Groups 13 And 14 Flashcards
What is Boron considered as and why? What structure does it form?
A metalloid
- non-metal with directional bonding
Forms icosahedral network
What are the elements of Group 13 (except Boron) considered as?
Metals - non-directional bonding
Why does Gallium have such a low MP?
Due to it forming sheets - similar to graphite
These intermolecular bonds can be broken easily
Why is Boron’s 1st IE so high?
Due to a lack of shielding and very small atomic radius
Why goes Ga have a higher IE than Al?
Because the 3D orbital provides poor shielding due to its shape
Greater effective nuclear charge
Hence requires more energy to remove an electron
Why does Tl have a higher IE than In?
4f orbital is even worse at shielding and so Zeff is larger than that seen in In
What’re the trends in electronegativity down Group 13?
Very similar to 1st ionisation energies
This is due to poor and even poorer shielding in Ga and Tl respectively
What’re the application of boron oxide?
Borosilicate glass - Pyrex
Borax - ‘noisy’ putty
What’re the application of aluminium?
Packaging - cans
Alloys used in aircraft and bikes
What’re the uses of gallium?
High temperature thermometers
Semiconductors (GaAs, GaP)
What’re the uses of indium?
Transparent conducting oxides
Semiconductors - glass coating in laptops
Why does lewis acidity increase when the halides of boron increase in size?
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
Why is BF3 the least Lewis acidic BX3?
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
What happens when BX3 forms an adduct with a Lewis base?
An adduct is formed
B has 8 valence electrons due to dative bond
B-X pi bonding is lost
What happens when BX3 is hydrolysed?
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
What is the trend of the rate of hydrolysis of BX3?
BBr3 > BCl3»_space; BF3
What’re the uses of BX3? What considerations must be met?
Widely used as Lewis acid catalysts in organic synthesis
Must be handled anhydrously due to reactivity with water
What structure does AlCl3 form?
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
How does GaCl3 and InCl3 differ to AlCl3?
They form a salt with mixed valency due to inert pair effect
Similar reaction for InCl3
What structure doe TlI3 posses?
Linear
- tri-iodide anion of iodine due to inert pair effect
NOT Tl(lll)
2s electrons are inert
What IS the inert pair effect?
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
Why do the bond enthalpies of M-X decrease down the group?
Increased promotion energy not compensated by M-X bond enthalpy
Combination of both case inert pair effect
Why does TlBr3 spontaneously lose bromine?
Because of the increased stability of the Tl(l) oxidation state - inert pair effect
Why is diborane spontaneously flammable in air?
Due to the positive enthalpy change of formation it is thermodynamically unstable
Hence spontaneously flammable and rapid hydrolysis by water
Why are LiH or NaH used to form M-X anions?
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
What are the uses or tetrahydrometallates?
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
What are the uses or borohydride?
Used as reducing agents in organic synthesis
Can be used as a store for hydrogen
- used in hydrogen powered cars
What are the dangers of tetrahydrometallates like tetraborohydride?
They are:
Toxic
Corrosive
Highly flammable
Why are aluminium oxides so stable?
Because of their very negative energy of formation
Why structure does a-alumina (corundum) form?
Hexagonal close packing
What structure does y-alumina form?
Cubic close packing
Why can aluminium oxides be described as amphoteric?
Because of their have acid-base behaviour
What does Al2O3 form when reacted with an acid/base respectively?
Acid - forms a hexagonal-aqua Al(lll) ion
Base - forms a tetra-hydroxyaluminate ion
What is ruby and how is it formed?
Ruby is an impure form of corundum (a-alumina)
Some Al(lll) is replaced with Cr(lll)
What is ruby used for?
Synthetic ruby used in lasers
Wheat is sapphire and how is it formed?
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
What’re the benefits of y-alumina?
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
How does y-alumina have both Lewis acidic and basic sites?
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
What does y-alumina catalyse?
Catalyses the dehydration of alcohols and isomerization of alkenes
Why does the Pauling electronegativity of Group 14 not just decrease steadily down the group?
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
What as the structure of Sn?
The diamond alpha structure is unstable above 13˚C
It then adopts its ß conformation below 13˚C - forming a distorted bcc structure
Describe the bonding structure in diamond
Sp3 hybridisation forming 4 ∂ bonds
C-C = 154pm - Very short bond length
Strong directional nature of bonds
Describe the bonding structure in graphite
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
Why does Si not have a graphite analogue?
Because pi bonding is too weak due to poorer orbital overlap because of increased atomic radii
What is the length of the average C-C bond in buckminsterfullerene?
Avg - C-C = 140pm
How are fullerenes made?
2 graphite electrodes in a partial vacuum and striking an electric discharge between them
Forms soot and fullerene mixture which must be separated
What is the bond length in graphene?
C-C = 142pm
What are the uses of graphene
Carbon nanotubes - cylindrical graphene
What are the uses of carbon?
Graphite - lubricant
Coal - reductant for metal extraction in blast furnace
Diamond - industrial cutting tool
What ar the uses of silicone and germanium?
Important semiconductors in electronics
What are the uses of tin?
‘Float glass’ process
Alloyed with Cu - forms bronze
What is lead used for?
Radiation shields - very dense metal
What are the bond enthalpy trends down group 14?
Homonuclear bonds decrease down the group
E-H decreases down the group
E-F > E-Cl
E-halogen maximum for Si
What are the general properties of group 14 halides?
They are electron precise compounds
All EX4 molecules are either liquids or low-melting solids
What are the properties of CX4
Non-polar
Not Lewis acidic
Not susceptible to hydrolysis
C-X bond strength C-F > C-Cl > C-Br > C-I
What are the properties of SiX4?
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
What are the trends of Ge,Sn and Pb halides?
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
What can SnCl2 be used as?
A mild reducing agent - Lewis acidic as Sn is electron deficient
What are the trends of the sum of the 3rd/4th ionisation potentials?
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
Why are the E-E and E-H bonds of silicon weaker than that of carbon?
Si-Si and Si-H bonds significantly weaker due to poorer orbital overlap for the larger 2nd row Si atoms
Larger atomic radii
What are silanes and their properties?
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
What’s the importance of SiH4?
Very important in the synthesis of highly pure Si for electronics industry
How can Si be formed from SiH4?
Either by electric discharge using photovoltaic devices
Or through thermal decomposition - 500˚C
Forming crystalline Si
