3.1.3 (b&s) intermolecular forces Flashcards
out of van der waals, permanent dipole-dipole forces and hydrogen bonds, which are the strongest?
hydrogen bonds
what’s another term for Van der waals forces?
temporary-induced dipole attractions
what does an odd equal electron distribution result in?
A temporary dipole in the molecule, which can induce an opposite dipole on a neighbouring molecule
The strength of van der waals forces increase if what increases?
- The size of the atom or molecule
- The area of contact between molecules
when did the boiling points of halogens increase?
As the group is descended
what do all the halogens exist as?
Diatomic molecules
what are intermolecular forces?
The forces that exist between simple, covalent molecules
what are the three types of intermolecular forces?
- van der waals
- permanent dipole-dipole
- hydrogen bonding
because the van der waals are the weakest intermolecular force what does this mean?
they are easily overcome
what happens to the melting point going down group 7?
- The melting points of the halogens increase
- Because, as the size of the molecule increases, the size of the electron cloud increases
- And so the extent of the temporary distortion of the electron clouds can increase
- Resulting in stronger van der waals forces between molecules
what happens to the melting point of halogens, the stronger the van der waals forces are?
The stronger the van der waals forces between the halogen molecules, the higher the boiling point of the substance
as more energy is needed to overcome these forces and release individual molecules of gases
what happens to the boiling point as the size of the alkane molecule increases?
as is the size of the alkane molecule increases
The area of contact between molecules increases
And so the number of temporary induced dipole-dipole attractions also increases therefore the boiling point of the alkane increases
when a covalent bond is polar, the pair of electrons in the bond are ……………….. …………………. by the atoms in the bond.
what does this result in?
shared equally
this results in the least electronegative atom, having a d+ charge, and the most electronegative atom having a d- charge
do partial charges have strong or weak, electrostatic attractions?
weak
what do these weak particles charges set up?
a permanent dipole-dipole force
what are the boiling points like for permanent dipoles?
Higher than van der waals
in diagrams, how are permanent dipole-dipole forces shown as?
dotted lines
when do hydrogen bonds occur?
hydrogen bonds ONLY OCCUR if a molecule has a hydrogen atom covalently bonded to a
- nitrogen, oxygen of fluorine atom
AND there is a lone pair of electrons available on the
- nitrogen, oxygen or fluorine atom
what do nitrogen, oxygen and fluorine form with hydrogen?
why?
very polar covalent bonds
as they pull electron density away from the hydrogen leaving the proton of the hydrogens nucleus exposed
why doesn’t chlorine form a hydrogen bond?
because the chlorine atom is too large to get close to a d+ hydrogen atom to form a hydrogen bond
in water, strong hydrogen bonds form between what?
the lone pair on electrons on an oxygen atom of one molecule of water and a d+ hydrogen atom of a neighbouring water molecule
what do the hydrogen bonds cause the water molecules to do in ice?
they cause the water molecules to be slightly further apart than in liquid water giving ice a lower density than liquid water
do compounds that exhibit hydrogen bonding have a higher or lower boiling/melting point?
Why?
higher
Because more energy is required to break the hydrogen bonds compared to permanent dipole-dipole and van der waals forces
