|
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:
- Atomic nuclei are not affected by chemical processes.
- Mass is conserved in chemical processes.
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:
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).
- Arrhenius Definition: This is the oldest definition,
and the one we will use.
- Acids release protons (H+) into water.
- Bases release hydroxide ions (OH-) into water.
- This is very limited - it only deals with acids and bases
in water, and many substances which chemists and others think
of as bases (such as ammonia) don't fit the definition. However,
it will work for most of our observations.
- Brønsted Definition: This is the definition
used by most scientists most of the time, particularly in life
scienses, environmental sciences etc.
- Acids are proton (H+) donors.
- Bases are proton (H+) acceptors (they will react
with protons).
- Note that there is no restriction as to solvent, and many
substances besides hydroxide ion can contribute basicity.
© R A Paselk
Last modified 17 September 2004
