CL 6- shapes of molecules and IMF

Question 1

electron pair repulsion theory

Answer 1

• electron pairs are negatively charged
• to minimise repulsion, the electron pairs move as far away from each other as possible
• lone pairs repel more than bonding pairs

Question 2

2 bonding electron pairs

shape, angle

Answer 2

linear, 180⁰

Question 3

3 bonding electron pairs

shape, angle

Answer 3

trigonal planar, 120⁰

Question 4

4 bonding electron pairs

shape, angle

Answer 4

tetrahedral, 109.5⁰

Question 5

6 bonding electron pairs

shape, angle

Answer 5

octahedral, 90⁰

Question 6

3 bonding electron pairs, 1 lone pair

shape, angle, reasoning

Answer 6

pyramidal, 107⁰, lone pairs repel more than bonding pairs

Question 7

4 bonding electron pairs, 2 lone pairs

shape, angle

Answer 7

square planar, still 90⁰

Question 8

electronegativity

definition

Answer 8

the ability of an atom to attract the bonding electrons in a covalent bond

Question 9

density of ice, melting/boiling points

anomalous properties of water

H bonding

Answer 9

• ice is less dense than water, as the H₂O molecules in ice are further apart/more spread
• water has a relatively high melting and boiling point, as it has london forces but also H bonds, which require a lot of energy to overcome

Question 10

permanent dipole

definition

Answer 10

a small charge difference between atoms at either end of a covalent bond

Question 11

how do you know a molecule is non-polar?

Answer 11

• dipoles cancel,
• molecule symmetrical (all BPs and no LPs

Question 12

how do you know a molecule is a polar?

Answer 12

• dipole doesn’t cancel
• molecule not symmetrical
• has LPs

Question 13

permanent dipole-dipole interactions

occurence

Answer 13

only occurs between 2 polar molecules

Question 14

NF₃ or BF₃ higher boiling point

Answer 14

NF₃- fluorine is more electronegative, stronger permanent dipole-dipole forces, more energy required to weaken the imfs

Question 15

hydrogen bonds

occurence

Answer 15

• H-F
• H-N
• H-O

Question 16

london forces

occurence

Answer 16

non-polar molecules/atoms

Question 17

formation of london forces

Answer 17

• electrons are constantly moving forming an instantaneous dipole
• induces dipole on neighbouring molecules causing repulsion

Question 18

boiling points of noble gases

trend

Answer 18

increases as you go down the group because
* no. of electrons increases
* larger induced dipole (london force)
* increased attraction between molecules
* more energy required to overcome the london forces

Question 19

straight chain/branched molecules

comment on attraction

Answer 19

straight chain molecules attract each other more than branched chains, because straight chains have more points of contact, so the london forces are stronger, more energy required to overcome