Question
Read the passage given below and answer the following questions from 1 to 5.
Oxidation state and trends in chemical Reactivity 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. But as we move from B to Al, the sum of the first three ionisation Enthalpies of Al considerably decreases, and Is therefore able to form $Al^{3+}$ ions. In fact, Aluminium is a highly electropositive metal. However, down the group, due to poor Shielding effect of intervening d and f orbitals, The increased effective nuclear charge holds ns Electrons tightly (responsible for inert pair Effect) and thereby, restricting their Participation in bonding. As a result of this, Only p-orbital electron may be involved in Bonding. In fact in Ga, In and Tl, both +1 and +3 oxidation states are observed. The relative Stability of +1 oxidation state progressively Increases for heavier elements: A l< Ga < In< Tl. In Thallium +1 oxidation state is predominant whereas the +3 oxidation state is highly Oxidising in character. The compounds in +1 oxidation state, as expected from energy Considerations, are more ionic than those in +3 oxidation state.
Important trends and anomalous properties of boron – certain important trends can be observed in the chemical behaviour of group 13 elements. The tri-chlorides, bromides and iodides of all these elements being covalent in nature are hydrolysed in water. Species like tetrahedral $[M(OH)_4]^–$ and octahedral $[M(H_2O)6]^{3+}$, except in boron, exist in aqueous medium. The monomeric trihalides, being electron deficient, are strong Lewis acids. Boron trifluoride easily reacts with Lewis bases such as $NH_3$ to complete octet around boron. It is due to the absence of d orbitals that the maximum covalence of B is 4. Since the d orbitals are available with Al and other elements, the maximum covalence can be expected beyond 4. Most of the other metal halides (e.g., $AlCl_3$) are dimerised through halogen bridging (e.g., $Al2Cl_6$). The metal species completes its octet by accepting electrons from halogen in these halogen bridged molecules.
i) Reactivity towards air Boron is unreactive in crystalline form. Aluminium forms a very thin oxide layer on The surface which protects the metal from Further attack. Amorphous boron and Aluminium metal on heating in air form $B_2O_3$ And $Al_2O_3$ respectively. With dinitrogen at high Temperature they form nitrides. The nature of these oxides varies down the Group. Boron trioxide is acidic and reacts with Basic (metallic) oxides forming metal borates. Aluminium and gallium oxides are amphoteric And those of indium and thallium are basic in Their properties.
ii) Reactivity towards acids and alkalies 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. Aluminium dissolves in dilute HCl and Liberates dihydrogen.
$2Al(s) + 6HCl (aq) \rightarrow 2Al_3^+ (aq) + 6Cl^– (aq) + 3H_2 (g)$
However, concentrated nitric acid renders Aluminium passive by forming a protective Oxide layer on the surface. Aluminium also reacts with aqueous alkali And liberates dihydrogen.
$2Al (s) + 2NaOH(aq) + 6H_2O(l) \rightarrow 2 Na+ [Al(OH)_4]^– (aq) + 3H_2(g)$
Sodium Tetrahydroxoaluminate(III).
iii) Reactivity towards halogens These elements react with halogens to form Trihalides (except TlI3). $2E(s) + 3 X_2 (g) \rightarrow 2EX_3 (s) (X = F, Cl, Br, I)$
Borax- It is the most important compound of boron. It is a white crystalline solid of formula $Na_2B_4O_7⋅10H_2O$. In fact it contains the Tetranuclear units and correct Formula; therefore, is $Na2 [B4O5 (OH) 4].8H2O$. Borax dissolves in water to give an alkaline Solution.
$Na_2B_4O7 + 7H_2O \rightarrow 2NaOH + 4H_3BO_3$
On heating, borax first loses water Molecules and swells up. On further heating it Turns into a transparent liquid, which solidifies Into glass like material known as borax Bead. $Na_2B_4O_7.10H_2O \rightarrow Na^2B_4O_7\rightarrow 2NaBO_2+ B2O_3$
Metaborate Boric Anhydride The metaborates of many transition metals Have characteristic colours and, therefore, Borax bead test can be used to identify them In the laboratory. For example, when borax is Heated in a Bunsen burner flame with CoO on A loop of platinum wire, a blue coloured Co(BO2) 2 bead is formed.
Orthoboric acid, $H_3BO_3$ is a white crystalline Solid, with soapy touch. It is sparingly soluble In water but highly soluble in hot water. It can Be prepared by acidifying an aqueous solution Of borax.
$Na_2B_4O_7 + 2HCl + 5H_2O \rightarrow 2NaCl + 4B(OH)_3$
It is also formed by the hydrolysis (reaction With water or dilute acid) of most boron Compounds (halides, hydrides, etc.). It has a layer structure in which planar $BO_3$ units are Joined by hydrogen.
