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(2) Main Group Chemistry > Group 13 > Flashcards

Group 13 Flashcards

(40 cards)

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1
Q

What is the outer orbital configuration?

A

ns2np1

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2
Q

What is the inert pair effect?

A

The increasing stability of oxidation states that are 2 less than the maximum oxidation state of the group

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3
Q

What is passivated?

A

This is making a metal unreactive by altering the surface layer and coating the surface with a thin inert layer

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4
Q

What is the max oxidation state?

A

+3

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5
Q

Which oxidation state becomes more important down the group?

A

+1 oxidation state becomes more important down the group
The max oxidation state is +3
This shows the inert pair effect

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6
Q

Why are there no ionic compounds containing B3+?

A

The 1st 3 ionisation energies of B are high so there are no ionic compounds containing it
Instead it will form covalent compounds

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7
Q

Why are group 13 elements Lewis acids?

A

They have a low energy empty p orbital that can accept a pair of electrons

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8
Q

How do they overcome their lack of valence electrons?

A

Many compounds form 3 centre, 2 electron bonds

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9
Q

What is boron best described as?

A

Non metal whereas other G13 are metal

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10
Q

What is boron extracted from?

A

Deposits of borax

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11
Q

What is Al extracted from?

A

Extracted from the ore bauxite

1) Bauer haber process
2) hall herout process

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12
Q

Why can Al be used in power lines?

A

It is passivated so it is resistance to corrosion

- oxide acts as a physical barrier

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13
Q

Describe the trend in metallic character down the group

A

Metallic character increases going down the group- reflected by Li and Ti dissolving in acid

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14
Q

What causes anomalies to I.E. Trend?

A

The d and f block contraction which is characterised by the poor shielding effects of the 3d and 4f orbitals

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15
Q

What do G13 form when they burn in air?

A

Oxides

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16
Q

What is the general formula of a compound ?

A

MX3

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17
Q

Why do they display Lewis acid behaviour ?

Study These Flashcards
A

They have 6 valence electrons and so can accept a pair of electrons

18
Q

Why do they have a lack of valency?

Study These Flashcards
A

They only have 6 valence electrons

19
Q

What is the bond in B2H6?

Study These Flashcards
A

3 centre, 2 electron bonding as B is electron deficient

20
Q

How is B2H6 synthesised?

Diborane

Study These Flashcards
A

4BF3 + 3NaBF4 -> 3NaBF4 + B2H6

21
Q

Explain why B2H6 forms 3 centre 2 electron bonds

Study These Flashcards
A

-Each B has 3 outer electrons and 4sp3 orbitals
-There are 12 electrons in total from 2B and 6H for bonding
-After the 4 terminal BH bonds have been made there are only 4 electrons left for last 2 hydrogen which require bridging bonds (2 bonds each)
This means 3 centre 2 electron bonds are formed (each contain 2 electrons)

22
Q

What hybridisation does boron have?

Study These Flashcards
A

Boron is tetrahedral and so has sp3 hybridisation

23
Q

Use MO theory to describe the 3 centre 2 electron bonding in diborane

Study These Flashcards
A 
There are 3 atomic orbitals involved in each 3 centre, 2 electron bond 
- an sp3 orbital from each B
- a 1s orbital from H
- This forms 3 molecular orbitals 
( bonding, antibonding and non bonding)
24
Q

How is the bonding, antibonding and non bonding MO in the 3 centre, 2 electron bonds formed in diborane?

Study These Flashcards
A

Bonding MO- all AO are in the same phase

Antibonding MO- the 2 B AOs are the same phase, H AO is a different phase

Non bonding MO- the 2 B AOs are different phases and so H AO has the wrong shape

25
What is a halide?
A binary compound of a halogen with another element or group
26
Describe the structure and hybridisation of boron halides
Trigonal planar structures with sp2 hybridisation
27
Why do B-X bonds in BX3 have double bond character?
Boron has a formally vacant p orbital which the halogens can donate their lone pairs into This means each B-X has a little double bond character and changes the charge distribution
28
What is the trend in acidity of boron halides?
BF3 < BCl3 < BBr3 | Opposite to electronegativity
29
What would be the expected trend in boron halide acidity and what is the actual trend caused by?
On basis of electronegativity you would expect the opposite (electronic effects making the most positive B atoms) However this is not true, it is due to pi bonding contribution
30
How does pi bonding contribution affect boron halide acidity?
Halogens have lone pairs they can donate to B empty p orbital - B 2p and F2p are similar sizes so good overlap & more electron density can be donated As halogens get bigger, the orbital size is less similar, poorer overlap and so less electron density is donated= more positive B atoms This is explains trend in acidity
31
What happens to structure and hybridisation when a BX3 reacts with a Lewis base?
Trigonal planar to tetrahedral | Sp2 to sp3
32
What ppi-ppi bonding?
In BX3, X can donate electron density from filled p orbitals to empty p orbitals on boron This creates ppi-ppi bonding
33
Why is ppi bonding strongest in BF3 and weakest in BI3?
Pi bonding is distance sensitive and so ppi-ppi bonding decreases in strength as the size of the halogen increases
34
Why does ppi-ppi bonding only occur with sp2 hybridised boron and not sp3?
There is no available orbital of the correct symmetry to interact with the halogen p orbital in sp3
35
Why do larger halogens form adducts more readily?
Ppi-ppi bonding is weaker in larger halogens | This bonding is less when the tetrahedral Lewis acid- Lewis base adduct forms
36
Why do boron halides not dimerise?
- this is because BX3 is much more stable than its dimer because : - the presence of ppi-ppi bonding would be lost in the B2X6 dimer - there would be a greater steric bulk due to increase in coordination number
37
Why do BX3 has shorter bond lengths than expected?
- ppi-ppi bonding - the large electronegativity difference between B and X leads to ionic character in the bond - There are only 6 valence electrons and so less electron repulsions
38
Describe the bridging bonding in Al2Br6
There are 3 centre, 4 electron interactions Each bridging bromide donates all 3 of its valence electrons, 1 in a normal covalent bond and the other 2 in a dative covalent bond
39
What drives the formation of the Al2X6 dimer?
This is driven by the high Lewis acidity of the aluminium centre (it wants to accept a pair of electrons from the bromine in the form a dative covalent bond)
40
Why can Al form Al2X6 dimers with halogens whereas boron can only form halides BX3
Aluminium is larger than boron and so steric strain is less

(2) Main Group Chemistry (11 decks)

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