Zn gives H2 gas with H2SO4 and HCl but not with HNO3 because
(a) Zn acts as an oxidising agent when it reacts with HNO3
(b) HNO3 is weaker acid than H2SO4 and HCl
(c) In electrochemical series, Zn is above hydrogen
(d) NO3- is reduced in preference to hydronium ion
Correct answer: (d) NO3- is reduced in preference to hydronium ion
Explanation:
Zinc gives H2 gas with dil H2SO4/HCl but not with HNO3 because in HNO3, NO3- ion is reduced and give NH4NO3, N2O, NO and NO2 (based upon the concentration of HNO3)
[Zn + 2HNO3 → Zn(NO3)2 + 2H] x 4 (nearly 6%)
HNO3 + 8H → NH3 + 3H2O
NH3 + HNO3 → NH4NO3
4Zn + 10HNO3 → 4Zn(NO3)2 + NH4NO3 + 3H2O
Zn is on the top position of hydrogen in electrochemical series. So Zn displaces H2 from dilute H2SO4 and HCl with liberation of H2.
Zn + H2SO4 → ZnSO4 + H2