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CHEM111 > Groups 13 And 14 > Flashcards

Groups 13 And 14 Flashcards

1
Q

What is Boron considered as and why? What structure does it form?

A

A metalloid
- non-metal with directional bonding

Forms icosahedral network

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2
Q

What are the elements of Group 13 (except Boron) considered as?

A

Metals - non-directional bonding

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3
Q

Why does Gallium have such a low MP?

A

Due to it forming sheets - similar to graphite

These intermolecular bonds can be broken easily

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4
Q

Why is Boron’s 1st IE so high?

A

Due to a lack of shielding and very small atomic radius

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5
Q

Why goes Ga have a higher IE than Al?

A

Because the 3D orbital provides poor shielding due to its shape

Greater effective nuclear charge

Hence requires more energy to remove an electron

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6
Q

Why does Tl have a higher IE than In?

A

4f orbital is even worse at shielding and so Zeff is larger than that seen in In

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7
Q

What’re the trends in electronegativity down Group 13?

A

Very similar to 1st ionisation energies

This is due to poor and even poorer shielding in Ga and Tl respectively

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8
Q

What’re the application of boron oxide?

A

Borosilicate glass - Pyrex

Borax - ‘noisy’ putty

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9
Q

What’re the application of aluminium?

A

Packaging - cans

Alloys used in aircraft and bikes

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10
Q

What’re the uses of gallium?

A

High temperature thermometers

Semiconductors (GaAs, GaP)

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11
Q

What’re the uses of indium?

A

Transparent conducting oxides

Semiconductors - glass coating in laptops

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12
Q

Why does lewis acidity increase when the halides of boron increase in size?

A

Due to disparity in size between full Cl/Br orbitals and empty 2p orbital on B

Leads to very poor overlap - B becomes more electron deficient

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13
Q

Why is BF3 the least Lewis acidic BX3?

A

Because e-density is donated from full 2p orbitals on F to empty 2p orbital on B

B becomes less e- deficient and so is less Lewis acidic

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14
Q

What happens when BX3 forms an adduct with a Lewis base?

A

An adduct is formed

B has 8 valence electrons due to dative bond

B-X pi bonding is lost

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15
Q

What happens when BX3 is hydrolysed?

A

Lewis acidity of BX3 allows H2O to coordinate to B

B-X bond polarity allows hydrolysis to form HX and boric acid B(OH)3

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16
Q

What is the trend of the rate of hydrolysis of BX3?

A

BBr3 > BCl3&raquo_space; BF3

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17
Q

What’re the uses of BX3? What considerations must be met?

A

Widely used as Lewis acid catalysts in organic synthesis

Must be handled anhydrously due to reactivity with water

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18
Q

What structure does AlCl3 form?

A

Forms dimer in gas phase or when in non-polar solvent

Cl forms bridges by donation of linen pair to Lewis acidic AlCl3
- Al no has 8 valence electrons and is sp3 hybridised

Solid state structure is polymeric with 6 coordinate Al

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19
Q

How does GaCl3 and InCl3 differ to AlCl3?

A

They form a salt with mixed valency due to inert pair effect

Similar reaction for InCl3

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20
Q

What structure doe TlI3 posses?

A

Linear
- tri-iodide anion of iodine due to inert pair effect

NOT Tl(lll)

2s electrons are inert

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21
Q

What IS the inert pair effect?

A

The stability of the n-2 oxidation state increase for heavier elements of p-block elements
(n = group number)

E.g. Tl(l) is more stable than Tl(lll)

S-orbital stability increases for heavier elements - due to relativistic effects

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22
Q

Why do the bond enthalpies of M-X decrease down the group?

A

Increased promotion energy not compensated by M-X bond enthalpy

Combination of both case inert pair effect

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23
Q

Why does TlBr3 spontaneously lose bromine?

A

Because of the increased stability of the Tl(l) oxidation state - inert pair effect

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24
Q

Why is diborane spontaneously flammable in air?

A

Due to the positive enthalpy change of formation it is thermodynamically unstable

Hence spontaneously flammable and rapid hydrolysis by water

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25
Q

Why are LiH or NaH used to form M-X anions?

A

Due to positive heats of formation and Lewis acidity they decompose to form tetrahyrdometallates - anions

The stability of LiCl or NaCl is the driving force of these reactions

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26
Q

What are the uses or tetrahydrometallates?

A

They can be used as reducing agents (a source of hydride ions) which react with aldehydes/ketones by attacking carbonyl group

Hence central metal forms MH3 as H- is lost to form a C-H bond, carbonyl becomes an O- ion, then forms an alcohol

27
Q

What are the uses or borohydride?

A

Used as reducing agents in organic synthesis

Can be used as a store for hydrogen
- used in hydrogen powered cars

28
Q

What are the dangers of tetrahydrometallates like tetraborohydride?

A

They are:

Toxic
Corrosive
Highly flammable

29
Q

Why are aluminium oxides so stable?

A

Because of their very negative energy of formation

30
Q

Why structure does a-alumina (corundum) form?

A

Hexagonal close packing

31
Q

What structure does y-alumina form?

A

Cubic close packing

32
Q

Why can aluminium oxides be described as amphoteric?

A

Because of their have acid-base behaviour

33
Q

What does Al2O3 form when reacted with an acid/base respectively?

