Intermolecular Forces and States of Matter AP Chemistry

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. H2OH2O

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

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

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

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

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) -- important for... solutions of ionic substances in polar liquids e.g., aq. soln. of KCl… K + H H O Cl – IDF “momentary kissage”

Dipole-Dipole Forces (DDFs) -- exist between neutral polar molecules that are close together -- weaker than IDFs -- as the dipole moment  (i.e., the polarity) increases... DDFs (thus, BP and MP ) Phosphine (PH 3 ), is used in pest control fumigation due to its high toxicity. In the liquid phase, phosphine is subject to DDFs.

London Dispersion Forces (LDFs) -- exist between all molecules, but are the ONLY forces between nonpolar molecules that are close together -- motion of e – causes instantaneous dipoles, which induce dipoles on nearby m’cules; LDFs with increasing molar mass -- 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

Hydrogen Bonding Forces (HBFs) exist between a hydrogen atom in a polar bond and an unshared electron pair on a nearby, small, highly electronegative ion or atom strongest IMF; special class of DDF (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 3.0 Cl 4.0 F 3.5 O 3.0 N 2.8 Br 2.5 C S 2.1 P 2.4 Se

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