Group 13

Question 1

What do the group 13 halides act as and why?

Answer 1

Lewis acids as they are electron deficient e.g. AX3 has an empty p orbital and 6 valence electrons so can accept electrons from a Lewis base to achieve a stable octet.

Question 2

What are the properties of BX3?

Answer 2

It is monomeric in nature (can undergo polymerisation), trigonal planar and is more volatile than the aluminium analogue as it is not as heavy and Al forms dimers.

Question 3

What is the trend in the B-X bond length from fluorine to iodine?

Answer 3

It increases.

Question 4

How can BF3 be synthesised?

Answer 4

B2O3 + 3CaF2 + 3H2SO4 —> 2BF3 + 3CaSO4 + 3H2O.

Question 5

In the synthesis of BF3, what does excess H2O lead to and give the equation.

Answer 5

The hydrolysis of BF3

4BF3 + 6H2O —> 3[H3O]+ + 3[BF4]- + B)OH)3. The last two molecules are equivalent to 3HBF4 and 3H2O.

Question 6

What are the properties of HBF4?

Answer 6

It is a commercially available strong acid and is non-isolable - it must be used as a solution.

Question 7

What is another method of synthesis of HBF4?

Answer 7

B(OH)3 + 4HF —-> H3O+ + BF4- + 2H2O

Question 8

What are the properties of [BF4]-?

Answer 8

Is is innocent (when ligands allow the oxidation state of the central atoms to be defined), it is weakly coordinating of strong acid and used to precipitate cations.

Question 9

What does BF3 form complexes with?

Answer 9

Ethers, such as diethyl ether.

Question 10

How does the stability of complexes of BX3 with ethers change down the halogen group and why is this trend present?

Answer 10

Stability increases down the group. This is as B-X bonds have partial pi character which are lost during the change from sp2 to sp3. This is endothermic reorganisation. The L–>B bond formation is exothermic, and F is the strongest stabiliser due to the smaller size and better overlap. There is a bigger energy deficit on the loss of the B-F pi bond character due to a smaller B-F bond length and covalent radius resulting in a stronger bond with more pi character.

Question 11

What are the structures of halides of Al, Ga, In and Tl?

Answer 11

They form dimers/oligomers/infinite networks.

Question 12

What is the structure of AlF3?

Answer 12

An octahedral structure with 6F atoms each linked to 2 Als.

Question 13

How can AlF3 be made?

Answer 13

Al2O3 + 6HF —> 2AlF3 + 3H2O.

Question 14

How can the infinite structure of AlF3 be broken up?

Answer 14

Forming an anion with [pyH]4.

Question 15

What are the properties of compounds made of AL, Ga and In with Cl, Br and I?

Answer 15

They are dimeric in solution and are formed from the elements.

Question 16

What is the structure of AlCl3?

Answer 16

It is a 3-centre 4-electron bond.

Question 17

What is the equation for when water is added to AlCl3?

Answer 17

Hydrolysis occurs: AlCl3 + 6H2O —> [Al(H2O)6]3+3Cl-.

Question 18

What happens when AlCl3 is in a coordinating solvent?

Answer 18

Adducts form - AlCl3 . EtO2.

Question 19

Is B(OH)3 acidic?

Answer 19

Yes.

Question 20

How is B(OH)3 acidic?

Answer 20

It forms B(OH)3OH2 in water which can lose a proton from the OH2 group.

Question 21

How can B(OH)3 be formed?

Answer 21

The hydrolysis of B2O3.

Question 22

What is the formula for alumina?

Answer 22

Al2O3.

Question 23

How can alumina be formed?

Answer 23

Heating Al(OH)3 —> Al2O3 + H2O.

Question 24

What is the most common soluble salt of aluminium and what are its properties?

Answer 24

Aluminium sulfate octadecahydrate (Al2(SO4)3 . 18H2O). It is acidic in aqueous solutions and in water forms a strong hydration complex that can hydrolyse.

Question 25

What is the formula of the hydration complex that aluminium sulfate octadecahydrate forms

Answer 25

[Al(H2O)6]3+.

Question 26

What is the equation for the hydration complex of aluminium sulfate octadecahydrate?

Answer 26

[Al(H2O)6]3+ + H2O [Al(H2O)5OH]2+ + H3O+.

Question 27

What happens if hydrolysis continues of the hydration complex of aluminium sulfate octadecahydrate?

Answer 27

Aluminium hydroxide can form, which is a white precipitate.

Question 28

What does amphoteric mean?

Answer 28

A compound that can act as a base or an acid.

Question 29

Give examples of how aluminium hydroxide is amphoteric.

Answer 29

With acids it acts as a base in a neutralisation reaction, whereas with bases a hydroxo ion forms and it acts as an acid.

Question 30

What do the amphoteric properties of aluminium show?

Answer 30

It is a reflection of the partial non-metal characteristics of aluminium.