## 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:

• 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.

 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

 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:

 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).

• 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.