10.4 Nitrogen dioxide or Di-nitrogen tetroxide $NO_2$ or $N_2O$

  p Block Elements

    1.0 Group $13$ – The Boron Family
    2.0 Boron
    3.0 Compounds of boron
    4.0 Compounds of Aluminium
    5.0 Group $14$ – The Carbon family
    6.0 Allotropes of Carbon
    7.0 Compounds of Carbon
    8.0 Properties of Silicon
    9.0 Group $15$-The Nitrogen Family
    10.0 Oxides of nitrogen
    11.0 Oxyacids of Nitrogen
    12.0 Phosphorus
    13.0 Oxygen
    14.0 Sulphur

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10.4 Nitrogen dioxide or Di-nitrogen tetroxide $NO_2$ or $N_2O$

This oxide $NO_2$ exists in the gaseous state while at low temperature, it exists as a dimer $N_2O_4$ (Solid state)\[2N{O_2}(Brown\;gas)\overset { - 11^\circ C} \leftrightarrows {N_2}{O_4}(Colourless\;solid)\]

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Preparation:

It is prepared in the laboratory either by heating nitrates of heavy metal or by the action of concentrated nitric acid on metals like copper, silver, lead etc.$$2Pb{(N{O_3})_2} \to 2PbO + 4N{O_2} + {O_2}$$

The mixture of nitrogen dioxide and oxygen is passed through a U-tube cooled by freezing mixture. Nitrogen dioxide condenses to a pale yellow liquid while oxygen escapes.$$\begin{equation} \begin{aligned} Zn{(N{O_3})_2} \to ZnO + N{O_2} + {O_2} \\ Ag(N{O_3}) \to Ag + N{O_2} + {O_2} \\ Cu + 4HN{O_3} \to Cu{(N{O_3})_2} + 2N{O_2} + 2{H_2}O \\\end{aligned} \end{equation} $$

It is obtained by air oxidation of nitric oxide.$$2NO + {O_2} \to 2N{O_2}$$

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Properties:

(a) It is brown colored gas with the pungent odor. Above $140^\circ C$, it is 100%$NO_2$. The liquid, as well as solid, is entirely $N_2O_4$ (dimer) at low temperature. The liquid boils at $22^\circ C$ and solid melts at $ - 11^\circ C$

(b) It decomposes completely into nitric oxide and oxygen at $620^\circ C$

$$N{O_2} + HCl \to NOCl + C{l_2} + {H_2}O$$ $$2N{O_2}\mathop \leftrightarrows \limits^{{{620}^ \circ }C} \underbrace {2NO + {O_2}}_{Gaseous\;mixture\;colorless}$$

(c) When reacted with cold water, it forms a mixture of nitrous acid and nitric acid.$$2N{O_2} + {H_2}O \to HN{O_2} + HN{O_3}$$on account of this, it is known as the mixed anhydride of these two acids. However, with an excess of warm water, it forms nitric acid and nitric oxide.$$3N{O_2} + {H_2}O \to 2HN{O_3} + NO$$

(d) When absorbed by alkalies, nitrites and nitrates are formed.$$2N{O_2} + 2NaOH \to NaN{O_2} + NaN{O_3} + {H_2}O$$

(e) It acts as an oxidizing agent. It oxidizes metals like sodium, potassium, mercury, tin copper, etc.$$N{O_2} + 2Na \to N{a_2}O + NO\quad \quad \quad \quad \quad N{O_2} + 2Cu \to C{u_2}O + NO$$none metals like carbon, sulphur, phosphorous when burnt in its atmosphere, are converted into corresponding oxides.$$\begin{equation} \begin{aligned} 5N{O_2} + 2P \to {P_2}{O_5} + 5NO \\ 2N{O_2} + S \to S{O_2} + 2NO \\ 2N{O_2} + C \to C{O_2} + 2NO \\\end{aligned} \end{equation} $$It liberated iodine from $KI$ turns starch-iodine paper blue.$$2KI + 2N{O_2} \to 2KN{O_2} + {I_2}$$

In aqueous solution, it oxidizes $SO_2$ to sulphuric acid.$$S{O_2} + {H_2}O + N{O_2} \to {H_2}S{O_4} + NO$$

This reaction is used for the manufacture of $H_2SO_4$ by lead chamber process.

$H_2S$ is oxidised to $S$ and $CO$ to $CO_2$.$${H_2}S + N{O_2} \to {H_2}O + S + NO\quad \quad \quad \quad \quad CO + N{O_2} \to C{O_2} + NO$$

(f) It behaves also as a reducing agent. It reduces ozone to oxygen.$$2N{O_2} + {O_3} \to {N_2}{O_5} + {O_2}$$

It decolorize acidified $KMnO_4$ solution.$$2KMn{O_4} + {H_2}S{O_4} + 10N{O_2} + 2{H_2}O \to {K_2}S{O_4} + 2MnS{O_4} + 10HN{O_3}$$

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Uses:

(i) It is used for the manufacture of nitric acid.

(ii) It is employed as a catalyst in the lead chamber process for the manufacture of sulphuric acid.