Humboldt State University ® Department of Chemistry

Richard A. Paselk

	Science 331
Fall 2004	Lecture/Activity	Office: SA560a
Notes: 10 November		Phone: x 5719 Home: 822-1116
		e-mail: rap1

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 juice green).
• 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:

• Hydrochloric acid, HCl;

• Sulfuric acid, H₂SO₄;

• the Acetic acid in vinegar, CH₃CO₂H

• Note the different behaviors of the H's in acetic acid.
• Examples
  • 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

• Potassium hydroxide, KOH

• 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:

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).

Molecular Geometry

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 shape.

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)
• "drugs"
• 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.
• Repulsion is strong at 90° and weaker at 120° (weakest at 180°).

Abbreviated VSEPR predicts geometry based on these assumptions in a few simple, sequential, steps:

1. Draw a correct Lewis Structure.
2. Determine the Steric Number = the number of bonded atoms + the number of lone pairs.
3. 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 geometry? What are they? Sketch each geometry Worked example 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.

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© R A Paselk

Last modified 10 November 2004