NCERT Exemplar

Hint

(c) The melting point of gallium is $30^{\circ} \mathrm{C}$ and boiling point is $2240^{\circ} \mathrm{C}$. Thus, the element exists in liquid state for a wide range of temperature.

Hint

(a) $\mathrm{AlCl}_3$ is electron-deficient and hence acts as a Lewis acid.

Hint

(a) Boron has die electronic configuration:
$1 s^2 2 s^2 2 p_x^1 2 p_y^0 2 p_z^0$
In the excited state, 2 s -orbital electrons are impaired and one electron is shifted to a p-orbital. Now, hybridisation occurs between one $s$-and three p -orbitals to give $s {p}^3$ hybridisation and tetrahedral geometry.
$1 s^2 \underbrace{2 s^1 2 p_x^1 2 p_y^1 2 p_z^0}_{\text {sp }{ }^3 \text {-hybridisation }}$

Hint

(a) $\mathrm{B}_2 \mathrm{O}_3$ is acidic in nature. It reacts with basic oxides to form metal borates. Acidic nature decreases on moving down the group.

$\underbrace{\mathrm{B}_2 \mathrm{O}_3}_{\begin{array}{c} \text { More acicic } \\ \text { less basic } \end{array}}, \underbrace{\mathrm{Al}_2 \mathrm{O}_3 \text { and } \mathrm{Ga}_2 \mathrm{O}_3}_{\text {Amphoteric }}, \underbrace{\mathrm{In}_2 \mathrm{O}_3 \text { and } \mathrm{Tl}_2 \mathrm{O}_3}_{\text {Basic }}$

Hint

(a) The lowest atomic number among the given is boron. Boron has atomic number 5 and do not have vacant $d$-orbital. The element $M$ in the complex ion $M F_6^{3-}$ has a coordination number of six. Boron can show maximum coordination number of 4 . Thus, B can not form complex of the type $M F_6^{3-}$.

Hint

(b) The tendency to form long open or closed atom chains by combination of some atoms in themselves is known as catenation. The catenation is maximum in carbon and decreases down the group.
$\mathrm{C} \gg \mathrm{Si}>\mathrm{Ge} \approx \mathrm{Sn}>\mathrm{Pb}$
This is due to high bond energy of $\mathrm{C}-\mathrm{C}$ bonds. Down the group, size increases and electronegativity decreases, thereby, tendency to show catenation decreases.

Hint

(d) On moving down the group from B to Tl , a regular decreasing trend in the ionisation energy values is not observed.
$\begin{array}{cccccc} \mathrm{B} & \mathrm{Al} & \mathrm{Ga} & \mathrm{In} & \mathrm{Tl} \\ 801 & 577 & 579 & 558 & 589 & \mathrm{~kJ} \mathrm{~mol}^{-1} \end{array}$
In Ga , there are ten d-electrons in the penultimate shell which screen the nuclear charge less effectively and thus, outer electron is held firmly. As a result, the ionisation energy of both Al and Ga is nearly the same. The increase in ionisation energy from In to Tl is due to poor screening effect of 14 4-f electrons present in the inner shell ( order of shielding effect $s>p>d>f$ ) and so effective nuclear charge increases, consequently ionisation enthalpies increase.

Hint

(b) Quartz, cristobalite and tridymite are some of the crystalline forms of silica and they are interconvertable at suitable temperature. Quartz is extensively used as a piezoelectric material.

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(d) Reducing agents are those substances which reduces other substances and it self oxidises.
$+4$ oxidation state of $\mathrm{Sn}$ is more stable than +2 oxidation state. Therefore, $\mathrm{Sn}^{2+}$ can be easily oxidised to $\mathrm{Sn}^{4+}$ and hence $\mathrm{SnCl}_2$ acts a reducing agent.
$\mathrm{SnCl}_2+2 \mathrm{Cl} \rightarrow \mathrm{SnCl}_4+2 \mathrm{e}^{-}$

Hint

(c) Carbon dioxide can be obtained as a solid in the form of dry ice allowing the liquified $\mathrm{CO}_2$ to expand rapidly.

Note Dry ice is also called cardice. It is obtained when $\mathrm{CO}_2$ is cooled under pressure (50-60 atm). Dry ice is used for making cold baths in the laboratory by mixing it with volatile organic solvents. It is also used as a coolant for preserving perishable articles in food industry, curing local burns and in hospitals for surgical operation of sores.

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(a, b) The additional 10 d-electrons offer poor screening effect for the outer electrons from the increased nuclear charge in Gallium. Hence, atomic radius of Gallium is less than that of aluminium.

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(b, c) The linear shape of $\mathrm{CO}_2$ is due to $p \pi-p \pi$ bonding between carbon and oxygen and sp hybridisation of carbon.

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(b, d)

$\text { The answer is the option (ii) } \mathrm{O}=\mathrm{C}=\mathrm{O} \text { and (iv) }{ }^{-} \mathrm{O}-\mathrm{C} \equiv \mathrm{O}^{+}$

Hint

(a) (i) $\rightarrow$ (e) (ii) $\rightarrow$ (c) (iii) $\rightarrow$ (d) (iv) $\rightarrow$ (a), (b)

(i) $\mathrm{BF}_4^{-}$: Tetrahedral shape, $\mathrm{sp}^3$ hybridisation, regular geometry.
(ii) $\mathrm{AlCl}_3$ : Octet of Al not complete, acts as Lewis acid.
(iii) $\mathrm{SnO}: \mathrm{Sn}^{2+}$ can show +4 oxidation state.
(iv) $\mathrm{PbO}_2$ : Oxidation.state of Pb in $\mathrm{PbO}_2$ is +4 . Due to inert pair effect, $\mathrm{Pb}^{4+}$ is less stable than $\mathrm{Pb}^{2+}$ and hence acts as strong oxidising agent.

Hint

(b) (i) $\rightarrow$ (c) (ii) $\rightarrow$ (d) (iii) $\rightarrow$ (a) (iv) $\rightarrow$ (e) (v) $\rightarrow$ (b)

– $\mathrm{BH}_3$ is unstable, forms diborane $\mathrm{B}_2 \mathrm{H}_6$ by 3 centre -2 electron bonds, shows banana bonds.
– Gallium with low melting point and high boiling point makes it useful to measure high temperatures.
– Borax is used as a flux for soldering metals.
– Aluminosilicate is used as catalyst in petrochemical industries.
– Quartz, is a crystalline form of silica.

Hint

(c) (i) $\rightarrow$ (b), (ii) $\rightarrow$ (c), (iii) $\rightarrow$ (b), (iv) $\rightarrow$ (a), (v) $\rightarrow$ (b), $(\mathrm{vi}) \rightarrow$ (c)

Boron in $\left[\mathrm{B}(\mathrm{OH})_4\right]^{-}$is $\mathrm{sp}^3$ hybridised.
Aluminium in $\left[\mathrm{A} 1\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{3+}$ is $\mathrm{sp}^3 \mathrm{~d}^2$ hybridised.
Boron in $\mathrm{B}_2 \mathrm{H}_6$ is $\mathrm{sp}^3$ hybridised.
Carbon in Buckminsterfullerene $\mathrm{sp}^2$ is hybridised.
Silicon in $\mathrm{SiO}_4{ }^{4-}$ is $\mathrm{sp}^3$ hybridised.
Germanium in $\left[\mathrm{GeCl}_6\right]^{2-}$ is $\mathrm{sp}^3 \mathrm{~d}^2$ hybridised.