A

Acid - forms a hexagonal-aqua Al(lll) ion

Base - forms a tetra-hydroxyaluminate ion

34
Q

What is ruby and how is it formed?

A

Ruby is an impure form of corundum (a-alumina)

Some Al(lll) is replaced with Cr(lll)

35
Q

What is ruby used for?

A

Synthetic ruby used in lasers

36
Q

Wheat is sapphire and how is it formed?

A

An impure form of corundum (a-alumina)

Some Al(lll) is replaced with Fe(ll) and Ti(lV)
- Fe loses e- to Ti due to photons

37
Q

What’re the benefits of y-alumina?

A

Can be made with very small particle size - massive SA : V ratio

Can be used as a catalyst support (transition metals) or catalyst alone

Has both acidic and basic sites when heated

38
Q

How does y-alumina have both Lewis acidic and basic sites?

A

Surface OH groups from atmosphere are heated, forming an oxygen bound to 2 Al and a terminal OH group

Terminal OH is bound to Al3+ which strongly polarises the bond - Bronsted acid

39
Q

What does y-alumina catalyse?

A

Catalyses the dehydration of alcohols and isomerization of alkenes

40
Q

Why does the Pauling electronegativity of Group 14 not just decrease steadily down the group?

A

Ge has a greater electronegativity because of the filling of 3d orbitals between Ca and Ga

Pb has a much greater affinity for electrons because of the filling of 5d and 4f orbitals between Ba and Tl

41
Q

What as the structure of Sn?

A

The diamond alpha structure is unstable above 13˚C

It then adopts its ß conformation below 13˚C - forming a distorted bcc structure

42
Q

Describe the bonding structure in diamond

A

Sp3 hybridisation forming 4 ∂ bonds

C-C = 154pm - Very short bond length

Strong directional nature of bonds

43
Q

Describe the bonding structure in graphite

A

Sheets of benzene rings - bonds shorter than that found in diamond, typical of aromatics

Sp2 hybridisation
∂ and pi bonding in plane o rings

Weak interplanar attractions - no effective bonding between sheets at 335pm

Layers are offset - ABAB arrangement

44
Q

Why does Si not have a graphite analogue?

A

Because pi bonding is too weak due to poorer orbital overlap because of increased atomic radii

45
Q

What is the length of the average C-C bond in buckminsterfullerene?

A

Avg - C-C = 140pm

46
Q

How are fullerenes made?

A

2 graphite electrodes in a partial vacuum and striking an electric discharge between them

Forms soot and fullerene mixture which must be separated

47
Q

What is the bond length in graphene?

A

C-C = 142pm

48
Q

What are the uses of graphene

A

Carbon nanotubes - cylindrical graphene

49
Q

What are the uses of carbon?

A

Graphite - lubricant

Coal - reductant for metal extraction in blast furnace

Diamond - industrial cutting tool

50
Q

What ar the uses of silicone and germanium?

A

Important semiconductors in electronics

51
Q

What are the uses of tin?

A

‘Float glass’ process

Alloyed with Cu - forms bronze

52
Q

What is lead used for?

A

Radiation shields - very dense metal

53
Q

What are the bond enthalpy trends down group 14?

A

Homonuclear bonds decrease down the group

E-H decreases down the group

E-F > E-Cl

E-halogen maximum for Si

54
Q

What are the general properties of group 14 halides?

A

They are electron precise compounds

All EX4 molecules are either liquids or low-melting solids

55
Q

What are the properties of CX4

A

Non-polar

Not Lewis acidic

Not susceptible to hydrolysis

C-X bond strength C-F > C-Cl > C-Br > C-I

56
Q

What are the properties of SiX4?

A

Si is a large atom and become hypervalent - SiX4 is Lewis acidic

Polarity of Si-X bond leads to ready hydrolysis - elimination of E-H
- hence must be handled in anhydrous conditions / inert atmosphere

57
Q

What are the trends of Ge,Sn and Pb halides?

A

All are Lewis acidic -just like SiX4

+2 oxidation state becomes more stable from Ge to Pb

Pb(IV) halides are very unstable - much prefer EX2 state

58
Q

What can SnCl2 be used as?

A

A mild reducing agent - Lewis acidic as Sn is electron deficient

59
Q

What are the trends of the sum of the 3rd/4th ionisation potentials?

A

There’s an increase in 3rd/4th IPs at Ge due to filling of 3d orbital

Also an increase in 3rd/4th IPs at Pb due to the extra stability of the s orbital

S-orbitals stabilised for heavy elements due to effects of special relativity

60
Q

Why are the E-E and E-H bonds of silicon weaker than that of carbon?

A

Si-Si and Si-H bonds significantly weaker due to poorer orbital overlap for the larger 2nd row Si atoms

Larger atomic radii

61
Q

What are silanes and their properties?

A

They are Si analogues of alkanes, no simple alkene analogues- very poor pi overlap

Lewis acidity of Si makes silanes highly reactive - inert atmosphere NEEDED

62
Q

What’s the importance of SiH4?

A

Very important in the synthesis of highly pure Si for electronics industry

63
Q

How can Si be formed from SiH4?

A

Either by electric discharge using photovoltaic devices

Or through thermal decomposition - 500˚C
Forming crystalline Si

CHEM111 (12 decks)

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