Oxidation state and trends in chemical Reactivity 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. But as we move from B to Al, the sum of the first three ionisation Enthalpies of Al considerably decreases, and Is therefore able to form $Al^{3+}$ ions. In fact, Aluminium is a highly electropositive metal. However, down the group, due to poor Shielding effect of intervening d and f orbitals, The increased effective nuclear charge holds ns Electrons tightly (responsible for inert pair Effect) and thereby, restricting their Participation in bonding. As a result of this, Only p-orbital electron may be involved in Bonding. In fact in Ga, In and Tl, both +1 and +3 oxidation states are observed. The relative Stability of +1 oxidation state progressively Increases for heavier elements: A l< Ga < In< Tl. In Thallium +1 oxidation state is predominant whereas the +3 oxidation state is highly Oxidising in character. The compounds in +1 oxidation state, as expected from energy Considerations, are more ionic than those in +3 oxidation state.
Important trends and anomalous properties of boron – certain important trends can be observed in the chemical behaviour of group 13 elements. The tri-chlorides, bromides and iodides of all these elements being covalent in nature are hydrolysed in water. Species like tetrahedral $[M(OH)_4]^–$ and octahedral $[M(H_2O)6]^{3+}$, except in boron, exist in aqueous medium. The monomeric trihalides, being electron deficient, are strong Lewis acids. Boron trifluoride easily reacts with Lewis bases such as $NH_3$ to complete octet around boron. It is due to the absence of d orbitals that the maximum covalence of B is 4. Since the d orbitals are available with Al and other elements, the maximum covalence can be expected beyond 4. Most of the other metal halides (e.g., $AlCl_3$) are dimerised through halogen bridging (e.g., $Al2Cl_6$). The metal species completes its octet by accepting electrons from halogen in these halogen bridged molecules.
i) Reactivity towards air Boron is unreactive in crystalline form. Aluminium forms a very thin oxide layer on The surface which protects the metal from Further attack. Amorphous boron and Aluminium metal on heating in air form $B_2O_3$ And $Al_2O_3$ respectively. With dinitrogen at high Temperature they form nitrides. The nature of these oxides varies down the Group. Boron trioxide is acidic and reacts with Basic (metallic) oxides forming metal borates. Aluminium and gallium oxides are amphoteric And those of indium and thallium are basic in Their properties.
ii) Reactivity towards acids and alkalies 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. Aluminium dissolves in dilute HCl and Liberates dihydrogen.
$2Al(s) + 6HCl (aq) \rightarrow 2Al_3^+ (aq) + 6Cl^– (aq) + 3H_2 (g)$
However, concentrated nitric acid renders Aluminium passive by forming a protective Oxide layer on the surface. Aluminium also reacts with aqueous alkali And liberates dihydrogen.
$2Al (s) + 2NaOH(aq) + 6H_2O(l) \rightarrow 2 Na+ [Al(OH)_4]^– (aq) + 3H_2(g)$
Sodium Tetrahydroxoaluminate(III).
iii) Reactivity towards halogens These elements react with halogens to form Trihalides (except TlI3). $2E(s) + 3 X_2 (g) \rightarrow 2EX_3 (s) (X = F, Cl, Br, I)$
Borax- It is the most important compound of boron. It is a white crystalline solid of formula $Na_2B_4O_7⋅10H_2O$. In fact it contains the Tetranuclear units and correct Formula; therefore, is $Na2 [B4O5 (OH) 4].8H2O$. Borax dissolves in water to give an alkaline Solution.
$Na_2B_4O7 + 7H_2O \rightarrow 2NaOH + 4H_3BO_3$
On heating, borax first loses water Molecules and swells up. On further heating it Turns into a transparent liquid, which solidifies Into glass like material known as borax Bead. $Na_2B_4O_7.10H_2O \rightarrow Na^2B_4O_7\rightarrow 2NaBO_2+ B2O_3$
Metaborate Boric Anhydride The metaborates of many transition metals Have characteristic colours and, therefore, Borax bead test can be used to identify them In the laboratory. For example, when borax is Heated in a Bunsen burner flame with CoO on A loop of platinum wire, a blue coloured Co(BO2) 2 bead is formed.
Orthoboric acid, $H_3BO_3$ is a white crystalline Solid, with soapy touch. It is sparingly soluble In water but highly soluble in hot water. It can Be prepared by acidifying an aqueous solution Of borax.
$Na_2B_4O_7 + 2HCl + 5H_2O \rightarrow 2NaCl + 4B(OH)_3$
It is also formed by the hydrolysis (reaction With water or dilute acid) of most boron Compounds (halides, hydrides, etc.). It has a layer structure in which planar $BO_3$ units are Joined by hydrogen.
- Boron is … in crystalline form.
- unreactive
- highly reactive
- less reactive
- only (a) or (c)
- Orthoboric acid is …
- Amorphous
- Crystalline
- Polyamorphous
- None of above
- Aluminium and gallium oxides are … in their properties.
- acidic
- Basic
- amphoteric
- None of above
- Indium and thallium are … in their properties.
- acidic
- Alkali
- amphoteric
- basic
- Aluminium is a highly … metal.
- electronegative
- Neutral
- electropositive
- None of above
