Question

Two non-metallic gaseous elements react in

1 : 3

ratio to give another gas X. A mixture of X and oxygen is taken in a jar and a burning match stick is introduced into it. Another sample of the same mixture containing a large excess of

O_{2}

is passed over heated platinum. The mixture of gaseous products is dissolved in water to give Y and Z which are colourless. Z on long standing turns to yellow. Identify X, Y and Z by giving reasons wherever necessary. Also, identify the exothermic reaction in the above sequence.

Accepted Answer

Two non-metallic gaseous elements react in $1 : 3$ ratio to give another gas X. The non-metallic gaseous elements are nitrogen and hydrogen and the gas formed is ammonia. A combination reaction takes place. So, X is ammonia. When a mixture of X (ammonia) and oxygen is taken in a jar and a burning match stick is introduced into it, burns with a pale blue flame.

$4 {NH}_{3} + 3 O_{2} \to 2 N_{2} + 6 H_{2} O$

When another sample of the same mixture containing a large excess of oxygen is passed over heated platinum, nitric oxide and water vapour are formed with the evolution of heat. So, it is an exothermic reaction.

$4 {NH}_{3} + 5 O_{2} \to_{900 ° C}^{P} 4 NO + 6 H_{2} O 2 NO + O_{2} \to 2 {NO}_{2} 2 {NO}_{2} + H_{2} O \to \underset{(Y)}{{HNO}_{2}} + \underset{(Z)}{{HNO}_{3}}$

Thus, Y is nitrous acid and Z is nitric acid, they are colourless. Z on long-standing turns to yellow due to chemical decomposition. When exposed to light, nitric acid decomposes to form brownish nitrogen dioxide that dissolves again in the acid to give it a yellowish colour.

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