State the observation for the following:
(a) Dry ammonia gas reacts with oxygen in the presence of a catalyst.
(b) Excess chlorine gas reacts with ammonia gas.
(c) Carbon reacts with hot concentrated nitric acid.
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(a) Dry ammonia gas reacts with oxygen in the presence of platinum (catalyst) to produce nitric oxide (NO), water and heat. Nitric oxide (NO) produced is oxidised to nitrogen dioxide (NO2) and reddish brown vapours are seen in the flask. As the reaction is exothermic, so the platinum (catalyst) continues to glow even after the heating is discontinued.
4NH3 + 5O2 $\xrightarrow[800 ^{\circ} \text{C}]{\text{Pt.}}$ 6H2O + 4NO↑ + Δ
2NO + O2 ⟶ 2NO2
(b) Colourless ammonia gas reacts with greenish yellow chlorine gas in excess producing hydrogen chloride and a yellow explosive liquid (nitrogen trichloride).
NH3 + 3Cl2 [excess] ⟶ 3HCl + NCl3
(c) Carbon reacts with hot concentrated nitric acid producing carbon dioxide, water and reddish brown fumes of nitrogen dioxide are evolved.
C + 4HNO3 ⟶ CO2 + 2H2O + 4NO2