intermolecular forces
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how does a polar bond form
when the electronegativity between the two bonded atoms has a large difference.
how does a polar molecule form
when the bonds are polar and add up (like vectors). the bonds can be polar but cancel out so the molecules aren’t polar.
what is an induced dipole-dipole force?
a temporary dipole in one atom/molecule that induces a temporary dipole in a neighbouring atom/molecule
what is a permanent dipole-dipole force?
the force between two molecules with a permanent dipole (polar molecules)
what is a hydrogen bond?
a strong dipole-dipole attraction between an δ+ H in an OH, NH or HF bond and the lone pair of electrons of O, N or F in another molecule
why can a hydrogen bond form
the δ+ H (polar bonded hydrogen) leaves hydrogens orbital mostly empty when bonded with O, F or N because they are the most electronegative elements. the lone pair of electrons almost form a dative bond
how is a hydrogen bond shown in a stick diagram?
the lone pair of electrons are shown as two dots. then there are straight lines drawn between the O, N or F and the hydrogen (e.g H llll:O)
whats the relative strength of the three types of dipole interactions
Hydrogen bond > permanent dipole-dipole > induces dipole-dipole
the relative strength of hydrogen bonds
HF > OH > NH
if HF hydrogen bonds are stronger than OH why does HF have a lower M.P than H2O?
there’s 1 hydrogen bond per HF molecule but there are two hydrogen bonds per H2O molecule so more energy is needed to separate the molecules
why does ice have a greater volume than water?
in ice, each water molecule is hydrogen-bonded to 4 others. theses bonds hold the molecules further apart than in water so the volume is larger.