|Notes: 10 November
||Phone: x 5719
Acids, bases and Chemical Change
So what kinds of changes do we see with acids
and bases. Review first.
Recall we defined acids and bases in two ways:
- Empirically (observationally) - acids turn
litmus red (cabbage juice red), bases turn litmus blue (cabbage
- Theoretically (molecular), Arrhenius definition
- acids provide protons (H+ ions) in aqueous solution,
bases provide hydroxide (OH-) ions in aqueous solution.
What does the chemical definition really mean?
Let's look at some common acids:
HCl -> H+ + Cl-
H2SO4 -> H+
H+ + CH3CO2-
- the Acetic acid in vinegar, CH3CO2H
NaOH -> Na+ + OH-
KOH -> K+ + OH-
- Note the different behaviors of the H's in
- Benzoic acid
- Amino Acids
- Note that for "organic" acids (acids
with carbon) we will see a common element = -COOH, a carbon with
two oxygens, one of which has a hydrogen attached. The hydrogen
on the oxygen is then the acidic hydrogen.
- Alcohol, with a -COH is not acidic.
- Sodium hydroxide, NaOH
Ca(OH)2 -> Ca+
+ 2 OH-
- Calcium hydroxide Ca(OH)2
What happens when an acid and a base are mixed?
Make "salt" water. Of course we can't see it happen,
because we are working in water, and the new water is a very small
fraction of the amount we already have!
Let's try hydrochloric acid and sodium hydroxide:
HCl + NaOH -> H+ + Cl-
+ Na+ + OH- -> H2O + Na+
so the net result is that water was
made (salty water in this case):
H+ + OH- ->
pH and pH scale
(Fig 1.20, p 32)
- Note that acids have low pH (< 7), bases
high pH (>7) and neutral solutions have a pH of 7 (in reality
a range of pH values near 7 are generally considered neutral).
One of the most important ideas of chemistry is that the properties
of substances depend on the arrangements of particular atoms in
those substances and how they are attached to each other. Bonding
describes how they are attached. What I want to look at next is
The importance of molecular shape: recognition at the
molecular level in organisms. Shape and electron density are extraordinarily
important to the interaction of biomolecules - Examples
- Sarin (nerve gas)
- estrogen mimics ("feminization" of various animal
populations - birth control complication)
Lewis Structures enable us to predict bonding patterns for
molecules of the representative elements, but how can we predict
their shapes? We will add another tool, VSEPR Theory, to our chemical
toolbox - a simple way to predict the geometry of bonds around
a central atom (for larger molecules predict one center at a time).
VSEPR (Valence Shell Electron Pair
Repulsion) Theory is based on three main assumptions
(there are more advanced versions). We are going to look at an
even more abbreviated version with just two assumptions (it will
still give us good predictions for common molecules):
- Electron pairs will orient around a central point to minimize
- Repulsion is strong at 90° and weaker at 120° (weakest
Abbreviated VSEPR predicts geometry based on these assumptions
in a few simple, sequential, steps:
- Draw a correct Lewis Structure.
- Determine the Steric Number = the number of bonded
atoms + the number of lone pairs.
- Maximize the angles between electron pairs, placing the lone
(unbonded) pairs at the extremes.
Illustrated with ball & Stick and space filling model kits
for a linear molecule (Carbon dioxide, CO2) and three
molecules which share a tetrahedral electronic geometry (SN =
4), but different molecular geometries:
- Methane (CH4 = tetrahedral molecule)
- Ammonia (NH3 = trigonal pyramidal molecule)
- Water (H2O = bent molecule)
With this background, let's now explore the geometries possible
for electrons and molecules with a single central atom and an
octet of electrons, using toothpiks and gumdrops/marshmallows.
Use your "molecular model kit" (gum drops, grapes
etc. and toothpicks) to build the possible electronic geometries
for atoms with two bonded atoms and no lone pairs, three bonded
atoms and no lone pairs, one bonded atom and one lone pair, one
bonded atom and two lone pairs, four bonded atoms, etc.
- How many geometries
can you find?
- What are they?
- Sketch each geometry
Let's now explore the molecular geometries possible
with these electroninc geometries. Build examples of the three
electronic geometries above (a gumdrop/grape central atom and
toothpick electron bonds). Now determine how many different molecular
geometries you can make by adding one or more atoms(gumdrops
or grapes) to the electron pairs (toothpicks). Widely used colors
for different elements are: Black = Carbon, Red = Oxygen, Blue
= Nitrogen, Yellow = Sulfur, Orange = Bromine, Green = Chlorine,
Purple = Iodine. Or you can be creative.
- How many molecular geometries can you find for each electronic
- What are they?
- Sketch each geometry
molecules: CO, CO2, NH3, CH3+,
H2O, CH2O, CH4
Other geometries occur when there are more than eight electrons
(as frequently occurs with transition metals), but we won't worry
about them in this course.
© R A Paselk
Last modified 10 November 2004