Why is hydrogen gas not produced when most metals react with nitric acid, even though it is produced when the same metals react with hydrochloric acid?
Generated by claude-sonnet-4-6 · 2026-06-26 01:03 · grounding rag
Model Answer
HNO₃ is a strong oxidising agent. When a metal reacts with nitric acid, the hydrogen gas produced is immediately oxidised to water by HNO₃, which itself gets reduced to nitrogen oxides (N₂O, NO, NO₂). HCl is not an oxidising agent, so hydrogen gas is released freely.
(Exception: Mg and Mn react with very dilute HNO₃ to produce H₂.)
Source: Chapter 3, Section 3.2.3
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Explanation
- The key phrase examiners look for: "HNO₃ is a strong oxidising agent" — this must appear.
- Then explain the consequence: H₂ is oxidised to water, and HNO₃ is reduced to nitrogen oxides.
- Contrast with HCl: HCl has no oxidising property, so H₂ escapes.
- Mentioning the exception (Mg/Mn with very dilute HNO₃) is a bonus point but not compulsory for 2 marks.