5. polarity and intermolecular forces Flashcards
electronegativity is
the atoms ability to attract the electron pair in a covalent bond
what is the most electronegative element
fluorine
what are the 3 most electronegative elements
Fluorine
Oxygen
Nitrogen
how does electronegativity affect covalent bonds
one atom more electronegative than another = polarise’s the bond
a permanent dipole occurs when
great difference between electronegativity of atoms causing a shift in electron density in the bond
if polar bonds are arranged symmetrically
e.g. CO2
the dipoles cancel each other out
molecule has no overall dipole and is non polar
if polar bonds aren’t arranged symmetrically
e.g. H2O
dipoles dont cancel eachother out
charge arranged unevenly across molecule
overall dipole and is polar
to work out if a molecule is polar you need to
draw molecule out in 3D
label partial charges
see if they cancel each other out
the higher the difference in electronegativity
the more ionic in character the bonding becomes
3 types of intermolecular forces
- induced dipole-dipole or London (dispersion) forces
- permanent dipole-dipole interactions
- hydrogen bonding
intermolecular forces are strong/weak?
very weak
what is the strongest type of intermolecular force
hydrogen bonding
london dispersion forces are found between
all atoms and molecules
how do london dispersion forces work
- electrons in charge clouds moving quickly.
- at any one moment electrons more likely to be more concentrated on one side than the other = temporary dipole.
- temporary dipole can cause another temporary (induced) dipole on a neighbouring atom = 2 dipoles then attracted to each other
- 2nd dipole can induce a dipole in 3rd atom and so on.
- constantly moving = dipoles created and destroyed all the time.
overall effect = atoms attracted to each other
what affects strength of london dispersion forces
- larger molecules = larger electron clouds = stronger ldf
2. greater surface area = bigger exposed electron cloud = stronger ldf
stronger LDF mean
higher bpt/mpt
need more energy to overcome stronger intermolecular forces
LDF can hold molecules in a
lattice
permanent dipole dipole interactions happen between
polar molecules
why do permenant dipole dipole interactions happen
the delta +/- charges on polar molecules cause weak electrostatic forces of attraction between molecules
when does H bonding happen
when H is covalently bonded to Fluorine Nitrogen or oxygen
H has a high charge density because it is so small. F O N are very electronegative
how does H bonding happen
bond is so polarised between a H and F/O/N
causes weak bond to form between the H of one molecule and a lone pair of electrons on FON on another molecule
what effect does H bonding have on the properties of substances
soluble in water
higher boiling and freezing points than molecules of a similar size that aren’t able to form H bonds
how is ice less dense than water
unusual most substances get denser when freeze.
ice = molecules held together in lattice by long H bonds water = less H bonds
in general, main factor that determines bpt of a substance
strength of LDF (unless molecules can form H bonds)
why do simple covalent compounds have low mpt/bpt
weak intermolecular forces
don’t need much energy to break
often liquid/gas at room temp
why are polar molecules soluble in water
water is polar tends to dissolve other polar substances
compounds with h bonds can form h bonds with water molecules and will be soluble
molecules with only ldf e.g. methane=insoluble
when talking about mpt/bpt of simple covalent compounds you refer to
the intermolecular forces between simple covalent compounds not the actual covalent bond.
