Question

The standard reduction potentials of \(2H^+ / H_2\), \(Cu^{2+} / Cu\), \(Zn^{2+} / Zn\), and \(NO_3^- / HNO_2\) are 0.0, +0.34, -0.76, and +0.97 V respectively. Observe the following reactions:
I. \( Zn + HCl \rightarrow \)
II. \( Cu + HCl \rightarrow \)
III. \( Cu + HNO_3 \rightarrow \)
Which reactions do not liberate \(H_2\) gas?

• A. II, III only
• B. I, II only
• C. I, III only
• D. I, II, III

Hint:

- Metals with a reduction potential higher than \( H_2 \) do not liberate hydrogen gas from acids. - Copper does not react with HCl but reacts with oxidizing acids like \( HNO_3 \), producing \( NO \) or \( NO_2 \) instead.

Answer 1

The Correct Option is A

Step 1: Understanding the reaction conditions - The ability of a metal to liberate \( H_2 \) from acid depends on its reduction potential. - A metal with a lower standard reduction potential than \( H_2 \) can displace \( H_2 \) from acid, whereas metals with a higher reduction potential cannot.
Step 2: Analysis of each reaction - Reaction I: \( Zn + HCl \rightarrow ZnCl_2 + H_2 \)
- Zinc has a lower reduction potential (-0.76 V) than \( H_2 \), so it can displace \( H_2 \). - \(\Rightarrow\) \( H_2 \) gas is liberated. - Reaction II: \( Cu + HCl \rightarrow \) No reaction.
- Copper has a higher reduction potential (+0.34 V) than \( H_2 \), so it does not react with HCl. - \(\Rightarrow\) \( H_2 \) gas is not liberated. - Reaction III: \( Cu + HNO_3 \rightarrow \) NO gas instead of \( H_2 \).
- \( NO_3^- \) acts as an oxidizing agent, reducing to \( NO \). - \(\Rightarrow\) \( H_2 \) gas is not liberated. 
Final Answer: Reactions II and III do not liberate \( H_2 \).