1. Solutions, Solubility and Polarity! Chapter 7.2 Pp 243 – 254

2. Why do some substances dissolve (in water) while others don’t? What do we mean by polarity? When atoms come together to form molecules bonds are formed. When there is an electronegativity (EN) difference between the atoms the bond is said to be polar. If the polar bonds in a molecule don’t cancel each other out then the molecule is said to be polar or have a dipole.

3. Oxygen is more electronegative than hydrogen. Therefore each bond is polar towards the O atom. This is what we mean when we say water is a polar solvent / molecule. This results in the molecule have a dipole moment (aka being a polar molecule).

4. What about non-polar? In methane (CH 4 ) there is essentially no difference in EN between C and H which results in a non- polar molecule. Oil is essentially a bigger version of methane, so why don’t they mix?

5. 3 step process: 1) Forces between the ions (ionic solid) or molecules (molecular solid) must be broken. Requires energy. 2) Intermolecular forces (dipole-dipole) in the liquid may need to be broken (Hydrogen Bonding in H 2 O). Requires energy. 3) Interaction between solid particle and liquid particle. Releases energy.

6. Dissolving sodium acetate (NaCH 3 COO) in water (H 2 O) “Negative” end of the water is attracted to the “positive” ion (Na + ) “Positive” end of the water is attracted to the “negative” ion (CH 3 COO - ) This solution will conduct electricity!

7. How do we dissolve covalent compounds in water, a polar solvent? Add polar functional groups... O’s, N’s, etc. Polar functional groups will allow them to interact with water. Non-electrolytes! No electrical conductivity.

8. LIKE DISSOLVES LIKE We will revisit ionic solubility rules in the near future as not all ions are soluble. Furthermore, with respect to covalent compounds, the larger the molecule the less soluble in water it will be. CH 3 CH 2 OH (ethanol, common alcohol) is much more soluble in water than CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 OH (octanol, perfume component) Please read pp- 243 – 248.

9. For solids, as temperature increases so does solubility. The opposite holds true for gases. We can examine this phenomenon quantitative with solubility curves. We can deduce from this chart that gas solubility will increase with pressure.

10. I recommend that you complete all of the problems on page 254 (Section Review) for Tuesday March 25 th. I will be collecting Questions 5, 6 and 8 on Wednesday March 26 th and marking them. Please have them available on a separate piece of paper than your other problems.