Why does nitric acid gives hydrogen only with magnesium and manganese
HNO3 is a very strong oxidising agent. On reacting with other metals, it oxidises their outer layer and renders them passive(unreactive)
HNO3(about 1% concentrated) doesn’t oxidise Mg and Mn a lot and so it reacts with them as you would expect an acid to react, releasing hydrogen.
For more details go through below description,
Technically, metal + acid ----> metal salt +water.
But when we look at HNO3, it has a tendency to form H2O and not H2 when it reacts with metals.
But, Magnesium and Mangenese are two metals that traditionally, liberate H2 and not H2O when they react with HNO3. This is because, when they initially react, they do form H2O, but the Mg or Mn metals are strong reducing agents and hence they reduce H2O and themselves oxidise. This leaves them with only H2 gas, instead of H2O.
Hence, when Mg/Mn reacts with Nitric acid to form a salt and H2O, but the H2O is quickly (almost instantaneously) reduced and converted to H2 gas.