唔明Nitric acid

唔係一有 conc.nitric acid 就出 NO2 ; dilute nitric acid 就出 NO ??

No.
Concentrated nitric acid gives NO2 gas ONLY when concentrated nitric acid acts as an oxidizing agent. Similarly, dilute nitric acid give NO gas ONLY when dilute nitric acid acts as an oxidizing agent.


Why dilute nitric acid add to magnessium carbonate power 唔出 NO(g)?

The reaction between dilute nitric acid and magnesium carbonate is NOT redox, and thus no NO gas is given off. In the reaction, nitric acid acts as an acid to give carbon dioxide with magnesium carbonate.
2HNO3(aq) + MgCO3(s) → Mg(NO3)2(aq) + H2O(l) + CO2(g)
or: 2H^+(aq) + MgCO3(s) → Mg^2+(aq) + H2O(l) + CO2(g)


Why conc.nitric acid is add dropwise to saturated solution of iron(2)sulphurate
出 NO(g) 而唔出 NO2 ?

Note that the iron(II) sulphate solution contains water. When concentrated acid is added, the acid is diluted by the water in the iron(II) sulphate solution to become dilute nitrate acid. Dilute nitric acid undergoes redox reaction with iron(II) sulphate solution, and the dilute nitric acid is reduced to NO gas.
3Fe^2+(aq) + 4H^+(aq) + NO3^-(aq) → 3Fe^3+(aq) + 2H2O(l) + NO(g)


Why conc.nitric acid is add dropwise to zinc granules 出 NO2 ?

Concentrated acid undergoes redox reaction with Zn, and the concentrated nitric acid is reduced to NO2 gas.
Zn(s) + 4H^+(aq) + 2NO3^-(aq) → Zn^2+(aq) + 2H2O(l) + 2NO2(g)

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