Chem Unit 5 - Polarity, IMFs & Bulk Properties

Unit 5 Bonding quiz prep (Covalt Honors Chem)

Electronegativity

how strongly an atom pulls shared electrons toward itself (F is the highest)

δ⁺ and δ⁻

partial charges: δ⁻ on the more electronegative atom, δ⁺ on the less

Dipole arrow direction

points toward the MORE electronegative atom (δ⁻ end)

What makes a MOLECULE polar?

polar bonds AND an asymmetric shape, so the dipoles don't cancel

Quick polar test

lone pairs on the central atom (bent, pyramidal) or different outer atoms → polar

Quick nonpolar test

symmetric shape, all outer atoms identical, no lone pairs on central atom → nonpolar

Is CO₂ polar?

No: polar bonds but linear/symmetric, dipoles cancel

Is H₂O polar?

Yes: bent shape, dipoles don't cancel

Is NH₃ polar?

Yes: trigonal pyramidal with a lone pair

Is CH₄ polar?

No: C–H nonpolar, symmetric tetrahedral

Is CCl₄ polar?

No: polar C–Cl bonds cancel in symmetric tetrahedral

Is CH₃Cl polar?

Yes: different outer atoms, dipoles don't cancel

Is BF₃ polar?

No: trigonal planar, symmetric

Is HCl polar?

Yes: one polar bond, nothing to cancel it

Intramolecular vs intermolecular forces

intra = bonds INSIDE a molecule (strong); inter = attractions BETWEEN molecules (weaker, IMFs)

IMF strength order (weakest → strongest)

LDF < dipole-dipole < hydrogen bonding < ion-dipole

London dispersion forces (LDF)

temporary dipoles from moving electrons; in ALL molecules; the ONLY IMF in nonpolar molecules

What makes LDF stronger?

more electrons / bigger molecule (more polarizable), more surface area

Dipole-dipole

attraction between polar molecules: δ⁺ end of one to δ⁻ end of another

Hydrogen bonding

H bonded DIRECTLY to F, O, or N, attracted to a lone pair on F/O/N of another molecule

FON rule

H-bonding only if H is attached to F, O, or N

Ion-dipole

an ion attracted to a polar molecule (NaCl dissolving in water)

IMFs in a nonpolar molecule

LDF only

IMFs in a polar molecule (no FON-H)

LDF + dipole-dipole

IMFs in a molecule with H on F/O/N

LDF + dipole-dipole + hydrogen bonding

IMFs in CH₄

LDF only

IMFs in HCl

LDF + dipole-dipole

IMFs in H₂O / NH₃ / HF

LDF + dipole-dipole + H-bonding

Does H₂S hydrogen bond?

No: S isn't F, O, or N (LDF + dipole-dipole only)

Does CH₃OH hydrogen bond?

Yes: it has an O–H

Stronger IMFs → boiling & melting point

HIGHER

Stronger IMFs → vapor pressure

LOWER (evaporates less)

Stronger IMFs → viscosity & surface tension

HIGHER

How to rank boiling points

compare strongest IMF first; if tied, bigger molecule (more e⁻ = more LDF) wins

Why does H₂O boil higher than H₂S?

H₂O has hydrogen bonding; H₂S doesn't

Halogens F₂, Cl₂, Br₂, I₂ at room temp

gas, gas, liquid, solid: bigger = more LDF = higher bp

Like dissolves like

polar dissolves polar/ionic; nonpolar dissolves nonpolar (oil and water don't mix)