Intermolecular forces Flashcards
What are the 3 types of intermolecular forces in order of weakest to strongest?
van der Waals/permanent dipole-dipole/hydrogen bonding
What are van der Waals forces?
they are temporarily induced dipole-dipole interactions
they are present between all molecules
they can be very weak
How are van der Waals forces formed?
at any one moment there may be an unequal distribution of the electron cloud as the electrons are constantly moving
this results in a temporary dipole on the molecule which can then induce a temporary dipole on another molecule
the attraction between these temporary dipoles are van der Waals forces
With regard to van der Waals, more electrons in a molecule result in…
a greater likelihood of temporary dipoles
With regard to van der Waals, the greater the molecule size…
the stronger the van der Waals
Why do straight chain molecules have higher melting/boiling points than branched ones?
they have more points of contact between molecules so stronger van der Waals forces between molecules- takes more energy to break the intermolecular forces
What is dipole-dipole bonding?
the attraction between a permanent dipole on one molecule and a permanent dipole on another
How are permanent dipole-dipole bonds formed?
when there are two atoms in a covalent bond that have different electronegativities, where the more electronegative atom attracts the bonding pair of electrons, giving one atom a positive charge and one atom a negative charge
With regard to bond polarity, the bigger the difference in electronegativity between the atoms, the more…
polar the bond
What is hydrogen bonding?
the attraction between a lone pair of electrons on a very electronegative atom (N/O/F) and a slightly positive hydrogen atom that is directly bonded to a N/O/F on a neighbouring molecule
Why only NOF elements in hydrogen bonding?
they are the only atoms small enough to approach a slightly positive H atom closely enough for a hydrogen bond to be formed
Cl has the same electronegativity as N. Why does it not form a hydrogen bond?
it is too large to get close enough to the hydrogen to form a hydrogen bond
What are the main rules when drawing hydrogen bonding?
1) draw the hydrogen bonding atoms
2) draw the bonding atoms in a straight line
3) when drawing hydrogen bonding you must always draw the lone pairs like this : and write in the δ
In liquid water, hydrogen bonds…
constantly break and reform
Why is ice less dense than water?
the hydrogen bonds cause the water molecules to be slightly further apart than in liquid water, resulting in ice having an open structure so the density is less. each oxygen is linked to 4 hydrogens (2 covalent 2 hydrogen) in a 3D tetrahedral arrangement
