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Balanced Equations
Next, we need to deal with balanced equations. A balanced equation shows how mass
is conserved because it shows how atoms are conserved.
A skeleton equation does not show conservation. Consider this skeleton
equation. Notice that water consists of two hydrogen atoms and one oxygen atom. On the
right hand or product side, the hydrogen consists of two hydrogen atoms hooked together
and oxygen consists of two oxygen atoms hooked together. Now notice that we have two
oxygen atoms on the right and only one on the left side of the arrowthat's not balanced. 
word equation 
water 

hydrogen 
+ 
oxygen 

skeleton equation 
H_{2}O 

H_{2} 
+ 
O_{2} 


Balanced equations take into account what substances are involved in the
reaction and how much of each is involved in the reaction. The balanced
equation for this reaction shows that you actually started with two molecules of water, 2
H_{2}O. You have to have two water molecules in order to get the two oxygen atoms
you need for the oxygen molecule on the right. Notice that on the left side of the arrow
there are altogether four hydrogens and two oxygens. On the right there are also a total
of four hydrogens and two oxygensthe same number of atoms. They have been rearranged,
they don't have the same bonding patterns, they are not hooked up together in the same
way; but they are the same in number, so that is a balanced equation. 
word equation 
water 

hydrogen 
+ 
oxygen 

skeleton equation 
H_{2}O 

H_{2} 
+ 
O_{2} 

balanced equation 
2 H_{2}O 

2 H_{2} 
+ 
O_{2} 


Same thing is true in the balanced equation for magnesium burning.
Magnesium combines with oxygen to form magnesium oxide. Note that the oxygen starts as O_{2},
two oxygen atoms hooked together. When magnesium oxide is formed, those oxygen atoms are
split apart and each one hooks up with a magnesium. There is a magnesium
for every oxygen. Well, if you start with two oxygens, then you need to end up with two
oxygens. And if you have a magnesium for every oxygen, then you have to start with two
magnesiums. So the balanced equation is 2 Mg + O_{2} 2 MgO. 
word equation 
magnesium 
+ 
oxygen 

magnesium oxide 

skeleton equation 
Mg 
+ 
O_{2} 

MgO 

balanced equation 
2 Mg 
+ 
O_{2} 

2 MgO 


These examples illustrate the three kinds of equations you will be dealing with: (1)
word equations, (2) skeleton equations, and (3) balanced equations. Note that there is a
progressioneach one tells you a little bit more.
The word equations tell you what chemicals are reacting by name. The skeleton equations
tell you the same thing but using the formulas, and the formulas tell you about the
composition of the chemicals that you are working with. The balanced equations tell you
how much of each chemical is involvedthat is, the proportions in which they react. For
example, two H_{2}O's become two H_{2}'s and one
O_{2}.
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Eden Francis
Clackamas Community College
©1998, 2002 Clackamas Community College, Hal Bender
