Why Magnesium , Manganese does react with dilute HNO3 to evolve hydrogen gas

while other metals dont produce hydrogen gas when reacted with HNO3

All metals are oxidizing in nature, they react with most of acids to produce hydrogen gas by adding electrons to hydrogen ion, while doing so they are getting oxidized to positive metal ion and are reducing the hydrogen ion to hydrogen gas.

But nitrate ions are also easily reduced to product like nitrogen oxide and nitrogen dioxide. So the metals reacting with HNO3 tend to produce oxides of nitrogen rather than Hydrogen gas.

3Cu + 4HNO3 ---->3 Cu (NO3)2 + H2O

i.e... Because nitric acid is a very strong oxidizing agent, hydrogen gas is rarely formed during the reaction with metals.

 

When nitric acid reacts with Mg or Mn it produces hydrogen gas

Mg + HNO-------> Mg (NO3)2 + H2

From the reaction we can see that,  loss of electron takes place which implies that HNO3 OXIDISES Mg. Mg is are reducing agent , hence hydrogen is reduced due to reducing nature of Mg .

Mg and Mn are highly reactive metals, they are able to displace hydrogen from the nitric acid, however, less reactive metals do not displace hydrogen gas but get oxidized due to strong oxidising nature of nitric acid.

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magnesium and maganese react with very dilute nitric acid to evolve hydrogen gas as weak nitric acid is not able to oxidize the hydrogen evolved in the reaction to water.

and the other reason is that maganese and magnesium are quite reactive ,metals so they are able to displace hydrogen from the acid 

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