3.1.3 (b&s) intermolecular forces

Question 1

out of van der waals, permanent dipole-dipole forces and hydrogen bonds, which are the strongest?

Answer 1

hydrogen bonds

Question 2

what’s another term for Van der waals forces?

Answer 2

temporary-induced dipole attractions

Question 3

what does an odd equal electron distribution result in?

Answer 3

A temporary dipole in the molecule, which can induce an opposite dipole on a neighbouring molecule

Question 4

The strength of van der waals forces increase if what increases?

Answer 4

• The size of the atom or molecule
• The area of contact between molecules

Question 5

when did the boiling points of halogens increase?

Answer 5

As the group is descended

Question 6

what do all the halogens exist as?

Answer 6

Diatomic molecules

Question 7

what are intermolecular forces?

Answer 7

The forces that exist between simple, covalent molecules

Question 8

what are the three types of intermolecular forces?

Answer 8

• van der waals
• permanent dipole-dipole
• hydrogen bonding

Question 9

because the van der waals are the weakest intermolecular force what does this mean?

Answer 9

they are easily overcome

Question 10

what happens to the melting point going down group 7?

Answer 10

• 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

Question 11

what happens to the melting point of halogens, the stronger the van der waals forces are?

Answer 11

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

Question 12

what happens to the boiling point as the size of the alkane molecule increases?

Answer 12

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

Question 13

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?

Answer 13

shared equally

this results in the least electronegative atom, having a d+ charge, and the most electronegative atom having a d- charge

Question 14

do partial charges have strong or weak, electrostatic attractions?

Answer 14

weak

Question 15

what do these weak particles charges set up?

Answer 15

a permanent dipole-dipole force

Question 16

what are the boiling points like for permanent dipoles?

Answer 16

Higher than van der waals

Question 17

in diagrams, how are permanent dipole-dipole forces shown as?

Answer 17

dotted lines

Question 18

when do hydrogen bonds occur?

Answer 18

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

Question 19

what do nitrogen, oxygen and fluorine form with hydrogen?

why?

Answer 19

very polar covalent bonds

as they pull electron density away from the hydrogen leaving the proton of the hydrogens nucleus exposed

Question 20

why doesn’t chlorine form a hydrogen bond?

Answer 20

because the chlorine atom is too large to get close to a d+ hydrogen atom to form a hydrogen bond

Question 21

in water, strong hydrogen bonds form between what?

Answer 21

the lone pair on electrons on an oxygen atom of one molecule of water and a d+ hydrogen atom of a neighbouring water molecule

Question 22

what do the hydrogen bonds cause the water molecules to do in ice?

Answer 22

they cause the water molecules to be slightly further apart than in liquid water giving ice a lower density than liquid water

Question 23

do compounds that exhibit hydrogen bonding have a higher or lower boiling/melting point?

Why?

Answer 23

higher

Because more energy is required to break the hydrogen bonds compared to permanent dipole-dipole and van der waals forces