Two iron rod A and B are taken. A is dipped in dilute H2SO4. B is first dipped in conc. HNO3 and later in dilute H2SO4. What will be the status of iron rods after some time? Justify.
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Solution
Generally, Iron rod when dipped in concentrated H2SO4, then ferric sulphate forms.
3Fe+8H2SO4→FeSO4+Fe2(SO4)3+4SO2+8H2O
The rod A is directly dipped. So A forms Fe2(SO4)3
But rod B does not form anything with concentrated H2SO4. Because a passive film of oxide is formed on the surface by reacting rod B with concentrated HNO3