Group 17 & 18

Question 1

Why is the ∆H of electron affinity and bond enthalpy of Fluorine less than Chlorine?

Answer 1

EA(F) < EA(Cl) due to the large e- repulsion’s in compact F atom

Same is true for the bond enthalpy of F2 and Cl2

Question 2

What atom in group 2 doesn’t form an ionic bond with chlorine?

Answer 2

Beryllium - forms polymeric solid

Question 3

Why is the BP of HF so much higher than other H-X molecules?

Answer 3

It is much higher than its H-X counterparts because it forms strong hydrogen bonding to neighbouring molecules

(Only has 1 hydrogen atom bound - so can’t form an extensive network like water)

Question 4

Why is HF a weak Bronsted acid with pKa of 3.45?

Answer 4

Because it has such a high bond enthalpy, not willing to lose H+

Question 5

Why are HCl, HBr and HI strong Bronsted acids?

Answer 5

Because their bond enthalpies are lower due to larger atomic radius, and are able to hold a -ve charge more effectively than F

Question 6

What is the hypochlorite oxoanion used in?

Answer 6

Used in alkaline bleach solution

Question 7

Why is perchloric acid such a strong acid?

Answer 7

Because it has 3 oxo groups with 1 OH - in line with Pauling’s rules

Question 8

Why is the perchlorate oxoanion such a weak Bronsted base?

Answer 8

Because perchloric acid is such a strong Bronsted acid

This makes it weakly coordinating due to it being a very poor ligand binder

Question 9

Why is the perchlorate oxoanion often used in coordination chemistry?

Answer 9

Often used as a counterion in metal coordination chemistry where you want to avoid having a competing anionic ligand.

Question 10

What’s so special about fluorine?

Answer 10

Being very small and very electronegative it is difficult to distort its electron distribution

Fluorine is said to be ‘hard’

Question 11

Why do fluoride compounds have such low BPs?

Answer 11

Because they have reduced VDW interactions

Very small and hard to polarise - VDW require transient polarisation of electron distribution, which is very difficult to achieve with F

Question 12

Why is uranium hexafluoride important?

Answer 12

It is used to separate 235U from 238U

235 is more fissionable

Question 13

Where is sulfur hexafluoride used?

Answer 13

Used as gaseous dielectric medium in electrical applications - well insulating

Also used in Nike Air running shoes - stopped use as it is VV bad greenhouse gas

Question 14

Where is PTFE used?

Answer 14

Polytetrafluoroethylene is used in non-stick pans

Doesn’t interact with fats, oils or proteins

Question 15

Hey is taking HF out of out of solution require a lot of energy?

Answer 15

Desolvation of HF is very polar and has a large salvation energy - requiring a vast amount of energy

Question 16

Why is the electron affinity enthalpy greater in F than in Cl?

Answer 16

There’s a greater increase in interatomic repulsion in F than when an e- is added to Cl

Question 17

Why is solvation of F- more favourable than solvation of Cl- ions?

Answer 17

Because F- is more polarising and will have greater H bonding interactions with the solvent

Question 18

Why is HF a weak acid?

Answer 18

Because the ∆G is thermodynamically unfavourable (+ve) overall

This is because of the unfavourable change of entropy

F- organises the H2O molecules round it - making it unfavourable overall

Question 19

Why are all anions of groups 18 unstable?

Answer 19

Because they have -ve electron affinity energies as extra e- would have to go into a new hell of higher energy, disturbing the octet

Question 20

How did they establish that Xe could react with F?

Answer 20

Because the 1st IP of O2 is 1177 kJ/mol

This is very similar to the 1st IP of Xenon

Question 21

What’re the direct reaction conditions of Xe and F?

Answer 21

XeF2(g):
- 400˚C / 1atm

XeF4(g):
- 600˚C / 6atm

XeF6(g):
- 300˚C / 60atm

Question 22

What’s the structure of XeF4?

Answer 22

XeF4 has 6 regions of electrons density, 2 of which are lone pairs

Square planar molecule - written with 2 ionic F atoms

Question 23

What’re the properties of XeF4?

Answer 23

Powerful oxidising agent - oxidises solid platinum to PtF4(s)

Lewis acidic - can react with F- ions to form [XeF5]-

Question 24

What’s the structure of [XeF4]- anion?

Answer 24

Square based pyramid due to 1 lone pair and 6 regions of e- density

Question 25

What’s the structure of XeF2?

Answer 25

Linear - 5 regions of e- density

Leaves 2 fluorides to be at axial sites of molecule