Chemical Reactions

Oxidation Numbers, cont.

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:

Let's try these rules on some examples:

• 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:

• NH₄⁺
• CO₃^2-
• NO₃^-
• NO₂^-
• PO₄^3-
• SO₃^2-
• SO₄^2-
• C₂O₄^2-
• HCO₃^-
• MnO₄^-
• HClO₄
• HClO₃
• HClO₂
• HClO

Click here to go to the key.

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

LN Demos

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.)

Charles' Law

The relationship between volume and temperature was determined much later because accurate thermometers had to be developed first. But once thermometers were available a number of workers determined that volume is directly proportional to temperature. Plotting data for the relation of volume of a gas to temperature between 0° C and 100 ° C gives a plot similar to that below:

Extrapolating this data to V = 0 we can find an absolute minimum value of temperature on the assumption that negative volumes can't exist:

The intercept on the volume axis is then taken as absolute zero = -273.15 °C = 0 K for an ideal or "perfect" gas with particles of zero volume and no interactions other than collisions.

Algebraically we then find that V = k'T, & V₁/T₁ = V₂/T₂.

Syllabus / Schedule

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© R A Paselk

Last modified 22 February 2009