Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2015

Lecture Notes 14: 23 February

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Acid-Base Reactions, cont.

Consider the reaction of a weak acid and strong base: 50.0 mL of 0.25 M acetic acid is reacted with 18.0 mL of 0.50 M sodium hydroxide. Find the number of moles of each of the reactants and products after reaction.

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.

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 the oxidation number 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:

LN Demos

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© R A Paselk

Last modified 23 February 2015