Lesson 6 - Chemical Bonds and Water

Student Performance Objectives
1. Define: covalent bond, polar covalent bond and hydrogen bond.
2. Define: intermolecular bond and intramolecular bond.
3. Describe an experiment that shows that water is not an elemental substance.
4. Draw diagrams illustrating the molecular structures of methane and water.
5. Draw diagrams illustrating the molecular interactions of water molecules with labels
    indicating the intramolecular (covalent bonds), and the intermolecular (hydrogen bonds).
6. Explain how a substance dissolves in water using the terms hydrophilic and hydrophobic,
    and "the ability to form hydrogen bonds with water molecules."
7. List and explain 3 functions of water that are important for the functioning of the human
8. Explain why the dissolving of salt in water is not the breaking of ionic bonds.

Lesson Outline
A. Review of ionic bonding.
B. Covalent bonding
    1. Role of carbon in living cells.
    2. Atomic structure of carbon and the sharing of electrons to satisfy the octet rule.
    3. Use of methane and glucose as examples of covalently bonded molecules.
C. Polar covalent bonds - the case of oxygen bonding to hydrogen.
D. Water 
    1. Intramolecular polar covalent bonds.
    2. Hydrogen bonds - based on the polar covalent bonds within water molecules. 
    3. DEMONSTRATION: the point is to show that water is not an elemental substance,
        as had been believed for thousands of years (recall the air, earth, fire, water, and aether
        subdivision of the universe). Water can be separated into hydrogen and oxygen.
        Utilize the Hoffman apparatus to carry out the electrolysis of water and test each of
        the collected gases with a glowing wooden splint to get the familiar (to you) "pop" in
        the hydrogen gas and the "bursting into flames" in the oxygen gas.
    4. Special properties of water based on its bonding patterns
        a. Universal solvent of biological systems
            (1) Hydrophilic and hydrophobic substances
            (2) To dissolve is to form good hydrogen bonds with water thus
                 disrupting the water-water hydrogen bonding. 
        b. Participant in many biologically important chemical reactions - digestion and
            dehydration syntheses.
        c. Cohesive properties causing surface tension and the laminar flow of blood
            within blood vessels.
        d. Adhesive properties causing water to form lubricating, protective films on
            interior body surfaces like the pleurae of the lungs, the pericardial sac and the
            peritoneum and its associated membranes - the omenta and mesentery.
        e. Thermal properties - high heat capacity - helps to stabilize body temperature.
E. Strength of bonds.
    1. Covalent bonds are strong bonds and they resist being broken in aqueous
        solution. So atoms in carbohydrates, lipids, proteins, and nucleic acids hold together
        in water. A lump of sugar dissolves in that the individual sucrose molecules separate
        from each other, but the covalent bonds holding the atoms together within the sucrose
        molecule hold. The individual sucrose molecules separated from each other in
        water because they were held together by weak, hydrogen bonds that can be
        separated by the polar water molecules, forming new hydrogen bonds.
    2. Ionic bonds are also strong bonds and do not break in water. The components of the
        ionic compound can sometimes separate (dissolve) in water; sometimes they do not
        separate. NaCl is one ionically bonded compound whose components easily separate
        in water - the Na + and Cl- ions separate from each other and become hydrated with
        water molecules, which means the separated Na+ and Cl- ions each bond with water
        molecules. Note that such separation of Na+ and Cl- ions from each other is not
        breaking the ionic bond; ion separation does not negate the transfer of an electron
        from a Na atom to a Cl atom, which is the ionic bond). On the other hand, the
        ionically bonded apatite salts (basically Ca and PO-3) of bone will not dissolve in
        water (fortunately for our bones). We can dissolve the Ca+2 and PO4-3 from bone
        substance only in a strong acid solution