Molecular Geometry, Polarity, and Intermolecular Forces Review

Vocabulary practice flashcards covering molecular shapes, bond angles, polarity, intermolecular forces (IMFs), and physical properties from Unit 6 Independent Review.

Polarity

An unequal sharing or "tug" of electrons where one part of a molecule or bond has a larger distribution of electrons than the other.

Polar Bond vs. Polar Molecule

A bond is polar when two atoms have different electronegativity values, causing one to hog electrons and become partially negative while the other becomes partially positive. A molecule is polar when its dipoles do not cancel out, resulting in an entire portion of the molecule being partially negative and another portion partially positive.

$CF_4$ Molecular Properties

Exhibits tetrahedral molecular shape with ideal bond angles of $109.5^\circ$; it is a nonpolar molecule whose strongest intermolecular force is London Dispersion Forces (LDF).

$NOCl$ Molecular Properties

Exhibits a bent molecular shape with ideal bond angles of $<120^\circ$; it is a polar molecule whose strongest intermolecular force is dipole-dipole.

$NF_3$ Molecular Properties

Exhibits a trigonal pyramidal molecular shape with ideal bond angles of $<109.5^\circ$; it is a polar molecule whose strongest intermolecular force is dipole-dipole.

$H_2CS$ Molecular Properties

Exhibits a trigonal planar molecular shape with ideal bond angles of $120^\circ$; it is a polar molecule whose strongest intermolecular force is dipole-dipole.

$CH_2F_2$ Molecular Properties

Exhibits tetrahedral geometry with bond angles of $109.5^\circ$; it is polar because in 3D, the two fluorine atoms are not directly across from each other, so their dipoles do not cancel out.

$H_2S$ Molecular Properties

Exhibits a bent molecular shape with ideal bond angles of $<109.5^\circ$; it is a polar molecule whose strongest intermolecular force is dipole-dipole.

$CO_2$ Molecular Properties

Exhibits a linear molecular shape with ideal bond angles of $180^\circ$; it is a nonpolar molecule held together by London Dispersion Forces (LDF).

Single-Bonded Diatomics ($O_2$ and $HF$)

Molecules such as $O_2$ (nonpolar, LDF) and $HF$ (polar, hydrogen bonding) have linear geometry with no bond angle because they contain only 1 bond.

IMF Strength Effects on Physical Properties

An increase in the strength of intermolecular forces causes both melting point and surface tension to increase.

Hydrocarbon Cohesion Forces

Among nonpolar hydrocarbons ($CH_4$, $C_2H_6$, $C_3H_8$, $C_4H_{10}$), $C_4H_{10}$ has the strongest cohesion forces because larger hydrocarbons have more electrons and a greater surface area, which increases their LDF.

Ion-Dipole Force

The type of intermolecular force involved when an ionic compound like $NaCl$ dissolves in $H_2O$.

$HF$ vs. $HCl$ Boiling Points

$HF$ has a much higher boiling point ($20^\circ\text{C}$) than $HCl$ ($-85^\circ\text{C}$) because $HF$ molecules are attracted by stronger hydrogen bonds (in addition to dipole-dipole and LDF), whereas $HCl$ molecules are held together by weaker IMFs (dipole-dipole forces and LDF).

$H_2O$ vs. $O_2$ Boiling Points

$H_2O$ has a much higher boiling point ($100^\circ\text{C}$) than $O_2$ ($-183^\circ\text{C}$) because $H_2O$ molecules are held together by hydrogen bonds, whereas $O_2$ molecules are held together by weak LDF.

Melting Point Ranking ($H_2$, $HI$, $HF$)

Ranked from lowest to highest melting point: $H_2$ (LDF only) < $HI$ (dipole-dipole) < $HF$ (hydrogen bonding).

Substance IMF Classifications

The strongest IMF present for each substance: $CBr_4$ (LDF), $H_2$ (LDF), $CO_2$ (LDF), $SO_2$ (Dipole-dipole), $HF$ (Hydrogen bonding), and $H_2O$ (Hydrogen bonding).