Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2011

Lecture Notes 13: 16 February

PREVIOUS

Oxidation Numbers

For simple elemental ions it is easy to determine the charge on an atom, but in many other circumstances this is not the case. In order to name compounds and understand reactions we frequently need this information which is obtained from oxidation numbers.

Oxidation numbers are in essence an electronic accounting method in which electrons are assigned to a particular atom in a bond or interaction. As such they give an approximate picture of where electrons actually reside in compounds. We will find this information very useful later when we look at particular types of chemical reactions. Oxidation numbers are essential for nomenclature.

For simple elemental ions it is easy to determine the charge on an atom, but in many other circumstances this is not the case. In order to name compounds and understand reactions we frequently need this information which is obtained from oxidation numbers.

Oxidation numbers are most readily assigned using a simple set of rules:

  1. In the formula for any substance the sum of the oxidation numbers of all the atoms in the formula is equal to the charge shown. Thus:
  2. In compounds fluorine is always assigned an oxidation number of -1.
  3. Alkali metals in compounds will always (for our class) be assigned an oxidation number of +1.
  4. Alkaline-earth metals in compounds will always (for our class) be assigned an oxidation number of +2
  5. In compounds oxygen is usually assigned an oxidation number of -2.
  6. In compounds hydrogen is usually assigned an oxidation number of +1
  7. Aluminum will always (for our class) be assigned an oxidation number of +3, other elements in this Group will usually be assigned an oxidation number of +3.

Let's try these rules on some examples:

Additional practice examples are available on the Study Module

Finally, note that in writing formulae, the element with the more positive oxidation number comes first. There are, of course, a few exceptions, the most well known being ammonia: NH3 (by the rules it should be H3N).

Gases

Gases: Briefly discussed overall properties of gases (fills container, compressible, lo density, lo viscosity).

What is Pressure? Pressure is the force/unit area. Due to collisions of particle with walls of container etc.

Units of Pressure:

 

Figure comparing open and closed manometers

 

 

Gas Laws

Gas Laws describe the relationships between the four properties characterizing any gas:

Boyle's Law

Boyle's Law describes the relationship between pressure and volume when the temperature and amount of substance are held constant.

PV = c @ constant T & n

Plotting pressure volume data (keeping n and T constant) gives a graph for a hyperbola (xy = c), as seen below:

 

Boyles Law Plot

 

Notice that we can rearrange this equation to give a straight-line relationship:

Divide both sides by V: (PV)/V = c/V

P = c (1/V)

This is now in the form of a straight line: y = ax + b, where b = 0

Thus, "At constant temperature the volume of any quantity of gas is inversely proportional to its pressure." V = k (1/P) & P1V1 = P2V2.

(Aside on straight-line plots: Very popular in science. Traditionally, we will do almost anything to get a straight line. Why? Because straight lines easy to recognize and evaluate. Also easy to evaluate statistically.)

NEXT


Syllabus / Schedule
home "refractometer" icon
C109 Home
lecture "spectroscope" icon

C109 Lecture Notes

© R A Paselk

Last modified 16 February 2011