2.6.3+4: intermolecular forces Flashcards
what are intermolecular forces?
weak interactions between dipoles of different molecules that hold molecules in covalent compounds together
what are IMF largely responsible for?
physical properties such as melting and boiling points
what are london forces?
the weakest IMF that acts between induced dipoles
how do london forces occur?
electrons in molecules are constantly moving and at any instant there may be an unequal distribution of electrons that results in a temporary dipole - this induces temporary dipoles on neighbouring molecules which then attract one another
what increases the strength of london forces?
the bigger the molecule
the more points of contact
the more electrons there are
what’s a permanent dipole-dipole?
the force of attraction between polar molecules with a large difference in electronegativity
what’s an example of permanent dipole-dipole?
hydrogen chloride
the chlorine is more electronegative and attracts the partially positive hydrogen
how is permanent dipole-dipole different to london forces?
they’re much stronger than temporary dipoles so they have a higher boiling point
what’s hydrogen bonding?
the strongest type of IMF that occurs between molecules that have a hydrogen atom covalently bonded to oxygen, nitrogen or fluorine
how does hydrogen bonding work?
the lone pair on an electronegative atom in one molecule pulls electrons away from the hydrogen atom in another molecule so the nucleus is unshielded
what anomalous properties does H2O have because of hydrogen bonding?
it has a relatively high melting/boiling point because more energy is required to break the hydrogen bonds on top of the london forces
ice is less dense than water because the hydrogen bonds hold the H2O molecules hold the H2O molecules apart in a lattice
what are simple covalent substances?
small units containing a definite number of atoms/molecular formula e.g. H2, CO2 that form simple molecular lattices when solid
the molecules are held together by weak IMF and the atoms within each molecule are bonded together by strong covalent bonds
properties of simple covalent lattices?
relatively low melting and boiling points (dependant on the type of IMF)
don’t conduct electricity because there’s no charged particles
do non-polar simple covalent structures dissolve in non-polar solvents?
yes because IMF’s form between the molecules and solvent which weaken and break the IMF in the lattice
do non-polar simple covalent structures dissolve in polar solvents?
no because the IMF in the polar solvent is too strong to be broken and theres little interaction between molecules in lattice and solvent
