When conc. H_(2)SO_(4) was added into an unknown nitrate salt present in a test-tube, a brown gas (A) was evolved. This gas intensified when copper turnings were also added into this test-tube. On cooling, the gas (A) changed intocolourless gas (B). (a) Identify the gases (A) and (B). (b) Write the equations for the reactions involved.

When conc. H_(2)SO_(4) was added into an unknown nitrate salt present

1.	When conc. H_(2)SO_(4) was added into an unknown nitrate salt present in a test-tube, a brown gas (A) was evolved. This gas intensified when copper turnings were also added into this test-tube. On cooling, the gas (A) changed intocolourless gas (B). (a) Identify the gases (A) and (B). (b) Write the equations for the reactions involved.
Answer» Solution :When a nitrate salt is heated with conc. `H_(2)SO_(4)`, vapours of `HNO_(3)` are first produced which undergo decomposition to produce nitrogen dioxide `(NO_(2))` which is a BROWN gas (A) When Cu turnings are added, the gas (A) intensified due to the reduction of `HNO_(3)` by Cu. `underset("sod. nitrate")(2NaNO_(3))+H_(2)SO_(4) overset("Heat")(rarr) underset(("Colourless"))underset("Nitric acid")(2HNO_(3))+Na_(2)SO_(4)` `4HNO_(3) overset("Heat")(rarr) underset(("Brown gas A"))underset("Nitrogen dioxide")(4NO_(2)) + O_(2)+2H_(2)O` `underset("Copper turnings")(CU) + 4HNO_(3) overset("Heat")(rarr) Cu(NO_(3))_(2) + underset("Nitrogen dioxide (A)")(2NO_(2))+2H_(2)O` `NO_(2)` is an odd electron species (7 + 8 + 8 = 23 electrons). In the valence SHELL, it has only 7 electrons and hence is less stable. To become morestable by having 8 electrons in the valence shell, it undergoes dimerization to form a colourless gas, `N_(2)O_(4)` Thus, (A) is nitrogen dioxide `(NO_(2))` and (B) is dinitrogen tetroxide `(N_(2)O_(4))`.