Polarity and Intermolecular Forces

NC Essential Standard 1.2.3 , 1.2.5

Electronegativity

• Measure of attraction for an electron– Most bonds are not completely ionic or

covalent, but somewhere in between– Difference in electronegativity determines the

nature of bonding• ΔEN > 2.0 ionic (complete transfer)• ΔEN = 0.5-2.0 polar covalent (unequal sharing)• ΔEN = 0-0.4 nonpolar covalent (equal sharing)

– The ΔEN #s above are strictly a guideline to go by, there is no significant difference between 1.9 and 2.0. Think of these as a gradual progression from completely nonpolar, ΔEN=0, to completely ionic, ΔEN=3.3 (the greatest possible difference)

Polar Bonds ΔEN = 0.4 - 2.0

• Unequal Sharing of electrons creates partially positive and negative ends, called a dipole (or simply polar)– Depending on geometry, polar bonds can cause the

entire molecule to exhibit polarity (polar molecule)– At the low end, 0.4-1.0: these molecules may be gases

or volatile liquids (example: ammonia, methanol)– At the high end, 1.0-2.0: these compounds will tend to

be solids, although with a much lower melting point than ionic solids (example: sugar, butter)

Nonpolar Bonds ΔEN < 0.4

• Nonpolar: the electronegativity, or pull on the electrons is close enough that no partially charged regions are created resulting in equal sharing of electrons in bonds

• Most nonpolar compounds are gases, but there are some exceptions

• Example: methane, oxygen

Electronegativity Table

Polar Bonds Sometimes Make Polar Molecules

• Looking at the ∆EN can tell you if a bond is polar• However, you must visualize the structure to

determine if the whole molecule will be polar• Like atoms bonded will always be nonpolar

– O2, S8

• Different atoms bonded will generally be polar (but need to look at ∆EN)

• If the structure has lone pairs of electrons on the central atom, different atoms bonded to the central atom, or anything asymmetrical, it will be POLAR– Symmetrical (shape and atoms) is NONPOLAR– Asymmetrical (shape and/or atoms) is POLAR

Symmetric Molecules are Symmetric Molecules are NonpolarNonpolar

Asymmetrical Molecules are Polar

“Like Dissolves Like”Polarity Effects Solubility

• This statement describes the general solubility of substances.

• Polar substances dissolve in polar solvents – (such as water)

• Nonpolar substances dissolve in nonpolar solvents – (such as gasoline)

• Polar and nonpolar substances do not mix – (oil and water)

Intermolecular Forces: Intermolecular Forces: The three types of forces The three types of forces

areare1.1. Dispersion forces Dispersion forces whichwhich occur between

nonpolar molecules. (Van der Waals)

2.2. Dipole-dipole Dipole-dipole forces whichforces which occur between polar molecules.

3.3. HydrogenHydrogen bonding bonding which occurs between molecules with an H-F, H-O, or H-N bond.

Intermolecular forces determine phase!Intermolecular forces determine phase!

Dipole-dipole Dipole-dipole forcesforces occur occur between between molecules with a molecules with a permanent permanent opposite charges.opposite charges.Ex:Ex:HCl and HBrHCl and HBr

Dipole-Dipole Forces

Hydrogen BondingHydrogen bonding is the strongest form of Intermolecular bonds. They are formed when the H of a molecule is attracted to the highly electronegative atom of a neighboring molecule.

Weakest intermolecular force are the dispersion forces. They occur between nonpolar molecules.1)Monatomic molecules: He, Ne, Ar, Kr, etc.2)Diatomics with 2 atoms of the same element. 3)Very symmetric molecules

Note: The larger the molecule the stronger the

dispersion forces.

Dispersion Forces

Melting pt, Boiling pt, HMelting pt, Boiling pt, Hff

and Hand Hvv and vapor and vapor

pressure depend on pressure depend on how hard it is to pull the how hard it is to pull the particles apart.particles apart.

Weak intermolecular Weak intermolecular forces – itforces – it’’s easy to pull s easy to pull them apart.them apart.

Strong intermolecular Strong intermolecular forces – itforces – it’’s hard. s hard.

Intermolecular bonds Effect M.P, BP, Hv and Hf

Which substance has the strongest Which substance has the strongest intermolecular forces?intermolecular forces?The weakest?The weakest?

WaterWaterDiethyl EtherDiethyl Ether