Correct Answer - 3
Nitrogenn tetroxide and dioxide are in equilibrium with each other:
`N_(2)O_(4) hArr 2NO_(2)`
The mixture is usually called by the name of the form which predominates at the temperature,neither form is a true peroxide, and it is incorrect to refer to this substance as nitrogen peroxide for that reason. The degree of dissociation is about `20%` at the boiling point, but vapour density measurements show that the proportion of dioxide increases rapidly as the temperature is raised, reaching nearly `100%` at `140^(@)C` . This is also apparent from the change in colour from yellow to dark brown as the temperature rises.
At higher temperature further dissociation occurs, which explains why nitrogen dioxide supports the combustion of strongly burning materials:
`N_(2)O_(4) hArr underset(at 150^(@)C)underset("complete")(2NO_(2)) hArr 2NO+underset(at 600^(@)C)underset("complete")(O_(2))`
The dark colour, which is caused by the `NO_(2)` molecules, is at its deepest at about `150^(@)C` and fades to colourless above `6006(@)C` reappearing on cooling. The arrows stress the reversibility of these changes, which provide good examples of thermal dissociation. The equilibrium mixture of the two oxides is strongly acidic, very soluble in water, forming a mixture of nitrous and nitric acide, of which it the mixed anhydride:
`N_(2)O_(4) hArr 2NO_(2) overset(H_(2)O(l))(rarr)HNO_(2)(aq.)+HNO_(3) (aq.)`
