Group 17 & 18 Flashcards
Why is the ∆H of electron affinity and bond enthalpy of Fluorine less than Chlorine?
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
What atom in group 2 doesn’t form an ionic bond with chlorine?
Beryllium - forms polymeric solid
Why is the BP of HF so much higher than other H-X molecules?
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)
Why is HF a weak Bronsted acid with pKa of 3.45?
Because it has such a high bond enthalpy, not willing to lose H+
Why are HCl, HBr and HI strong Bronsted acids?
Because their bond enthalpies are lower due to larger atomic radius, and are able to hold a -ve charge more effectively than F
What is the hypochlorite oxoanion used in?
Used in alkaline bleach solution
Why is perchloric acid such a strong acid?
Because it has 3 oxo groups with 1 OH - in line with Pauling’s rules
Why is the perchlorate oxoanion such a weak Bronsted base?
Because perchloric acid is such a strong Bronsted acid
This makes it weakly coordinating due to it being a very poor ligand binder
Why is the perchlorate oxoanion often used in coordination chemistry?
Often used as a counterion in metal coordination chemistry where you want to avoid having a competing anionic ligand.
What’s so special about fluorine?
Being very small and very electronegative it is difficult to distort its electron distribution
Fluorine is said to be ‘hard’
Why do fluoride compounds have such low BPs?
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
Why is uranium hexafluoride important?
It is used to separate 235U from 238U
235 is more fissionable
Where is sulfur hexafluoride used?
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
Where is PTFE used?
Polytetrafluoroethylene is used in non-stick pans
Doesn’t interact with fats, oils or proteins
Hey is taking HF out of out of solution require a lot of energy?
Desolvation of HF is very polar and has a large salvation energy - requiring a vast amount of energy
Why is the electron affinity enthalpy greater in F than in Cl?
There’s a greater increase in interatomic repulsion in F than when an e- is added to Cl
Why is solvation of F- more favourable than solvation of Cl- ions?
Because F- is more polarising and will have greater H bonding interactions with the solvent
Why is HF a weak acid?
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
Why are all anions of groups 18 unstable?
Because they have -ve electron affinity energies as extra e- would have to go into a new hell of higher energy, disturbing the octet
How did they establish that Xe could react with F?
Because the 1st IP of O2 is 1177 kJ/mol
This is very similar to the 1st IP of Xenon
What’re the direct reaction conditions of Xe and F?
XeF2(g):
- 400˚C / 1atm
XeF4(g):
- 600˚C / 6atm
XeF6(g):
- 300˚C / 60atm
What’s the structure of XeF4?
XeF4 has 6 regions of electrons density, 2 of which are lone pairs
Square planar molecule - written with 2 ionic F atoms
What’re the properties of XeF4?
Powerful oxidising agent - oxidises solid platinum to PtF4(s)
Lewis acidic - can react with F- ions to form [XeF5]-
What’s the structure of [XeF4]- anion?
Square based pyramid due to 1 lone pair and 6 regions of e- density
What’s the structure of XeF2?
Linear - 5 regions of e- density
Leaves 2 fluorides to be at axial sites of molecule
