Intermolecular Forces and States of Matter

1. AP Chemistry Intermolecular Forces and States of Matter

2. H2O Chemical properties are related only to chemical composition; physical properties are related to chemical composition AND the physical state of the substance at the time.

3. intermolecular forces (IMFs): forces between molecules -- largely determine the physical properties of molecular liquids and solids covalent STRENGTH OF IMFs… < < GASES LIQUIDS SOLIDS Solids w/highly-ordered structures are crystalline. ionic and metallic

4. H H O Intermolecular Forces (IMFs) -- these are much weaker than ionic or covalent bonds (< 15% as strong) -- In vaporizing water, we overcome the IMFs between water molecules, but... NOT the bonds holding the molecule together. Boiling (or freezing) doesn’t affect these

5. State of matter is determined by two opposing influences. If KE of particles is… then the state of matter is… and IMFs are… solid LOW “STRONG” liquid MEDIUM MEDIUM HIGH WEAK gas Pres. affects… IMFs (proximity) Temp. affects… KE -- BP and FP/MP depend on IMFs strong IMFs  high BP, high MP/FP weak IMFs  low BP, low MP/FP

6. Types of IMFs When ions are present: ion-dipole forces For neutral molecules: dipole-dipole forces electrostatic forces London dispersion forces hydrogen bonding forces There are different bonds between people: parent/child, boss/employee, friends, lovers, etc. We’re not talking about bonds holding an individual together.

7. H H O KCl solution is given intravenously in cases of capital punishment by lethal injection. The KCl causes the heart muscle cells to cease contracting. Ion-Dipole Forces (IDFs) -- exist between an ion and a partial charge on the end of a polar molecule (i.e., a dipole) solutions of ionic substances in polar liquids -- important for... e.g., aq. soln. of KCl… Cl– IDF K+ “momentary kissage”

8. Phosphine (PH3), is used in pest control fumigation due to its high toxicity. In the liquid phase, phosphine is subject to DDFs. Dipole-Dipole Forces (DDFs) -- exist between neutral polar molecules that are close together -- weaker than IDFs -- as the dipole moment m (i.e., the polarity) increases... DDFs (thus, BP and MP )

9. motion of e– causes instantaneous dipoles, which induce dipoles on nearby m’cules; LDFs with increasing molar mass London Dispersion Forces (LDFs) -- exist between all molecules, but are the ONLY forces between nonpolar molecules that are close together -- -- polarizability: the ease with which the charge distribution in a molecule can be distorted by an external magnetic field -- large atoms w/lots of e– exhibit high polarizability

10. C N O F 2.5 3.0 3.5 4.0 P S Cl 2.1 2.5 3.0 Se Br 2.4 2.8 Hydrogen Bonding Forces (HBFs) -- strongest IMF; special class of DDF -- exist between a hydrogen atom in a polar bond and an unshared electron pair on a nearby, small, highly electronegative ion or atom (e.g., F, O, N, Cl) -- “bare proton” of H is attracted to unshared e– pairs; its small size allows close proximity/strong bond -- important in structures of proteins and DNA

11. H H H H H H H H H H H H H H H H H H H H H H H H O O O O O O O O O O O O HBFs act between water/ice molecules. Water expands 9% when it freezes. water ice