1.5 intermolecular forces - london, hydrogen, dipole-dipole and simple molecular properties

what are intermolecular forces (IMFs)?

intermolecular forces are forces of attraction between molecules, weaker than covalent bonds, and determine physical properties like melting and boiling points.

what are the main types of IMFs in simple molecules?

1⃣ van der waals (london dispersion) forces

2⃣ permanent dipole–dipole forces

3⃣ hydrogen bonding

what are london dispersion (induced dipole-dipole interaction) forces?

temporary induced dipoles in molecules due to momentary uneven electron distribution, present in all molecules, stronger in larger molecules with more electrons.

what are permanent dipole–dipole forces?

attractions between permanent dipoles in polar molecules. stronger than london forces, weaker than hydrogen bonds.

what is hydrogen bonding?

strong type of dipole–dipole interaction where hydrogen is covalently bonded to highly electronegative atoms (f, o, n) and attracted to lone pair of electrons on another electronegative atom.

how do IMFs affect melting and boiling points?

stronger imfs → higher melting/boiling points because more energy is needed to overcome attractions.

why do simple molecular substances have low melting/boiling points?

molecules are held together by weak IMFs, while covalent bonds within molecules are strong. only intermolecular forces need to be overcome when melting/boiling.

why are simple molecular substances poor conductors?

they have no mobile electrons or ions to carry charge in solid or liquid state.

what affects the strength of london forces?

1⃣ size of molecule / number of electrons → larger molecules have stronger london forces.

2⃣ shape of molecule → more surface contact → stronger london forces

give an example of a hydrogen bonded simple molecule and its properties.

H2O → hydrogen bonding gives high boiling point, high surface tension, and solvent properties compared to similar-sized molecules like H2S.

how does molecular size/electron number affect boiling points in simple molecules?

as molecular size and number of electrons increase, london forces become stronger. this means more energy is needed to overcome IMFs, so boiling point increases slightly down the group or with larger molecules.

solubility in polar and non polar - give reasons

• polar dissolves in polar substances - IMFs match (london)

• non-polar dissolves in non-polar substances - hydrogen on dipole-dipole interactions occur