Question
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.
✓
Answer
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. For example,
In $2^{nd}$ period $B_2O_3 < CO_2 < N_2O_3$ (acidic nature increases).
In $3^{rd}$ period $A1_2O_3 < SiO_2 < P_4O_{10} < SO_3 < C1_2O_7$ (acidic character increases).
On moving down the group, acidic character decreases and basic character increases, e.g.,
Among the oxides of same element, higher the oxidation state of the element, stronger is the acid. For example, $SO_3$ is a stronger acid than $SO_2$.
$B_2O_3$ is weakly acidic and on 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.
$\text{B}_2\text{O}_3\ \ \ \ \ \ \ \ \ \ +\ \ \ \ \ \ \ 3\text{H}_2\text{O}\ \ \ \rightleftharpoons\ \ \ \ 2\text{H}_3\text{BO}_3\\^\text{Boron trioxide}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{Orthoboric acid}$
$\text{B(OH)}_3\ +\ \text{H}-\text{OH}\xrightarrow{\ \ \ \ \ \ \ \ \ \ \ \ }\big[\text{B}(\text{OH})_4\big]^{-}+\text{H}^+$
$Al_2O_3$ is amphoteric in nature. It is insoluble in water but dissolves in alkalies and reacts with acids.
$\text{Al}_2\text{O}_3\ \ +\ \ 2\text{NaOH}\ \xrightarrow{\ \ \ \ \ \ \ {\Delta} \ \ \ \ \ \ \ \ }\ 2\text{NaAlO}_2\ \ +\ \ \text{H}_2\text{O}\\ ^\text{Aluminium}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{Sodium meta}\\ \ \ ^\text{trioxide}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{chloride}$
$\text{Al}_2\text{O}_3\ \ +\ \ 6\text{HCl}\ \xrightarrow{\ \ \ \ \ \ \ {\Delta} \ \ \ \ \ \ \ \ }\ 2\text{AlCl}_3\ \ +\ \ 3\text{H}_2\text{O}\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{Aluminium}\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{chloride}$
$Tl_2O$ is as basic as NaOH due to its lower oxidation state (+1).
$\text{Tl}_2\text{O}\ +\ 2\text{HCl}\ \xrightarrow{\ \ \ \ \ \ \ \ \ \ \ \ }2\text{TlCl}\ +\ \text{H}_2\text{O}$
In $2^{nd}$ period $B_2O_3 < CO_2 < N_2O_3$ (acidic nature increases).
In $3^{rd}$ period $A1_2O_3 < SiO_2 < P_4O_{10} < SO_3 < C1_2O_7$ (acidic character increases).
On moving down the group, acidic character decreases and basic character increases, e.g.,
- Nature of oxides of 15 group elements.
Among the oxides of same element, higher the oxidation state of the element, stronger is the acid. For example, $SO_3$ is a stronger acid than $SO_2$.
$B_2O_3$ is weakly acidic and on 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.
$\text{B}_2\text{O}_3\ \ \ \ \ \ \ \ \ \ +\ \ \ \ \ \ \ 3\text{H}_2\text{O}\ \ \ \rightleftharpoons\ \ \ \ 2\text{H}_3\text{BO}_3\\^\text{Boron trioxide}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{Orthoboric acid}$
$\text{B(OH)}_3\ +\ \text{H}-\text{OH}\xrightarrow{\ \ \ \ \ \ \ \ \ \ \ \ }\big[\text{B}(\text{OH})_4\big]^{-}+\text{H}^+$
$Al_2O_3$ is amphoteric in nature. It is insoluble in water but dissolves in alkalies and reacts with acids.
$\text{Al}_2\text{O}_3\ \ +\ \ 2\text{NaOH}\ \xrightarrow{\ \ \ \ \ \ \ {\Delta} \ \ \ \ \ \ \ \ }\ 2\text{NaAlO}_2\ \ +\ \ \text{H}_2\text{O}\\ ^\text{Aluminium}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{Sodium meta}\\ \ \ ^\text{trioxide}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{chloride}$
$\text{Al}_2\text{O}_3\ \ +\ \ 6\text{HCl}\ \xrightarrow{\ \ \ \ \ \ \ {\Delta} \ \ \ \ \ \ \ \ }\ 2\text{AlCl}_3\ \ +\ \ 3\text{H}_2\text{O}\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{Aluminium}\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{chloride}$
$Tl_2O$ is as basic as NaOH due to its lower oxidation state (+1).
$\text{Tl}_2\text{O}\ +\ 2\text{HCl}\ \xrightarrow{\ \ \ \ \ \ \ \ \ \ \ \ }2\text{TlCl}\ +\ \text{H}_2\text{O}$
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