Interactions between molecules

1. Interactions between molecules Three phases of matter Solid ordered structure fixed volume and shape independent of container Strong interaction between molecules Liquid loosely ordered fixed volume, but not fixed shape - dependent on container Moderate interaction between molecules Gas No order Volume dependent on container Always occupies all of its container Almost no interaction between molecules

2. Role of Kinetic Energy and types of matter Kinetic energy is the amount of energy needed to bring a body to rest or impart motion on a resting body On the molecular level kinetic energy is dependent only on temperature Heating a compound will raise its kinetic energy (the temperature will rise)

3. Phase Transitions - change from one physical state to another When enough energy is added to a solid, the molecules are no longer rigid and the solid melts (turns from solid to liquid) The reverse (liquid to solid) is called freezing. When enough energy is added to a liquid, the molecules are no longer moderately attracted to each other vapourises (turns from liquid to gas) The reverse (gas to liquid) is called condensation. When enough energy is added to a solid, the molecules sometimes sublime (go directly to the gas phase) Dry Ice, Iodine (lab)

4. Question: Why do do molecules have different melting and boiling points? Water (H2O) boils at 100°C and is a liquid at room temperature Methane (CH4), with almost the exact molecular weight, boils at -164°C and is a gas at room temp. Answer: Interactions between water molecules are stronger than methane’s.

5. Attractive Forces Between Molecules (Van der Waal’s) Dipole-Dipole Interactions Attraction between polar molecules Hydrogen Bond (strongest) special type of dipole-dipole Found in molecules with HF, HO or HN bonds difference between electronegativities of these molecules is large • • Induced Dipole dipole (weakest) • - Attraction from temporary dipole moments • •temporary uneven distribution of electrons • • Increase with size of molecule

7. Like Dissolves Like Why don’t oil and water mix? A) Oil’s molecular mass is much greater than water’s. B) Oil has only induced dipole forces and water has H-bonding. Water will dissolve polar compounds (H-bonding or dipole-dipole) but not non-polar. Oil will dissolve non-polar compound (induced dipole forces) but not polar. When attractive forces between different molecules are similar, they will dissolve When attractive forces between different molecules are not similar, they will not dissolve

8. Soap The polar part of soap is hydrophilic (water loving) and interacts with water The non-polar part is hydrophobic (water hating) and interacts with oil & grease