intermolecular forces Flashcards
where do london forces occur
occur between all molecular substances and noble gases. they do not occur in ionic substances.
what are london forces also known as
instantaneous induced dipole-dipole interactions
how do london forces form?
- the electrons are moving constantly and randomly
- the electron density can fluctuate and so there can be an uneven distribution of electrons
- these temporary dipoles can cause dipoles to form in neighbouring molecules.
- these are called induced dipoles
what is the main factor affecting size of london forces
the more electrons there are in the molecule, the higher the chance that temporary dipoles will form.
this makes the london forces stronger between the molecules and more energy is needed to break them so boiling points will be greater
explain the increasing boiling points down group 7
increasing number of electrons in the bigger molecules causing an increase in the size of the london forces between the molecules.
This is why I2 is a solid an Cl2 is a gas
explain the increasing boiling points of the alkane homologous series
increasing number of electrons in the bigger molecules causing an increase in the size of the london forces between molecules
how does the shape of the molecule have an effect on the size of the london forces?
long straight chain alkanes have a larger surface area of contact between the molecules for london forces to form than compared to spherical shaped branched alkanes and so have stronger london forces
where do permanent dipole-dipole forces occur?
between polar molecules which are asymmetrical and have a bond where there is a significant difference in electronegativity between the atoms
what is the order of strongest intermolecular forces?
weakest strongest
london permanent dipole-dipole hydrogen bonding
forces forces
where does hydrogen bonding occur?
occurs in compounds that have a hydrogen atom attached to one of the 3 most electronegative atoms of nitrogen, oxygen and fluorine, which must have an available lone pair of electrons.
e.g . -O-H, -N-H, F-H
what is the bond angle of the hydrogen bond and why?
180 degrees because there are two pairs of electrons around the H atom involved in the hydrogen bond. these pairs of electrons repel to a positive of minimum repulsion, as far as possible
why can water form 2 hydrogen bonds per molecule?
the electronegative oxygen atom has two lone pairs of electrons on it. it can therefore form stronger hydrogen bonding and needs more energy to break the bonds, leading to a higher boiling point
can alcohols form hydrogen bonds?
yes, this means they have higher boiling poinds and relatively low volatility compared to alkanes with a similar number of electrons
what affect does hydrogen bonding have on ice?
the molecules are held further apart by hydrogen bonds than in liquid water and this explains the lower density of ice
why does H2O, NH3 and HF have anomalously high boiling points?
caused by the hydrogen bonding between these molecules in addition to their london forces. the additional forces require more energy to break and so have higher boiling points
why is there a general increase in boiling point for H2S to H2Te or from HCl to HI?
caused by increasing london forces between molecules due to an increasing number of electrons
