Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2011

Lecture Notes 13: 16 February

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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:
   • For elements, such as Ar, O₂, S₈, etc. in the uncombined state the oxidation number for each atom must be 0, since no charge is shown and the atoms are equal to each other.
   • For monoatomic ions the oxidation number equals the charge.
   • For a compound the sum of the oxidation numbers of the atoms equals 0.
   • For a polyatomic ion the sum of the oxidation numbers of the atoms equals the charge on the ion.
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.
   • Exception 1: in peroxides it is -1 while in superoxides it is -1/2. These will generally be obvious due to other rules (or the names).
   • Exception 2: in combination with fluorine oxygen can be positive due to Rule 2 above, thus for OF₂ oxygen is assigned an oxidation number of +2.
6. In compounds hydrogen is usually assigned an oxidation number of +1
• Exception: in metallic hydrides hydrogen is assigned an oxidation number of -1. These exceptions will be fairly obvious: NaH, CaH₂, etc.
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:

• H₂O₂: Rule 1 & 6 exception (H cannot be more + than +1) H = +1, O = -1
• KO₂: Rule 1, 3 & 5 exception 1 K = +1, O = -1/2
• OH^-: Rule 1 & 5 -2 + H = -1, H = +1 - (-2) = -1
• MgH₂: Rule 1 & 3 +2 + 2H = 0, 2H = -2, H = -1
• C₂H₃O₂^-: Rule 1 & 5 & 6 2C + 3(+1) + 2(-2) = -1, 2C = -1 - (+3) - (-4) = 0, C = 0
• MnO₂: Rule 1 & 5 Mn + 2(-2) = 0, Mn = - (-4) = +4

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).

• Gas particles exert pressure.
  • implication - gas particles have momentum (mass and velocity).

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

Units of Pressure:

• mmHg - based on manometers. Two types:
  • open tube - measures pressure relative to current atmospheric pressure.
  • closed tube - measures pressure relative to contents of the enclosed volume at the closed end. (A barometer is an example where the enclosed space is "empty", that is it contains a vacuum since the vapor pressure of mercury is very low.)

• atm = 760 mmHg at 1 gravity = 1.01 Bar
• others include: psi (pounds/square inch), pascal, Torr (= 1 mmHg), millibar, etc.

 

Gas Laws

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

• Amount of substance, n  (in moles)
• Volume, V (in Liters)
• Pressure, P (in atm, though often measured in mmHg)
• Temperature (in K)

Boyle's Law

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

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

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

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.)

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© R A Paselk

Last modified 16 February 2011