Humboldt State University ® Department of Chemistry

Richard A. Paselk

Science 331
Fall 2004 Lecture/Activity Office: SA560a
Notes: 15 September Phone: x 5719
Home: 822-1116
e-mail: rap1

Chemical Equations, Atoms, and Matter

Remember:

This means that for any chemical reaction we must have the same number of atoms of each type at the beginning and at the end!! Thus the term, "Chemical equation" - the two sides are equal in terms of atoms (though they will have been rearranged).

Let's look at some chemical reactions.

Hydrogen gas plus oxygen gas gives water
 H2 + O2 -> H2O    
 Now we need to have the same number of each kind of atoms on each side. Multiplying hydrogen and water by 2 will give us:
 2 H2 +  O2 -> 2H2O    
 2x2=4H + 1x2=2O = 2x2=4H +2x1=2O    

And eveything is balanced!

Let's try another reaction

Methane gas (natural gas, CH4) gas plus oxygen gas gives carbon dioxide plus water
 CH4 + O2 -> CO2  +  H2O
 Now we need to have the same number of each kind of atoms on each side. Let's count the atoms:
Carbons  1x1=1C = 1x1=1C
Hydrogens  1x4=4H            1x2=2H
Oxygens      1x2=2O    1x2=2O  +  1x1=1O
Carbon is balanced (thus the = sign), but hydrogen and oxygen are not. The best strategy for balancing is to start with the molecule containing atoms other than oxygen, in this case, methane. Comparing the two sides we again see we have the same number of carbons, but we need two more hydrogens on the right, so multiply the water by two:
 CH4  +  O2  -> CO2  +  2 H2O
Carbons  1x1=1C      =  1x1=1C    
Hydrogens  1x4=4H      =       2x2=4H
Oxygens       1x2=2O     1x2=2O     2x1=2O
Comparing the two sides again, we now have the same number of carbons, and the same number of hydrogens, but we need two more oxygens on the left, so multiply the oxygen gas by two:
  CH4 +   2 O2  ->  CO2 +   2 H2O
 Carbons  1x1=1C      =  1x1=1C    
 Hydrogens  1x4=4H      =       2x2=4H
 Oxygens       2x2=4O  =   1x2=2O     2x1=2O
voila! We have now balanced the entire equation. We have different substances on the two sides, but the same number of atoms of each type - elements and mass are conserved!

Let's try one more reaction:

Propane gas (C3H8) gas plus oxygen gas gives carbon dioxide plus water
 C3H8 + O2 -> CO2  +  H2O
 Now we need to have the same number of each kind of atoms on each side. Let's count the atoms:

 

Acids and Bases

What are acids and bases? There are three major definitions. We will look at two (the third is not needed for our study).

 


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© R A Paselk

Last modified 17 September 2004