Group 15

Question 1

What structure does white phosphorus adopt?

Answer 1

P4 molecules - triangular-based pyramid

Question 2

What structure does black phosphorus adopt?

Answer 2

Sheets of puckered fused P6 rings - fused cyclohexane rings

Most stable conformation

Question 3

Why does nitrogen have such a low MP?

Answer 3

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

Question 4

What’s the structure of elemental arsenic?

Answer 4

Same as phosphorus - layers of puckered As6 rings

Similar structures for Sb and Bi

Question 5

What does N2 react with?

Answer 5

Electropositive elements such as Mg or Li

Both are powerful reducing agents

Form Li3N and Mg3N2 respectively

Question 6

What’s the reactivity of white phosphorus?

Answer 6

White phosphorus P4 is highly reactive - spontaneously flammable in air
- P-P bond enthalpy ≈ 200 KJ/mol

Question 7

What conditions are needed to convert white phosphorus into red phosphorus and P=-P triple bond?

Answer 7

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

Question 8

What is the thermodynamic stability of NX3s?

Answer 8

Apart from NF3, all NX3 are thermodynamically unstable

Question 9

What’s the chemistry of PX3?

Answer 9

P can become hypervalent

Hence PX3 are hydrolysed by H2O
- hydrolysis for X=F is slow
- very violent hydrolysis for X = Cl, Br & I

Question 10

What’re the uses of PX3 compounds?

Answer 10

Used as starting materials in organophosphorus compounds

Question 11

What does hydrolysis of PX3 result in?

Answer 11

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

Question 12

Why are PX5 all Lewis acidic?

Answer 12

Because P can become hypervalent

• form octahedral anion

Question 13

What does hydrolysis of PCl5 result in?

Answer 13

Phosphoric acid + 5HCl

pKa ≈ 2.2

Pauling predicts = 8-5p = 3

Question 14

What’s the chemistry of arsenic and antimony halides?

Answer 14

All halides are known and all are hydrolysed similar to PX3

Question 15

What are the properties of arsenic and antimony halides when X = F, Cl?

Answer 15

As and Sb can become hypervalent - hence all are Lewis acids

Question 16

What does SbCl5 + AlCl3 form?

Answer 16

[AlCl2]+ cation - V.V good Lewis acid

[SbCl6]- octahedral anion - completes salt formation

Question 17

What does 2HF + SbF5 form?

Answer 17

H2F+ - Super Bronsted acid

[SbF6]- - octahedral anion

Question 18

Why do BiX5 not exist?

Answer 18

Because higher oxidation states are hard to achieve

Another example of the inert par effect

Question 19

What structure does SbCl6 form in solid state?

Answer 19

Forms dimers with Sb-Cl-Sb bridges

Question 20

Why is there over 50˚C difference in BP between NH3 and PH3?

Answer 20

Because of intermolecular hydrogen bonding found in NH3

Question 21

What’s the bond angle of NH3 and PH3 respectively? Why?

Answer 21

NH3 = 107˚
- orbitals are sp3 hybridised

PH3 = 93˚
- s & p orbitals not hybridised

Question 22

What’s the difference in the lone pair found in NH3 and PH3?

Answer 22

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

Question 23

What’re the uses of PH3 and where is it found?

Answer 23

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

Question 24

What’re the properties of hydrazine?

Answer 24

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

Question 25

What’re the uses of hydrazine and why?

Answer 25

Used as rocket fuel with N2O4 oxidant due to very exothermic enthalpy of reaction ∆rxnH˚ = -626.8 KJ/mol

Question 26

What are all oxides of N?

Answer 26

All are thermodynamically unstable N2O5 is most stable ∆fH˚ = +11.3 KJ/mol

Question 27

What role does lightning play in the chemistry of atmospheric nitrogen oxides?

Answer 27

Lightning catalyses the reaction to form NOx NOx is then hydrolysed —> HNO3 + HNO2

Question 28

What does NO2 and sunlight form?

Answer 28

NO + O - highly reactive oxygen O + O2 —> O3

Question 29

What do hydrocarbons form in the presence of ozone?

Answer 29

Photochemical smog - compounds irritating to eyes and respiratory system

Question 30

What do catalytic converters in cars do in relation to nitrogen?

Answer 30

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

Question 31

Why is P=O pi bonding much weaker then N=O bonding?

Answer 31

Due to disparity in p-orbital sizes Similar to C vs Si

Question 32

What does P4 from in limited oxygen form?

Answer 32

Forms a P(lll) oxide (P4O6) which can be hydrolysed to form phosphorous acid (H3PO3)

Question 33

What does P4 form in excess oxygen?

Answer 33

Produces P(V) oxide (P4O10) which can be further hydrolysed to form phosphoric acid (H3PO4)

Question 34

What is the ‘phosphate’ ion officially called?

Answer 34

Orthophosphate oxyanion [PO4]3-

Question 35

Where are the main sources of phosphate minerals?

Answer 35

Algeria, China & Morocco - 85% of stocks

Question 36

What’s the main use of oxyanions of phosphorus?

Answer 36

Agricultural fertilisers

Question 37

What does the mineral source apatite (Ca3(PO4)2) form?

Answer 37

Forms Ca5(PO4)(OH) - hydroxyapatite Found in bones and teeth

Question 38

What’re the uses of apatite? (Ca3(PO4)2

Answer 38

Converted into fertiliser Can be used to isolate white phosphorus P4

Question 39

What’re the uses of phosphoric acid?

Answer 39

Food industry: soft drinks & frozen meat Metal cleaning Water softening

Question 40

Describe the chemistry of the condensation of phosphorus oxyanions

Answer 40

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

Question 41

What problems can arise if farmers aren’t careful and if sewage works don’t treat water properly?

Answer 41

Toxic algal blooms formed from phosphate pollution - eutrophication of water Often replaced by zeolites