Download Using Coefficients to Balance Equations

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12/6/05
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MAZER
Page 284
Using Coefficients to Balance Equations
The balanced equation for the combustion of methane shows that one
molecule of methane reacts with two molecules of oxygen to produce
one molecule of carbon dioxide and two molecules of water. The equation can be simplified by writing 2O2 instead of O2 + O2, and 2H2O
instead of H2O + H2O.
The numbers in front of the chemical formulas are called coefficients. Coefficients indicate how many molecules take part in the
reaction. If there is no coefficient, then only one molecule of that type
takes part in the reaction. The balanced equation, with coefficients, for
the combustion of methane is shown below.
Balanced Equation with Coefficients
reminder
A subscript shows the
number of atoms in a
molecule. If a subscript is
changed, the molecule
represented by the formula
is changed.
CH 4
+
2O 2
coefficient
CO 2
+
2H 2 O
subscript
Chemical formulas can have both coefficients and subscripts.
In these cases, multiply the two numbers together to find the number
of atoms involved in the reaction. For example, two water molecules
(2H2O) contain 2 • 2 = 4 hydrogen atoms and 2 • 1 = 2 oxygen atoms.
Remember, coefficients in a chemical equation indicate how many
molecules of each type take part in the reaction.
Only coefficients can be changed in order to balance a chemical
equation. Subscripts are part of the
chemical formula for reactants or
products and cannot be changed
to balance an equation. Changing
a subscript changes the substance
represented by the formula.
For example, the equation for
the combustion of methane cannot be balanced by changing the
formula CO2 to CO. The formula
CO2 represents carbon dioxide gas,
which animals exhale when they
breathe. The formula CO represents carbon monoxide gas, which
is a very different compound
from CO2. Carbon monoxide gas
is poisonous, and breathing too
much of it can be fatal.
Check Your Reading
284 Unit 3: Chemical Interactions
Why are coefficients used
to balance equations?
The combustion of
methane (CH4) is used
to melt glass.
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