4.4 Intermolecular forces

Question 1

what is another name for intermolecular forces?

Answer 1

van der waals forces

Question 2

what are intermolecular forces?

Answer 2

the attractive forces present between covalent molecules

Question 3

what are the general properties of intermolecular forces?

Answer 3

• weak
• broken when melting or boiling a substance

Question 4

what are the 3 main types of intermolecular forces?

Answer 4

1. london dispersion force
2. dipole dipole forces
3. hydrogen bonds

Question 5

describe the strength of london dispersion forces

Answer 5

• weakest type of intermolecular forces
• they form between all covalent molecules

Question 6

describe the movement of electrons when there is the presence of london dispersion forces

Answer 6

• electrons are constantly moving
• so at any point in time the electrons within an atom will be present in a higher concentration on one side than the other
• creates a dipole

Question 7

define instantaneous dipole

Answer 7

a dipole created by having a high concentration of electrons in a certain area at any point in time, found in london dispersion forces

Question 8

what is an induced dipole-dipole attraction and how does it link to london dispersion forces?

Answer 8

a temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles

Question 9

why does the dipole constantly appear and disappear in london dispersion forces?

Answer 9

• this is due to electron movement
• but maintains a continual attraction between atoms of different molecules

Question 10

how does the increase in molecular mass affect london dispersion forces?

Answer 10

• the molecule has more electrons
• increase the chance of having an uneven distribution
• this distribution will be of greater strength
• london force strength increases as molecular mass increases

Question 11

how does the shape/ structure of a molecule affect the strength of the london dispersion force?

Answer 11

• as chain is longer, ldf increases
• longer chain has more contact points so more ldf can develop
• making the overall effect of the force stronger

Question 12

what is the difference between an induced dipole and a dipole-dipole force?

Answer 12

induced dipoles are temporary
dipole dipole forces are permanent

Question 13

define dipole-dipole force

Answer 13

the attraction between oppositely charged dipoles of 2 polar molecules

Question 14

describe the strength of dipole-dipole forces

Answer 14

• the second weakest form of intermolecular forces
• forms purely between polar molecules

Question 15

how are molecules attracted to each other in dipole-dipole forces?

Answer 15

• polar molecules have a dipole
• the positive end of a molecule is attracted to the negative end of another molecule

Question 16

describe the strength of hydrogen bonds

Answer 16

• the strongest type of intermolecular forces
• form when a negative lone pair of a nitrogen, oxygen or fluorine atom is attracted towards a positive hydrogen atom

Question 17

explain the existence of a hydrogen bond

Answer 17

• h bonds have a directional component towards the hydrogen atom
• the electronegative atom withdraws electron density, polarizing the bond

Question 18

rank the 3 intermolecular forces from weakest to strongest

Answer 18

ldf < d-d force < h bonds

Question 19

how does the strngth of the intermolecular forces have an affect on the substances’ melting and boiling point?

Answer 19

substances containing only ldf have the lowest mp and bp, whereas substances containing h bonds have the highest mp and bp
* note that this only applies when molecular mass or area if roughly similar

Question 20

what can change the default ranking of the 3 intermolecular forces?

Answer 20

if a non-polar substance has a large molecular mass or area, the subsequent increases in ldf can change this ranking

Question 21

what is the affect on solubility as alcohol chain length increases?

Answer 21

• as alcohol chain increases, solubility decreases
• number of contact point increases, increasing ldf
• the energy input to break the ldf and dissolve the substance thus increases whilst the energy output remains the same
• energy output is less than the energy inout, the substance will not dissolve in a solvent