First slide

Standard EMF of a cell

Question

Zn gives H₂ gas with dil.H₂SO₄ and HCl but not with dil.HNO₃ because

Moderate

A

Zn acts as oxidising agent when reacts with HNO₃
B

HNO₃ is weaker acid than H₂SO₄ and HCl
C

In electrochemcial series Zn is above hydrogen
D

${NO}_{3}^{-}$ is reduced in preference to $H^{+}$ ion

Solution

Zn + H₂SO₄ → ZnSO₄ + H₂↑
Zn + 2HCl → ZnCl₂ + H₂↑
$E_{H^{+} / \frac{1}{2} H_{2,} (pt)}^{°}$ = 0 (Arbitrarily taken as zero)
Where as $E_{{NO}_{3}^{-} / {NO}_{2}}^{°}; E_{{NO}_{3}^{-} / N_{2} O}^{°}$ are positive. Therefore, NO₃^- reduces to oxide nitrogen but not H⁺ to H_2.
Thus NO₃^- ion reduces in preference to H⁺ ion.
4Zn + 10 HNO₃(dil ) $\to$ 4 Zn(NO₃)₂ + 5H₂O +N₂O
Zn + 4 HNO₃ (conc) $\to$ Zn(NO3)₂ + 2H₂O +2NO₂

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