Zn gives $H_2$ gas with $H_2SO_4$ and HCl but not with $HNO_3$ because :-
A
Zn acts as oxidising agent when it reacts with $HNO_3$
B
$HNO_3$ is weaker acid than $H_2SO_4$ and HCl
C
In electrochemical series Zn is above hydrogen
D
$NO_3^-$ is reduced in preference to hydronium ion
The answer is D.
Solution
With dilute $H_2SO_4$ or HCl, zinc reduces $H^+$ (hydronium) ions to hydrogen gas: $Zn + 2H^+ \rightarrow Zn^{2+} + H_2$. Nitric acid is a strong oxidising agent. The nitrate ion is reduced more easily than $H^+$ (it has a higher reduction potential). So zinc reduces $NO_3^-$ to oxides of nitrogen, $N_2O$, or $NH_4^+$ instead of releasing hydrogen; any hydrogen formed is also oxidised to water. Example: $4Zn + 10HNO_3(dil) \rightarrow 4Zn(NO_3)_2 + N_2O + 5H_2O$ $HNO_3$ is a strong acid, and Zn acts as a reducing agent, not an oxidising agent. So $NO_3^-$ is reduced in preference to the hydronium ion.
ConceptNitrate reduced instead of H+
The p-Block Elements in past papers
77 questions from this chapter have appeared across 23 exam years.
