Group 15 Flashcards
What structure does white phosphorus adopt?
P4 molecules - triangular-based pyramid
What structure does black phosphorus adopt?
Sheets of puckered fused P6 rings - fused cyclohexane rings
Most stable conformation
Why does nitrogen have such a low MP?
Due to nitrogen’s highly stable triple bond it exists as small molecules which are inert, instead of polymeric solids like rest of group 15
What’s the structure of elemental arsenic?
Same as phosphorus - layers of puckered As6 rings
Similar structures for Sb and Bi
What does N2 react with?
Electropositive elements such as Mg or Li
Both are powerful reducing agents
Form Li3N and Mg3N2 respectively
What’s the reactivity of white phosphorus?
White phosphorus P4 is highly reactive - spontaneously flammable in air
- P-P bond enthalpy ≈ 200 KJ/mol
What conditions are needed to convert white phosphorus into red phosphorus and P=-P triple bond?
Red phosphorus:
300˚C in inert atmosphere
Much less reactive than P4
P=-P triple bond:
>800˚C in vacuum
Much weaker bond enthalpy relatively
What is the thermodynamic stability of NX3s?
Apart from NF3, all NX3 are thermodynamically unstable
What’s the chemistry of PX3?
P can become hypervalent
Hence PX3 are hydrolysed by H2O
- hydrolysis for X=F is slow
- very violent hydrolysis for X = Cl, Br & I
What’re the uses of PX3 compounds?
Used as starting materials in organophosphorus compounds
What does hydrolysis of PX3 result in?
Phosphorous acid which then sets up an equilibrium between itself and phosphoric acid
Has a pKa of 1.1 yet Pauling’s rules predict 8, this is due to the forwards reaction being more favourable, more acidic anion
Why are PX5 all Lewis acidic?
Because P can become hypervalent
- form octahedral anion
What does hydrolysis of PCl5 result in?
Phosphoric acid + 5HCl
pKa ≈ 2.2
Pauling predicts = 8-5p = 3
What’s the chemistry of arsenic and antimony halides?
All halides are known and all are hydrolysed similar to PX3
What are the properties of arsenic and antimony halides when X = F, Cl?
As and Sb can become hypervalent - hence all are Lewis acids
What does SbCl5 + AlCl3 form?
[AlCl2]+ cation - V.V good Lewis acid
[SbCl6]- octahedral anion - completes salt formation
What does 2HF + SbF5 form?
H2F+ - Super Bronsted acid
[SbF6]- - octahedral anion
Why do BiX5 not exist?
Because higher oxidation states are hard to achieve
Another example of the inert par effect
What structure does SbCl6 form in solid state?
Forms dimers with Sb-Cl-Sb bridges
Why is there over 50˚C difference in BP between NH3 and PH3?
Because of intermolecular hydrogen bonding found in NH3
What’s the bond angle of NH3 and PH3 respectively? Why?
NH3 = 107˚
- orbitals are sp3 hybridised
PH3 = 93˚
- s & p orbitals not hybridised
What’s the difference in the lone pair found in NH3 and PH3?
In NH3 lone pair is highly directional in sp3 orbital - available for bonding
In PH3 there is little s-p hybridisation, lone pair in lower energy s orbital close to P atom - hence much less available for bonding
What’re the uses of PH3 and where is it found?
PH3 used as a biomarker detected by rotational spectroscopy
Found in:
Sewage works
Landfill sites
Paddy fields
Penguin guts
- due to anaerobic bacteria breaking organic phosphates down into PH3
What’re the properties of hydrazine?
N in -2 oxidation state
Hence strong reducing agent - thermodynamically unstable as ∆fH˚ = +50.6 KJ/mol
Monomethylhydrazine is highly toxic - produced by fungi on heating
What’re the uses of hydrazine and why?
Used as rocket fuel with N2O4 oxidant due to very exothermic enthalpy of reaction
∆rxnH˚ = -626.8 KJ/mol
What are all oxides of N?
All are thermodynamically unstable
N2O5 is most stable ∆fH˚ = +11.3 KJ/mol
What role does lightning play in the chemistry of atmospheric nitrogen oxides?
Lightning catalyses the reaction to form NOx
NOx is then hydrolysed —> HNO3 + HNO2
What does NO2 and sunlight form?
NO + O - highly reactive oxygen
O + O2 —> O3
What do hydrocarbons form in the presence of ozone?
Photochemical smog - compounds irritating to eyes and respiratory system
What do catalytic converters in cars do in relation to nitrogen?
Converts reactive N oxides into less reactive species
NO + NO2 —> N2 + O2
Reduces effect of photochemical smog in urban areas as all N oxides are thermodynamically unstable
Why is P=O pi bonding much weaker then N=O bonding?
Due to disparity in p-orbital sizes
Similar to C vs Si
What does P4 from in limited oxygen form?
Forms a P(lll) oxide (P4O6) which can be hydrolysed to form phosphorous acid
(H3PO3)
What does P4 form in excess oxygen?
Produces P(V) oxide (P4O10) which can be further hydrolysed to form phosphoric acid
(H3PO4)
What is the ‘phosphate’ ion officially called?
Orthophosphate oxyanion
[PO4]3-
Where are the main sources of phosphate minerals?
Algeria, China & Morocco - 85% of stocks
What’s the main use of oxyanions of phosphorus?
Agricultural fertilisers
What does the mineral source apatite (Ca3(PO4)2) form?
Forms Ca5(PO4)(OH) - hydroxyapatite
Found in bones and teeth
What’re the uses of apatite? (Ca3(PO4)2
Converted into fertiliser
Can be used to isolate white phosphorus P4
What’re the uses of phosphoric acid?
Food industry: soft drinks & frozen meat
Metal cleaning
Water softening
Describe the chemistry of the condensation of phosphorus oxyanions
In acidic conditions it forms pyrophosphate and water - same as pyrosilicate
- lowers oxidation of P atoms per atom
Further acid forms metaphosphate - same as cyclic silicate
Tripolyphosphate [P3O10]3- used as water softener
What problems can arise if farmers aren’t careful and if sewage works don’t treat water properly?
Toxic algal blooms formed from phosphate pollution - eutrophication of water
Often replaced by zeolites
