Question

Which of the following ions can reduce dilute acid to give hydrogen gas?

• A. \(Ti^{2+},\ Cr^{2+}\)
• B. \(Mn^{3+},\ Ti^{2+}\)
• C. \(Mn^{3+},\ Cr^{2+}\)
• D. \(Co^{3+},\ Cr^{2+}\)

Hint:

Lower oxidation state transition metal ions like \(Ti^{2+}\) and \(Cr^{2+}\) often act as strong reducing agents.

Answer 1

The Correct Option is A

Step 1: Condition for liberation of hydrogen gas.
An ion can reduce dilute acid to liberate hydrogen gas if it is a strong reducing agent.
Such ions must have highly negative reduction potentials so that they can reduce: \[ H^+\rightarrow H_2 \]

Step 2: Analyze \(Ti^{2+}\).
\[ Ti^{3+}+e^-\rightarrow Ti^{2+} \] The \(Ti^{2+}\) ion is a strong reducing agent and readily gets oxidized to: \[ Ti^{3+} \] Hence, it can reduce dilute acids to liberate hydrogen gas.

Step 3: Analyze \(Cr^{2+}\).
\[ Cr^{3+}+e^-\rightarrow Cr^{2+} \] \(Cr^{2+}\) is also a strong reducing agent and reduces: \[ H^+ \] to: \[ H_2 \]

Step 4: Analyze remaining ions.
\[ Mn^{3+} \] and \[ Co^{3+} \] are oxidizing rather than reducing ions.
Hence, they cannot liberate hydrogen gas from dilute acids.

Step 5: Final conclusion.
Therefore, the ions that can reduce dilute acids to give hydrogen gas are \[ \boxed{Ti^{2+},\ Cr^{2+}} \]