Download Balancing Chemical Equations

Chemistry 141
Fall 2003
Balancing chemical equations
Balancing Chemical Equations
What does a chemical equation tell us?
The chemical formulas used in chemical equations can represent units of individual
chemicals or represent moles of these chemicals.
It is commonly understood that the coefficients in an equation represent moles of the
reactants or products.
There are several typical steps we have to follow to write a balanced chemical equation.
Many equations can be balanced by trial and error while many require a lot of careful
work.
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Equations can be balanced by adjusting the number of atoms of each element so
that their number is the same on both sides of the equation.
Do not change any formulas to balance an equation.
The general procedure is as follows:
1). Identify the reaction: Write a description or word equation for the reaction. Consider
the following example where mercury (II) oxide decomposes (breaks down) to
mercury and oxygen when heated:
Mercury (II) oxide
mercury + oxygen
2). Write the unbalanced equation using the correct formulas, which should be
determined from using the rules of naming. For our example:
HgO
Hg + O2
The oxygen has to written as O2 since it is a diatomic gas.
3). Balance the equation: Use the following steps as necessary:
a). Count and compare the number of atoms of each element on each side of the equation
and decide those that must be balanced. In our example Hg is balanced since there is
1 Hg atom on each side, the oxygen is unbalance since there are 2 atoms of oxygen on
the products side while there is only 1 oxygen atom on the reactant side.
b). Balance each element by placing coefficients (whole numbers that are always greater
than zero, leave blank if 1) in front of the formulas containing the unbalanced
element. It is usually best to balance in the order of metals, non-metals, hydrogen and
then oxygen. In our example we need to place a 2 in front of the HgO to balance of
oxygen:
2 HgO
Hg + O2
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Chemistry 141
Fall 2003
Balancing chemical equations
c). A coefficient multiplies every atom in the formula by that number. Select the
smallest coefficients that will give the same number of atoms of the elements on each
side.
d). Check all other elements, after each individual element is balanced to see whether, in
balancing one element, other elements have become unbalanced. Make adjustments
as needed. In our example the Hg is now unbalanced and we need to put a 2 in front
of the Hg on the right.
2 HgO
2 Hg + O2 (balanced)
4). Do a final check to ensure that each element is balanced and the smallest possible set
of whole-numbered coefficients is used. Thus:
2 HgO
4 HgO
2 Hg + O2 (correct form)
4 Hg + 2 O2 (INCORRECT)
Consider the following equations for practice:
1. Write the balanced equation for the reaction when Magnesium metal (s) is burned in
air to produce magnesium oxide (s).
2. Benzene (C6H6) an explosive liquid when burned in air produces carbon dioxide (g)
and water (g).
3. Nitrogen (g) reacts with hydrogen (g) to produce ammonia (g).
4. Silver nitrate (aq) reacts with hydrogen sulfide (g) to produce silver sulfide (s) and
nitric acid (HNO3 (aq)).
5. Sodium hydroxide (aq) when mixed with sulfuric acid (aq) produces sodium sulfate
(aq) and water (l).
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