I know that specific metals like $\ce{Pb}, \ce{Sn}$ and $\ce{Al}$ become passive in conc. nitric acid. My questions are:

  • Why does this happen only with conc. nitric acid, and not other conc. acids?
  • Why is the passivity specific to these metals? Do they have something in common or are there individual reasons?

1 Answer 1



This link states that:

Aluminium metal dissolves readily in dilute sulphuric acid to form solutions containing the hydrated Al(III) ion together with hydrogen gas, $\ce{H2}$. The corresponding reactions with dilute hydrochloric acid also give the hydrated Al(III) ion. Concentrated nitric acid passivates aluminium metal (due to formation of thin protective oxide layer, $\ce{Al2O3}$ on surface of metal which cuts off further reaction. )

$$\ce{2Al(s) + 3H2SO4(aq) → 2Al^3+(aq) + 2SO4^2-(aq) + 3H2(g)}$$ $$\ce{2Al(s) + 6HCl(aq) → 2Al^3+(aq) + 6Cl-(aq) + 3H2(g)}$$


This link states that:

The surface of metallic lead is protected by a thin layer of lead oxide, $\ce{PbO}$. This renders the lead essentially insoluble in sulphuric acid. Lead reacts slowly with hydrochloric acid and nitric acid. In the latter case, nitrogen oxides are formed together with lead(II) nitrate, $\ce{Pb(NO3)2}$.

$$\ce{Pb + HCl -> H2[PbCl4] + H2}$$ $$\ce{Pb + 4 HNO3 → Pb(NO3)2 + 2 NO2 + 2 H2O}$$


This link states that:

Tin is attacked only slowly by dilute acids such as hydrochloric acid and sulfuric acid. [...]Tin dissolves easily in concentrated acids.

$$\ce{Sn + 2H2SO4 → SnSO4 + SO2 + 2H2O}$$ $$\ce{Sn + 3HCl → H[SnCl3] + H2}$$ $$\ce{Sn + 4HNO3 → H2SnO3 + 4NO2 + H2O}$$

Summarizing as a table, enter image description here

Do they have something in common or are there individual reasons?

The three metals do not have common properties and reacts with different acids differently as we can see in the table.


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