Chem 11E Unit 7 - Molecular Polarity and Solubility

what are the van Der Waals forces?

• London forces

• dipole-dipole forces

• hydrogen bonds (stronger version of dipole-dipole)

what’s the distinction between the 2 types of forces due to?

the presence or absence of permanent dipoles

what’s a dipole?

a partial separation of charge existing when one end of molecule or bond has slight excess of positive charge and the other end has slight excess of negative charge

what are London forces?

weak attractive forces existing from temporary dipolar attraction between neighbouring atoms

what are dipole-dipole forces?

bonding forces from an electrostatic attraction between molecules having permanent dipoles

if a permanent dipole is absent what forces are present? when is it present? if a permanent dipole is present?

• only London forces

• London forces is ALWAYS present (even when permanent dipoles or ionic)

• dipole-dipole + London forces

what happens when 2 atoms bond?

since most atoms bonding have different electronegativites, this gives rise to a dipole (ex: H - Cl)

𝛿⁺ and 𝛿^–?

• slight excess in positive charge

• slight excess in negative charge

what happens to the atoms with low electronegativites?

tend to form positive ions, are electropositive (alkali metals, alkali earth metals)

what’s molecular polarity?

a molecule is polar if the distribution of charge within the molecule is uneven

what affects molecule polarity (5 things)?

• electronegativity of the atoms

• polarity of the individual bonds within the molecule

• intermolecular forces

• shape of the molecule

• symmetrical shape or not

if a molecule has polar bonds that are arranged symmetrically what is it?

non polar, because bonds must be polar and angles not equal to be polar molecule as symmetrical = forces resolved, charge is evenly distributed throughout molecule (rock and ropes example)

what are dipole-dipole forces weak equivalents of? why?

ionic bonds, because both are forms of attraction between a more positively charged atom or molecule and a more negative atom or molecule

relative strengths of interactions?

ionic bond » dipole-dipole forces ≈ London forces

what are hydrogen bonds?

a relatively strong type of dipole-dipole attraction occurring when H is bonded with N, O, or F (due to large difference in electronegativities)

when will a hydrogen bond form when bonded with N, O, or F?

• HF ONLY

• NH, NH₂, NH₃

• OH, H₂O, H₂O₂

what happens to viscosity when molecules have stronger intermolecular bonds vs. weaker? why?

stronger intermolecular bonding = higher viscosity (more resistant to flow) due to higher melting points and lessened ability for molecules to “slide“ and “flow“

polar or nonpolar - H₂O?

• bent

• charge not evenly distributed so polar

methanol (CH₃OH)

• polar

ethanol (CH₃CH₂OH)

• combo of tetrahedral and bent

• hydrogen bonding

• polar

Benzene (C₆H₆)

• non-polar

ethoxyethane aka diethyl ether, ether, hospital ether

• polar

• (C₂H₅)₂O

acetone (CH₃COCH₃)

• nail polish remover

• polar

heptane (C₇H₁₆)

• non-polar

ammonia (NH₃)

• polar

CF₄

• tetrahedral

• all angles 109.5 degrees

• bonds evenly distributed so non-polar

chloroform (CHCl₃)

• polar

carbon tetrachloride (CCl₄)

• non-polar

dissolving process in 3 attractions?

explanation

why does polar dissolve polar? why does non-polar dissolve non-polar?

explanation

water, methanol, and ethanol dissolving abilities?

explanation

polar dissolving polar 2 examples + 1 non example?

• sugar dissolves in water

• NaCl dissolves in water

• oil does not dissolve in water

non-polar dissolving non-polar 1 example + 1 non example?

• iodine dissolves in oil

• iodine does not dissolve in water