Ncert Exercises & Solutions

p-block elements form acidic, basic and amphoteric oxides. Explain each property by giving two examples and also write the reactions of these oxides with water.

In p-block, when we move from left to right in a period, the acidic character of the oxides increases due to increase in electronegativity. e.g.,

(i) 2nd period

B_{2} O_{3} < {CO}_{2} < N_{2} O_{3}

acidic nature increases.

(ii) 3rd period

{Al}_{2} O_{3} < {SiO}_{2} < P_{4} O_{10} < {SO}_{3} < {Cl}_{2} O_{7}

acidic character increases, On moving down the group, acidic character decreases and basic character increases. e.g.,

(a) Nature of oxides of 13 group elements

$\underset{Weakly acidic}{B_{2} O_{3}} \underset{Amphoteric}{{Al}_{2} O_{3} {Ga}_{2} O_{3}} \underset{Basic}{{In}_{2} O_{3}} \underset{Strongly}{{Tl}_{2} O}$
$Basic$

(b) Nature of oxides of 15 group elements

\underset{Strongly}{N_{2} O_{5}}

acidic \underset{Moderately}{P_{4} O_{10}}

acidic \underset{Amphoteric}{{As}_{4} O_{10}} \underset{Amphoteric}{{Sb}_{4} O_{10}} \underset{Basic}{{Bi}_{2} O_{3}}

Among the oxides of same element, higher the oxidation state of the element, stronger is the acid. e.g., SO₃ is a stronger acid than SO₂.

B₂O₃ is weakly acidic and n dissolution in water, it forms orthoboric acid. Orthoboric acid does not act as a protonic acid (it does not ionise) but acts as a weak Lewis acid.

\underset{Boron trioxide}{B_{2} O_{3}} + 3 H_{2} O ⇌ \underset{Orthoboric acid}{2 H_{3} {BO}_{3}}

B {(OH)}_{3} + H - OH \to {[B {(OH)}_{4}]}^{-} + H^{+}

Al₂O₃ is amphoteric in nature. It is insoluble in water but dissolves in alkalies and reacts with acids

\underset{Aluminium}{{Al}_{2} O_{3}}

trioxide + 2 NaOH \overset{∆}{\to} \underset{Sodium meta}{2 {NaAlO}_{2}}

aluminate + H_{2} O

\leftarrow {Al}_{2} O_{3} + 6 HCl \overset{∆}{\to} \underset{Aluminium}{2 {AlCl}_{3}}

chloride + 3 H_{2} O

Tl₂O is as basic as NaOH due to its lower oxidation state (+1)

{Tl}_{2} O + 2 HCl \to 2 TlCl + H_{2} O

P₄O₁₀ on reaction with water gives orthophosphoric acid

\underset{Phosphorus}{P_{4} O_{10}}

pentoxide + 6 H_{2} O \to \underset{Orthophosphoric}{4 H_{3} {PO}_{4}}

acid

Cl₂O₇ is strongly acidic in nature and on dissolution in water, it gives perchloric acid.

\underset{Dichlorine heptoxide}{{Cl}_{2} O_{7}} + H_{2} O \to \underset{Perchloric acid}{2 {HClO}_{4}}

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