Question

metals generally do not give hydrogen gas when react with HNO3 why

Answer 1

Heya
Here's your answer !!

Generally when metals are treated with mineral acids hydrogen gas is liberated but when metals (except Mn and Mg) are treated with HNO3 , hydrogen is not liberated.

This is because Nitric acid ( HNO3 ) is a powerful oxidising agent , and the oxygen formed due to its decomposition oxidises the hydrogen to give water thus defeating the purpose of the reaction .

3Zn + 8HNO3 -----> 3Zn(NO3)2 + 4H20 + 2NO (g) .

Even Magnesium and manganese react with very dilute nitric acid ( 1% nitric acid ) to liberate hydrogen . The oxidising action of the acid is much reduced due to its over dilution .

Mg + 2HNO3 ----> Mg(NO3)2 + H2 (g) .

Hope it helps !!