Why Magnesium and Manganese React with Dilute HNO3 to Evolve H2 Gas

Introduction:
Magnesium (Mg) and manganese (Mn) are two metals that react with very dilute nitric acid (HNO3) to evolve hydrogen gas (H2). This is in contrast to other metals that do not react with nitric acid to form hydrogen gas. The reason behind this lies in the unique properties of nitric acid as a strong oxidizing agent.

Properties of Nitric Acid:
1. Nitric acid is a strong oxidizing agent that readily donates oxygen atoms.
2. It can accept electrons from other substances and reduce them.
3. Nitric acid is capable of oxidizing most metals, converting them into their respective metal ions.

Reaction with Magnesium and Manganese:
When magnesium or manganese reacts with very dilute nitric acid, the following reactions occur:

1. Magnesium (Mg):
- Magnesium reacts with nitric acid to produce magnesium nitrate (Mg(NO3)2) and hydrogen gas (H2).
- The balanced chemical equation for this reaction is:
2Mg + 4HNO3 → 2Mg(NO3)2 + H2

2. Manganese (Mn):
- Manganese also reacts with nitric acid to produce manganese nitrate (Mn(NO3)2) and hydrogen gas (H2).
- The balanced chemical equation for this reaction is:
Mn + 2HNO3 → Mn(NO3)2 + H2

Reason for Reaction:
The reaction between magnesium/manganese and nitric acid occurs because nitric acid acts as both an acid and an oxidizing agent. In the case of magnesium and manganese, these metals are oxidized by nitric acid. The nitrate ion (NO3-) from nitric acid acts as an oxidizing agent, accepting electrons from the metal atoms.

Formation of Hydrogen Gas:
The oxidation of magnesium and manganese leads to the formation of metal ions, such as Mg2+ and Mn2+. Simultaneously, hydrogen gas is evolved as a result of the reduction of nitric acid. The hydrogen ions (H+) present in nitric acid are reduced to form hydrogen gas (H2) by accepting electrons.

Other Metals and Nitric Acid:
Other metals, such as copper, silver, gold, and platinum, do not react with nitric acid to produce hydrogen gas. This is because these metals are more resistant to oxidation by nitric acid due to their inherent stability and low reactivity. Nitric acid is not strong enough to oxidize these metals and convert them into metal ions.

Conclusion:
The unique properties of nitric acid as a strong oxidizing agent allow it to react with metals like magnesium and manganese. These metals are oxidized by nitric acid, leading to the evolution of hydrogen gas. However, other metals do not react with nitric acid in the same way due to their inherent stability and low reactivity.