Main Group 15 - Pnictogens

Question 1

Compare electron-electron repulsions

Answer 1

2 lone pairs > lone pair, bonding pair > 2 bonding pairs

(in terms of repulsion)

Question 2

Compare electron repulsions between multi-bonds

Answer 2

Larger the bond number, larger repulsion

X(trip.bond)Y X-Y > X=Y X-Y > X-Y X-Y

Question 3

Summarise trend in sum of first 3 ionisation energies down group 15

Answer 3

• decreases down group
• P > As > Sb > Bi
• due to increasingly stable E+3 ( far from nucleus, decreased electronegativity)

Question 4

Who discovered phosphorous and how?

Answer 4

• Hennig Brandt 1669 alchemist

- reductive distillation of putrefied horse urine

Question 5

Write the equation for the commercial production of phosphorous

Answer 5

2 Ca3(PO4)2 + 6 SiO2 + 10 C -> P4 + 6 CaSiO3 + 10 CO

Question 6

Define pyrophoric

Answer 6

Spontaneously combusts

Question 7

Summarise P4

Answer 7

• waxy yellow solid
• white/yellow phosphorous
• tetrahedral molecular so soluble
• pyrophoric (spontaneously combusts)

Question 8

Summarise red phosphorous

Answer 8

• Polymer so kinetically inert

- not soluble bc not volatile

Question 9

How to get from white phosphorous to red phosphorous

Answer 9

• Heat in a vacuum

Question 10

How to get black phosphorous and what is it?

Answer 10

• heat to even higher temps with pressure
• cross linked polymer, even more stable
• discovered 1916

Question 11

Can we ever get P=_P (trip bond)

Answer 11

• 900ºC
  P4(g) 2 P2(g)
• very unstable and cannot be isolated