Section 5.6—Intermolecular Forces & Properties

IMF’s and Properties IMF’s are Intermolecular Forces  London Dispersion Forces  Dipole interactions  Hydrogen bonding The number and strength of the intermolecular forces affect the properties of the substance. It takes energy to break IMF’s Energy is released when new IMF’s are formed

IMF’s and Changes in State Some IMF’s are broken to go from solid  liquid. All the rest are broken to go from liquid  gas. Breaking IMF’s requires energy. The stronger the IMF’s, the more energy is required to melt, evaporate or boil. The stronger the IMF’s are, the higher the melting and boiling point

Water Water is a very small molecule In general small molecules have low melting and boiling points Based on it’s size, water should be a gas under normal conditions However, because water is polar and can form dipole interactions and hydrogen bonding, it’s melting point is much higher This is very important because we need liquid water to exist!

IMF’s and Viscosity Viscosity is the resistance to flow  Molasses is much more viscous than water Larger molecules and molecules with high IMF’s become inter-twined and “stick” together more The more the molecules “stick” together, the higher the viscosity

Solubility In order from something to be dissolved, the solute and solvent must break the IMF’s they form within itself They must then form new IMF’s with each other

Solubility -+-+ -+-+ -+-+ -+-+ -+-+ Solvent, water (polar) ++ -- -+-+ Solute, sugar (polar) Water particles break some intermolecular forces with other water molecules (to allow them to spread out) and begin to form new ones with the sugar molecules.

Solubility Solvent, water (polar) ++ -- -+-+ Solute, sugar (polar) As new IMF’s are formed, the solvent “carries off” the solute—this is “dissolving” -+-+ -+-+ -+-+ -+-+ -+-+

Solubility If the energy needed to break old IMF’s is much greater than the energy released when the new ones are formed, the process won’t occur  An exception to this is if more energy is added somehow (such as heating)

Oil & Water Water has London Dispersion, Dipole and hydrogen bonding. That takes a lot of energy to break Water can only form London Dispersion with the oil. That doesn’t release much energy Much more energy is required to break apart the water than is released when water and oil combine. Water is polar and can hydrogen bond, Oil is non-polar. Therefore, oil and water don’t mix!

Surface Tension Surface tension is the resistance of a liquid to spread out.  This is seen with water on a freshly waxed car The higher the IMF’s in the liquid, the more the molecules “stick” together. The more the molecules “stick” together, the less they want to spread out. The higher the IMF’s, the higher the surface tension.

Soap & Water Soap has a polar head with a non-polar tail The polar portion can interact with water (polar) and the non-polar portion can interact with the dirt and grease (non- polar). Polar head Non-polar tail Soap

Soap & Water The soap surrounds the “dirt” and the outside of the this Micelle can interact with the water. The water now doesn’t “see” the non-polar dirt. Dirt

Soap & Surface Tension The soap disturbs the water molecules’ ability to form IMF’s and “stick” together. This means that the surface tension of water is lower when soap is added. The lower surface tension allows the water to spread over the dirty dishes.

What did you learn about soap?

Soap Inter-molecular forces Works based on Molecular Geometry Bonding types & Structures Determined by