3.5 Forces acting between molecules

Question 1

Types of intermolecular forces

Answer 1

Van der waals
Dipole-dipole
Hydrogen bonding

Question 2

What are dipole-dipole forces?

Answer 2

intermolecular forces between polar molecules
- the +ve end of one molecule is attracted to the -ve end of another

Question 3

What are dipole dipole moments?

Answer 3

when two polar molecules interact
- +ve end of one molecule attracts the -ve end of the other

Question 4

Does every molecule with polar bonds have permanent dipoles? Explain

Answer 4

No
- when polar bonds cancel out (symmetrical)

Question 5

What are van der Waals forces?

Answer 5

Forces between all molecules
- caused by shifting of electron density
- creates temporary dipoles that attract other molecules

Question 6

How do van der Waals forces work?

Answer 6

• electron density might shift to one side of the molecule
• causing a temporary dipole moment
• causes shifts in other molecules
• causing induced dipole forces

Question 7

What atoms/ molecules do v.d.W act and when?

Answer 7

Act between all atoms/ molecules at all times
- in addition to any other intermolecular forces

Question 8

When is there a larger instantaneous dipole(

Answer 8

When there are more electrons

Question 9

What concepts does v.d.W explain?

Answer 9

increasing boiling point of noble gases
- atomic number increases

increasing boiling point of hydrocarbons with the increase of their chain length

Question 10

What are hydrogen bonds?

Answer 10

dipole-dipole between an H atom bonded to a highly electronegative atom and a lone pair of e- on another

Question 11

Give an example of a molecule with hydrogen bonding

Answer 11

H2O

Question 12

When do H bonds form? (What is needed)

Answer 12

• H bond bonded to a very electronegative atom
  = strong partial +ve charge on H atom
• very electronegative atom with a lone pair of e-
  = attracted to the H atom in another

= forms H bond

Question 13

What is the importance of H bonds

Answer 13

• only about 10% of the strength of covalent bonds
• can make/break under conditions where covalent bonds are unaffected

Question 14

Explain the structure of ice

Answer 14

In ice
- water molecules fixed in position by H bonds
- resembles structure of diamond
- less packed than in liquid water

Question 15

What does the structure of ice do to its density

Answer 15

Less dense than water
Forms on top of bodies of water rather than at the bottom

Question 16

How does the density of ice help aquatic environments

Answer 16

Forms on top of bodies of water
- insulates ponds
- allows fish to survive the winter