationship
Never ever ever change the subscripts! You can only add coefficients
in front of a formula showing that you now have multiple of that formula.
Examples with coefficients:
3H2
3 x 2 = 6 H’s
Means you have:
H2
4CO2
4 x 1 = 4 C’s
4 x 2 = 8 O’s
2Mg3(PO4)2
2 x 3 = 6 Mg’s
2 x 2 = PO4’s Mg3(PO4)2
H2
H2
CO2 CO2 CO2 CO2
Mg3(PO4)2
Example 1:
Word Equation:
Formula Equation:
sodium + chlorine  sodium chloride
Na + Cl2  NaCl
Balanced Chemical Equation:
– Begin with the formula equation:
Na + Cl2  NaCl
– Now count the number of atoms of each element on each side of the arrow:
Reactant Side
Product Side
1 Na
1 Na
2 Cl
1 Cl
– There is only 1 Cl on the product side and 2 Cl’s on the reactant side; therefore, a
coefficient of 2 must be placed in front of NaCl. DO NOT change the formula to
NaCl2 because that is not the formula for sodium chloride.
Na + Cl2  2NaCl
– Now, recount the number of atoms of each element on each side of the arrow:
Reactant Side
Product Side
1 Na
2 Na
2 Cl
2 Cl
– Now, there are the same amount of Cl’s on both the product and the reactant
side; however, Na is unbalanced. To fix this, change the coefficient in front of the
Na to 2.
2Na + Cl2  2NaCl
– Final Check:
Reactant Side
1 Na
2 Cl
Product Side
2 Na
2 Cl
Example 2:
Word Equation:
Formula Equation:
water  hydrogen + oxygen
H2O  H2 + O2
Balanced Chemical Equation:
– Begin with the formula equation:
H2O  H2 + O2
– Now count the number of atoms of each element on each side of the arrow:
Reactant Side
Product Side
2H
2H
1O
2O
– See that currently the H’s are balanced, but there is only 1 O on the reactant side
and 2 O’s on the product side. Put a coefficient of 2 in front of H2O.
2H2O  H2 + O2
– Now, recount the number of atoms of each element on each side of the arrow:
Reactant Side
Product Side
4H
2H
2O
2O
– Now, the O’s are balanced; however, H is unbalanced. To fix this, change the
coefficient in front of the H2 to 2.
2H2O  2H2 + O2
– Final Check:
Reactant Side
4H
2O
Product Side
4H
2O
To make a chemical equation complete you must add the states of matter of each
chemical substance.
(g) = gas
(l) = liquid
(s) = solid
(aq) = aqueous (dissolved in water)
Examples:
2Na (s) + Cl2 (g)
2H2O (l)


2H2 (g)
2NaCl (s)
+
O2 (g)
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