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chem111 > Group 15 > Flashcards

Group 15 Flashcards

1
Q

What structure does white phosphorus adopt?

A

P4 molecules - triangular-based pyramid

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2
Q

What structure does black phosphorus adopt?

A

Sheets of puckered fused P6 rings - fused cyclohexane rings

Most stable conformation

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3
Q

Why does nitrogen have such a low MP?

A

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

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4
Q

What’s the structure of elemental arsenic?

A

Same as phosphorus - layers of puckered As6 rings

Similar structures for Sb and Bi

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5
Q

What does N2 react with?

A

Electropositive elements such as Mg or Li

Both are powerful reducing agents

Form Li3N and Mg3N2 respectively

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6
Q

What’s the reactivity of white phosphorus?

A

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

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7
Q

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

A

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

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8
Q

What is the thermodynamic stability of NX3s?

A

Apart from NF3, all NX3 are thermodynamically unstable

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9
Q

What’s the chemistry of PX3?

A

P can become hypervalent

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

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10
Q

What’re the uses of PX3 compounds?

A

Used as starting materials in organophosphorus compounds

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11
Q

What does hydrolysis of PX3 result in?

A

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

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12
Q

Why are PX5 all Lewis acidic?

A

Because P can become hypervalent

  • form octahedral anion
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13
Q

What does hydrolysis of PCl5 result in?

A

Phosphoric acid + 5HCl

pKa ≈ 2.2

Pauling predicts = 8-5p = 3

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14
Q

What’s the chemistry of arsenic and antimony halides?

A

All halides are known and all are hydrolysed similar to PX3

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15
Q

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

A

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

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16
Q

What does SbCl5 + AlCl3 form?

A

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

[SbCl6]- octahedral anion - completes salt formation

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17
Q

What does 2HF + SbF5 form?

A

H2F+ - Super Bronsted acid

[SbF6]- - octahedral anion

18
Q

Why do BiX5 not exist?

A

Because higher oxidation states are hard to achieve

Another example of the inert par effect

19
Q

What structure does SbCl6 form in solid state?

A

Forms dimers with Sb-Cl-Sb bridges

20
Q

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

A

Because of intermolecular hydrogen bonding found in NH3

21
Q

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

A

NH3 = 107˚
- orbitals are sp3 hybridised

PH3 = 93˚
- s & p orbitals not hybridised

22
Q

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

A

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

23
Q

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

A

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
24
Q

What’re the properties of hydrazine?

A

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

25
Q

What’re the uses of hydrazine and why?

A

Used as rocket fuel with N2O4 oxidant due to very exothermic enthalpy of reaction

∆rxnH˚ = -626.8 KJ/mol

26
Q

What are all oxides of N?

A

All are thermodynamically unstable

N2O5 is most stable ∆fH˚ = +11.3 KJ/mol

27
Q

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

A

Lightning catalyses the reaction to form NOx

NOx is then hydrolysed —> HNO3 + HNO2

28
Q

What does NO2 and sunlight form?

A

NO + O - highly reactive oxygen

O + O2 —> O3

29
Q

What do hydrocarbons form in the presence of ozone?

A

Photochemical smog - compounds irritating to eyes and respiratory system

30
Q

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

A

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

31
Q

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

A

Due to disparity in p-orbital sizes

Similar to C vs Si

32
Q

What does P4 from in limited oxygen form?

A

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

(H3PO3)

33
Q

What does P4 form in excess oxygen?

A

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

(H3PO4)

34
Q

What is the ‘phosphate’ ion officially called?

A

Orthophosphate oxyanion

[PO4]3-

35
Q

Where are the main sources of phosphate minerals?

A

Algeria, China & Morocco - 85% of stocks

36
Q

What’s the main use of oxyanions of phosphorus?

A

Agricultural fertilisers

37
Q

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

A

Forms Ca5(PO4)(OH) - hydroxyapatite

Found in bones and teeth

38
Q

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

A

Converted into fertiliser

Can be used to isolate white phosphorus P4

39
Q

What’re the uses of phosphoric acid?

A

Food industry: soft drinks & frozen meat

Metal cleaning

Water softening

40
Q

Describe the chemistry of the condensation of phosphorus oxyanions

A

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

41
Q

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

A

Toxic algal blooms formed from phosphate pollution - eutrophication of water

Often replaced by zeolites

chem111 (12 decks)

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