Polarity

Covalent bonds are classified as being either polar or nonpolar. Polarity a physical property of the compound is important because it determines other physical properties of the compound such as boiling point melting point solubility and intermolecular interactions.

\ Atoms using nonpolar bonds equally share the electrons between them. Polar bonds have unequal electrons between atoms. All ionic bonds are polar bonds.

\ Ionic: metal + nonmetal, full charges

Polar: non metal + different nonmetal, partial charges

Non polar: nonmetal + same nonmetal, no charges

Covalent: nonmetal + nonmetal

Metallic: metal + metal

\ If a negative charge is evenly distributed around the molecule, it is not polar.

If there is an uneven distribution of negative charge, the molecule is polar.

\ Are any bonds polar?

No - molecule is not polar

Yes - are atoms symmetrically distributed around central atom?

No - molecule is polar

Yes - are all outside atoms the same?

        No - molecule is likely polar (check individual bond polarity, if less than 5, then non polar)

        Yes - molecule is non polar

\ Slightly polar covalent: polarity less than or equal to 25

Very polar covalent: polarity greater than or equal to 75

\ Why do two nonpolar molecules mix?

  • There are only weak or no attractions between nonpolar molecules
  • Nature favors entropy
  • No nonpolar molecules will rearrange because all attractions are similar

\ Why do two polar substances mix?

  • Strong attractions between all types of polar molecules
  • Favors scrambling because they get more disordered (more entropy)

\ Why don’t polar and nonpolar molecules mix?

  • All polar molecules are attracted to each other
  • Nonpolars have only weak or no attractions
  • They separate because all polar molecules are attracted to each other
  • Nonpolars are left alone
  • Strong attractions between polars force the liquids to separate out even though there’s less entropy

\ 5 properties of matter affected by IMFs:

\ boiling point/melting point:

  • The bp is the temp at which the vapor pressure of a liquid equals the external pressure surrounding the liquid
  • The bp of a liquid depends on atmospheric pressure
  • The stronger the IMFs, the higher the bp

vapor pressure:

  • The vapor pressure of a liquid is the equilibrium pressure of a vapor above its liquid (or solid)
  • The pressure of the vapor resulting from evaporation of a liquid (or solid) above the sample of the liquid (or solid) in a closed container
  • The lower the vapor pressure the higher the bp
  • The lower the vapor pressure the colder the liquid and the stronger the IMF because strong bonds make it harder for molecules to separate so they move slower and have less pressure/energy

viscosity:

  • A liquid’s resistance to flow
  • When the intermolecular forces of attraction are strong within a liquid, there is a larger viscosity
  • Hot water, molecules move faster, higher-pitched sound

\ solubility:

  • The ability of a solid, liquid, or gas to dissolve in a solid, liquid, or gas solvent

\ surface tension:

  • The tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid which tends to minimize surface area
  • The molecules at the surface can only bond with H2O molecules next to them or below them because there are fewer of these H-bonds, these bonds are stronger
  • Stronger IMF, stronger surface tension

\ Intermolecular forces are attractions between molecules

Intramolecular forces (covalent) are attractions within a molecule

Intermolecular forces break when we melt or boil and are weaker than covalent bonds, ionic bonds, and metallic bonds

\ Dipole-dipole:

  • Attractions between polar molecules
  • Partial positive and partial negative

\ Hydrogen Bonds:

  • Attractions between polar molecules that have a particularly high difference in EN (strong dipole-dipole attractions)
  • A molecule can only do hydrogen bonding with covalent bonds of H-O, H-N, H-F
  • Hydrogen bonds are particularly strong dipole-dipole attractions because N, O, F have the highest EN and the EN change for H-N, H-O, H-F are very large so they have large dipoles
  • H partial positive is very strong and F, O, N are very partial negative
  • Hydrogen is the smallest atom and N, O, F are also very small so molecules get very close to each other

\ Dispersion:

  • Attractions between neutral atoms or nonpolar molecules
  • Exists in all molecules including polar
  • The electrons in a roughly symmetrical electron cloud can randomly shift more to one side
  • This induces the electron cloud in another atom to shift as well
  • Now the two attract
  • Are stronger when molecules are more polarizable
  • Atoms with larger electron clouds are more polarizable because larger electron clouds can make more significant shifts
  • Dispersion attraction can be comparable or even greater than dipole dipole

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