Hydrogen gas is not evolved when most metals react with nitric acid, yet it IS evolved when magnesium reacts with very dilute nitric acid. What property of nitric acid accounts for the usual absence of hydrogen gas, and what does the exception with very dilute HNO₃ and magnesium suggest about the conditions needed for this property to operate?
Generated by claude-sonnet-4-6 · 2026-06-26 01:05 · grounding rag
Model Answer
Property of HNO₃: Nitric acid is a strong oxidising agent. It oxidises the hydrogen gas produced to water and itself gets reduced to nitrogen oxides (N₂O, NO, NO₂). Therefore, H₂ is not evolved when metals react with HNO₃.
Exception with Mg and very dilute HNO₃: When the acid is extremely dilute, its oxidising ability is insufficient to oxidise all the hydrogen produced. Magnesium is also highly reactive (above H in activity series), reacting fast enough that H₂ escapes before it can be oxidised. This shows that the oxidising property of HNO₃ operates effectively only at sufficient concentration.
Source: Chapter 3, Section 3.2.3
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Explanation
- The key term examiners look for is "strong oxidising agent" — this is the direct reason H₂ is not evolved.
- Mention that H₂ is oxidised to water and HNO₃ is reduced to nitrogen oxides.
- For the exception part, link it to very dilute conditions reducing the oxidising power, and high reactivity of Mg. Both parts together earn full marks.
- Don't write lengthy chemistry; one equation is not required here — the explanation in words suffices for 3 marks.