Intermolecular Forces Flashcards
What does the VSEPR Theory stand for?
valance shell electron pair repulsion theory
What is the VSEPR Theory?
- molecular shape results from repulsion between groups of electrons, both bonding and non-bonding, in the valence shell of the central atom
- allows valence electron groups to have greatest possible angle of separation
1 or 2 groups of electrons
linear
180 degrees
3 groups of electrons
triangular planar
120 degrees
4 groups of electrons
tetrahedral
109.5 degrees
4 groups of electrons, 1 lone pair
pyramidal
107.5 degrees
4 groups of electrons, 2 lone pairs
bent/V shape
104.5 degrees
Electronegativity
the ability of an atom to attract electrons to itself and is measured on a scale of 0 to 4
higher electronegativity = greater tendency to attract electrons
Electronegativity in covalent bonds
- different electronegativity = uneven sharing
- shared electrons will spend more time near the more highly electronegative element
More electronegative element
delta minus
Less electronegative element
delta plus
A bond with charged ends
Bond dipole
Polar molecules
molecules with slight positivity and slight negative charged ends
said to have net dipole
all molecules with a single bond dipole
If a molecule has several bond dipoles they may….
reinforce one another or cancel each other’s effect (depends on strength and direction of individual bond dipoles)
Non-polar
If bond dipoles are equal of size and act in symmetrically opposing directions (cancel out each other’s effects)
has zero net dipole
Polar
non-symmetrically arranged
have net dipole
pull in a particular direction
dipole-dipole forces
attraction occurs between oppositely charged ends of a polar molecule
electrons are pulled towards the more electronegative molecule
Effects/Properties of dipole-dipole
increase melting/boiling points
solubility
ability to dissolve ionic solutes
Increasing melting/boiling points (DIPOLE-DIPOLE)
the higher the polarity the higher the melting/boiling points (than other substances of similar molecular size)
Solubility (DIPOLE-DIPOLE)
polar substances (dipole-dipole) will dissolve in one another but not in non-polar substances (ones that can't interact by dipole-dipole forces) LIKE DISSOLVES LIKE
Ability to dissolve ionic solutes
due to attraction between the dipole of a polar molecule and an ions charge
ion-dipole forces (NOT type of intermolecular force)
Dispersion forces
occurs in ALL molecular materials
only intermolecular force present in NON-POLAR substances eg. hydrocarbons, N2, O2, CO2
typically weak for molecules with few electrons
show increasing strength for molecules with greater number of electrons (seen in NON-POLAR)
Stronger in molecules who’s shape allows max surface contact between molecules (e.g. linear vs branched or spherical - hight melt/boil points)
dispersion forces increases with molar mass/molecular size
Temporary dipoles
random motion of electrons within a molecule produce short lived dipoles
can make non-polar molecules temporarily polar
cause nearby molecules to form similar temporary dipoles (INDUCED dipoles)
Hydrogen bonding
strong as 12% of the carbon to carbon covalent bond
only occur in H-F, H-O, H-N
due to the electrostatic attraction between a lone pair of electrons of F,O,N and H atom already covalently bonded to another F,O,N
Melt/boil points are much higher
extremely soluble in other substances with can also hydrogen bond
Intermolecular forces and physical properties
melting/boiling points
solubility
equilibrium vapour pressure/vapour pressure
melt/boil points (PHYSICAL PROPERTIES)
as intermolecular forces present in a substance increase in strength so does its melting/boiling points
substances that interact by dispersion forces as molecular mass increases so does melting/boiling points
substance that interact by dipole-dipole forces will have slightly higher melting/boiling points
hydrogen bonds = even higher melting/boiling points
Solubility (PHYSICAL PROPERTIES)
Like dissolves like
Equilibrium vapour pressure/vapour pressure (PHYSICAL PROPERTIES)
a measure of the tendency of a substance to evaporate and is measured in kPa
weak intermolecular forces correspond to liquids that easily evaporate and have a high vapour pressure
increases with temp
weakest intermolecular forces has the highest vapour pressure
