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.
- First recall that for the reaction of a weak acid and a strong base the reaction will go until one of the reactants is completely consumed.
- Writing the net ionic reaction we get (note that since it is a weak acid, we write it in the undissociated state):
CH3COOH + OH- H2O + CH3COO-
- First find moles of each reactant:
- acid = (50.0 mL)(1 L/1000 mL)(0.25 mole/L) = 1.25 x 10-2moles
- base = (18.0 mL)(1 L/1000 mL)(0.50 mole/L) = 0.900 x 10-2moles
- After reaction all of the base is consumed, so:
- base = 0
- acetate = 0.900 x 10-2moles
- acid = 1.25 x 10-2moles - 0.900 x 10-2moles = 0.35 x 10-2moles
- Water synthesized = 0.900 x 10-2moles.
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.
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: 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:
- 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 describe the relationships between the four properties characterizing any gas:
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© R A Paselk
Last modified 23 February 2015