intro to P block / G13

Question 1

how does metallic character trend through the p block

Answer 1

increases down a group due to decrease in IE

Decreases L to R across period due to increase in IE

Question 2

why doesn’t atomic radii increase uniformly down the group

Answer 2

This is due to the D and F block

Question 3

Define lewis acids and bases

Answer 3

acid: e pair acceptor
Base: e pair donor

Question 4

which elements can expand their octet

Answer 4

elements heavier than second period elements in group 14 and 15

Question 5

which groups of p block can act as lewis bases

Answer 5

15 16 and 17 as they have lone pairs strength of the base depends on the type of orbitals the electron pair is in

Question 6

What is the structure and bonding of boron

Answer 6

Boron is very stable inert macromolecular structure, bonding is between metallic and covalent so acts as semi-conductor.
lack of metallic character of B reflects its reluctance to form positive ions due to its high IE

Question 7

What are allotropes?

How many allotropes of boron?

Answer 7

different forms of an element

5 different allotropes all crystalline allotropes form B12 cluster units

Question 8

What is the structure of the rest of G13 other than B

Answer 8

Rest are metallic crystalline solids
Al - cubic close packing
Ti - Hexagonal close paxking
Ga and In more complex packing

Question 9

How is Al produced

Answer 9

found in aluminosilicates and aluminium oxides (bauxite)
It is extracted from bauxite - AlO(OH) + OH- + H2O -> [Al(OH)4]-
[Al(OH)4]- -> Al(OH)3 + OH-
2Al(OH)3 -> Al2O3 + 3H2O
Hydrolysis of Al2O3 to extract Al

Question 10

what are the properties and uses of Al

Answer 10

Al has high tensile strength, low density, high thermal conductivity, corrosion resistance and easily cast
Used in aircraft fuselage, construction industry, cans and kitchen utensils.

Corrosion resistant because forms adherant and impermeable oxide layer: 4Al + 3O2 -> 2Al2O3

Question 11

what are the uses of boron and gallium/indium

Answer 11

used in glass industry, borosilicate glass formed by fusing B2O3 and SiO2, high refractive index and high chemical/thermal resistance

Ga and In phosphides arsenides and antimonides used in the semiconductor industry, used as transistor materials in LEDs

Question 12

what are the oxidation states trends of g13

Answer 12

all compounds of B and Al contain elements in OS +3, Few Ga/ In and many Ti contain OS +1 - referred as inert pair effect

Question 13

what is an example of inert pair effect

Answer 13

6s2 Electrons in Ti are tightly bound, IE for these is higher than expected, the energy required for promotion greater than the energy that would be released by forming two new bonds, so lower OS preferred down the group as bond energies decrease down as atomic size increases

Question 14

how to determine OS of G13 elements

Answer 14

2 bond energies > promotional energy = 3+

2 bond energies < promotional energy = +1

Question 15

how to form boron trihalides

Answer 15

halogenation of boron oxides
B2O3 + 3C + 3Cl2 -> 3CO + 2BCl3

or by direct reaction with halogens:
LiBH4 + 2I2 -> LiI + BI3 + 2H2

BX3 species are monomeric and have trig planar structure (SP2)

Question 16

why are B-X bonds shorter than expected in BX3

Answer 16

Due to it being SP2 empty p orbital orthogonal to molecule plane and is parallel to corresponding p orbitals on halides so BX bond is between single and double bond. Pi donation from X to B due to good 2p 2p overlap

Question 17

Why is BX3 lewis acid

Answer 17

still electron-deficient and will accept two electrons into vacant p orbital to form an octet.

Question 18

How is boric acid formed from BI3, BCl3, BBr3

Answer 18

BX3 is hydrolysed with water, LP on H2O dative bonds in p orbitals on B, Loss of HX, this happens two more times to give B(OH)3 , Strong B-O bonds due to strong 2p-2p pi overlap

Question 19

why does BF3 not form boric acid

Answer 19

Strong B-F bonds hydrolysis reaction stops at the adduct with H2O dative bonded
BF3 +2H2O -> BF3OH2.H2O when solid, when molten will be [BF3OH]-[H3O]+

Question 20

How does lewis acidity of MX3 species of G13 reflect hardness of the G13 element

Answer 20

Towards hard base, lewis acidity of halides weaken as softness of element increases

but towards soft lewis base lewis acidity increases as softness of element element increases

Question 21

how is BH3 differently structured

Answer 21

it isn’t monomeric and exists as a dimer B2H6, each boron is SP3 hybridised and forms 2e 2 centre bonds with terminal H atoms, B-H-B bridging bonds form 2 electron 3 centre

Question 22

what are the properties of B2H6

Answer 22

highly reactive with O2 and H2O forming boron oxides B2O3, or hydroxides [B(OH)3] highly stable B-O bonds are the thermodynamic driving force behind reactions.

Question 23

What are propertirs of AlCl3

Answer 23

AlCl3 is a volatile solid giving Al2Cl6 Molecules in the gas phase,
dimerisation to relieve the electron deficiency is preferred to formation of pi bonds as seen in BCl3 as 3p-3p pi overlap in AlCl3 is not nearly as good a the 2p-3p overlap in BCl3