Chemistry CHEMICAL PROPERTIES OF GROUP 13 ELEMENTS

### Topics covered

color{brown}(★) Chemical properties

### Chemical Properties

In this topic we will be discussing some of the important chemical properties of group 13 elements.

### Oxidation state and trends in chemical reactivity

color{green}(★) Due to small size of boron, the sum of its first three ionization enthalpies is very high. This prevents it to form +3 ions and forces it to form only covalent compounds.

color{green}(★) But as we move from color{red}(B) to color{red}(Al), the sum of the first three ionisation enthalpies of color{red}(Al) considerably decreases, and is therefore able to form color{red}(Al^(3+)) ions.

color{green}(★) In fact, aluminium is a highly electropositive metal.

color{green}(★) However, down the group, due to poor shielding effect of intervening color{red}(d) and color{red}(f) orbitals, the increased effective nuclear charge holds color{red}(ns) electrons tightly (responsible for inert pair effect) and thereby, restricting their participation in bonding. As a result of this, only color{red}(p)-orbital electron may be involved in bonding. In fact in color{red}(Ga, In) and color{red}(Tl), both +1 and +3 oxidation states are observed.

color{green}(★) The relative stability of +1 oxidation state progressively increases for heavier elements: color{red}(Al < Ga < In < Tl ). In thallium +1 oxidation state is predominant whereas the +3 oxidation state is highly oxidising in character.

color{green}(★) The compounds in +1 oxidation state, as expected from energy considerations, are more ionic than those in +3 oxidation state.

color{green}(★) In trivalent state, the number of electrons around the central atom in a molecule of the compounds of these elements (e.g., boron in color{red}(BF_3) ) will be only six. Such electron deficient molecules have tendency to accept a pair of electrons to achieve stable electronic configuration and thus, behave as Lewis acids.

color{green}(★) The tendency to behave as Lewis acid decreases with the increase in the size down the group. color{red}(BCl_3) easily accepts a lone pair of electrons from ammonia to form color{red}(BCl_3⋅NH_3).

color{green}(★) In trivalent state most of the compounds being covalent are hydrolysed in water. For example, the trichlorides on hyrolysis in water form tetrahedral color{red}([M(OH)_4]^(-)) species; the hybridisation state of element color{red}(M) is color{red}(sp^3). Aluminium chloride in acidified aqueous solution forms octahedral color{red}([Al(H_2O)_6]^(3+)) ion. In this complex ion, the color{red}(3d) orbitals of color{red}(Al) are involved and the hybridisation state of color{red}(Al) is color{red}(sp^3d^2).
Q 3275001866

Standard electrode potential values, E^(⊖) for Al^(3+)// Al is –1.66 V and that of Tl^(3+) // Tl is +1.26 V. Predict about the formation of M^(3+) ion in solution and compare the electropositive character of the two metals.

Solution:

Standard electrode potential values for two half cell reactions suggest that aluminium has high tendency to make Al^(3+)(aq) ions, whereas Tl^(3+) is not only unstable in solution but is a powerful oxidising agent also. Thus Tl^(+) is more stable in solution than Tl^(3+). Aluminium being able to form +3 ions easily, is more electropositive than thallium.

### Reactivity of group 13 elements towards air

color{green}(★) Boron is unreactive in crystalline form.

color{green}(★) Aluminium forms a very thin oxide layer on the surface which protects the metal from further attack.

color{green}(★) Amorphous boron and aluminium metal on heating in air form color{red}(B_2O_3) and color{red}(Al_2O_3) respectively. With dinitrogen at high temperature they form nitrides.

color{red}(2E (s) + 3O_2 (g) overset(Delta)→ 2E_2O_3 (s))

color{red}(2E(s) +N_2 (g) overset(Delta)→ 2EN (s)) (E = element)

color{green}(★) The nature of these oxides varies down the group.

color{green}(★) Boron trioxide is acidic and reacts with basic (metallic) oxides forming metal borates

color{green}(★) Aluminium and gallium oxides are amphoteric and those of indium and thallium are basic in their properties.

### Reactivity towards acids and alkalies

color{green}(★) Boron does not react with acids and alkalies even at moderate temperature; but aluminium dissolves in mineral acids and aqueous alkalies and thus shows amphoteric character.

color{green}(★) Aluminium dissolves in dilute color{red}(HCl) and liberates dihydrogen.

color{red}(2Al (s) + 6HCl (aq) →2Al^(3+) (aq) + 6 Cl^(-) (aq) + 3H_2(g))

color{green}(★) However, concentrated nitric acid renders aluminium passive by forming a protective oxide layer on the surface.

color{green}(★) Aluminium also reacts with aqueous alkali and liberates dihydrogen.

### Reactivity towards halogens

color{green}(★) These elements react with halogens to form trihalides (except color{red}(Tl I_3)).

color{red}(2E(s) +3X_2 (g) →2EX_3(s) \ \ \ \ \ \ \ (X = F , Cl , Br , I))

Q 3285001867

White fumes appear around the bottle of anhydrous aluminium chloride. Give reason.

Solution:

Anhydrous aluminium chloride is partially hydrolysed with atmospheric moisture to liberate HCl gas. Moist HCl
appears white in colour.