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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 arrow--that's not balanced.
word equation
water rtarrow.gif (850 bytes) hydrogen + oxygen
skeleton equation
H2O rtarrow.gif (850 bytes) H2 + O2

 

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 H2O. 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 oxygens--the 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 rtarrow.gif (850 bytes) hydrogen + oxygen
skeleton equation
H2O rtarrow.gif (850 bytes) H2 + O2
balanced equation
2 H2O rtarrow.gif (850 bytes) 2 H2 + O2

 

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 O2, 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 + O2 rtarrow.gif (850 bytes) 2 MgO.
word equation
magnesium + oxygen rtarrow.gif (850 bytes) magnesium oxide
skeleton equation
Mg + O2 rtarrow.gif (850 bytes) MgO
balanced equation
2 Mg + O2 rtarrow.gif (850 bytes) 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 progression--each 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 involved--that is, the proportions in which they react. For example, two H2O's become two H2's and one O2.

 

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E-mail instructor: Eden Francis

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