Balancing Equations

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Balancing Equations

• AVHS - Chemistry

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• The Law of Conservation of Mass states that
• in a chemical reaction there is no loss of mass.
• So, this means that every atom that is present
• in the beginning of a reaction must be present
• at the end of the reaction.
• Remember, reactions are simply moving
• atoms around. Balancing is just keeping track
• of them.

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• Let's now look at some examples of writing
• and balancing chemical equations.
• Problem  Hydrogen (H2) burns in oxygen
• (O2) to form water (H2O).
• Hmmm. What kind of reaction is this?

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• Lets begin by writing a word description of
• the problem.  From the information given
• above, we have
• hydrogen      oxygen   ----------gt   water
• This will not be necessary forever, but it is a
• nice Baby step in the beginning!

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• The next step is to replace each name with its
• corresponding chemical formula.
• The formulas were all given in the problem,
• so we have,
• H2      O2   ----------gt   H2O

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• H2      O2   ----------gt   H2O
• Looking at the equation we have so far, we
• can see it is not balanced.  Keep in mind that a
• chemical reaction is just a rearrangement of
• atoms, so every atom you start out with has to
• be somewhere in the end.  And you can't end
• up with any atoms you didn't have in the
• beginning.  The equation above does not meet
• these requirements.

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H2      O2   ----------gt   H2O

• We can see that the left side of this equation
• has 2 atoms of hydrogen and 2 atoms of
• oxygen, while the right hand side has 2 atoms
• of hydrogen and only 1 atom of oxygen.  At
• this point, hydrogen is ok, but oxygen is not. 

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H2      O2   ----------gt   H2O

• Beginners are tempted to solve problems by
• adjusting the subscripts, but that is NEVER
• the thing to do!  For example, one incorrect
• approach someone might try is to drop the
• subscript of 2 in O2 and write
• H2     O   ----------gt   H2O

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H2     O   ----------gt   H2O

• Why is this wrong??
• This DOES balance the equation, but that's
• not our only objective.  We also want our
• equation to be a correct description of what
• really happens in nature.  Oxygen does not
• exist as a monatomic gas.

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• Another approach -- also incorrect that
• someone might try -- is to balance oxygen by
• adding a subscript 2 to the oxygen in water
• and write the equation as
• H2      O2   ----------gt   H2O2

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H2      O2   ----------gt   H2O2

• Why is this wrong?
• Again, we have a balanced equation, but things
  are
• still not right.  Our original objective was to
  describe
• the formation of WATER, and H2O2 is not
• water.  Often, when you start changing
  subscripts,
• you get some non-sense formula that does not
• exist.  In this case we do get a real substance. 
  H2O2
• is known as hydrogen peroxide.

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• The bottom line is NEVER ADJUST
• SUBSCRIPTS!  Every chemical substance
• has a definite chemical formula.  When you
• change subscripts, what you are doing is
• changing the chemical formula, which is
• never the correct course of action.

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• Rather than adjusting the subscripts, you can
• adjust the numbers that go in front of the
• formulas.  These numbers are called
• coefficients, and they tell us how many of
• those formula units we have.  The correctly
• balanced equation for the formation of water
• from hydrogen and oxygen is
• 2H2     O2  ----------gt   2H2O