van der Waals and dipole-dipole forces

Question 1

State the three types of inter-molecular forces from weakest to strongest

Answer 1

van der Waals forces dipole-dipole forces hydrogen bonding

Question 2

What are van der Waals forces?

Answer 2

all atoms are made up of positive and negative charges, even though they are neutral overall these charges produce very weak electrostatic attractions between all atoms and molecules these are called van der Waals forces

Question 3

How do van der Waals forces work?

Answer 3

in a helium atom, it has two negatively charged electrons at any moment the electrons could be anywhere this mean that the distribution of charge is changing at every instant this means the atom has a diople that can change at any point in time, although it almost certainly will have a dipole (temporary dipole) this affects electron distribution in nearby atoms, so they are attracted to the original helium for that instant, which induces dipoles in the nearby atoms

Question 4

What are the rules about van der Waals? (3)

Answer 4

van der Waals act between all atoms or molecules at all times they are in addition to any intermolecular forces the dipole is caused by changing position of the electron cloud, so the more electrons there are, the larger the instantaneous dipole will be

Question 5

As larger molecules produce larger van der Waals, what does this explain about boiling points

Answer 5

this explains why the boiling point of noble gases increase as the atomic numbers of the noble gas increase and it explains why the boiling points of hydrocarbons increase with increased chain length

Question 6

What are dipole-dipole forces?

Answer 6

molecules with polar bonds may have a dipole moment, this sums up the effect of the polarity of all the bond in the molecule

Question 7

When do dipole-dipole forces act between molecules?

Answer 7

when the molecules have a permanent dipole e.g. in the hydrogen chloride molecule, chlorine is more electronegative, so the electrons are pulled towards the chlorine atom a polar molecule has an ovarall dipole, when you take in any account any dipoles across the board

Question 8

When do dipoles cancel?

Answer 8

it is dependant on the shape of the molecule if there is a even distribution of dipoles they will cancel, e.g. O^δ- = C^δ+ = O^δ- individual dipoles cancel out as they are operating in opposing directions

Question 9

How does perfume evaporate as it contacts the skin?

Answer 9

solvent needs heat to overcome intermolecular forces

it has relatively weak intermolecular forces

diffuses through the room

energy is taken from the skin (so it cools)