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 liberating H2 from dilute acid.
An ion can reduce \( H^+ \) to \( H_2 \) only if its standard reduction potential is sufficiently negative (the ion is a strong reducing agent).
Step 2: Check Ti2+.
The \( Ti^{3+}/Ti^{2+} \) reduction potential is about \( -0.37 \) V. This is negative, so \( Ti^{2+} \) CAN reduce \( H^+ \) to \( H_2 \).
Step 3: Check Cr2+.
The \( Cr^{3+}/Cr^{2+} \) reduction potential is about \( -0.41 \) V. This is negative, so \( Cr^{2+} \) also CAN liberate \( H_2 \) from dilute acid.
Step 4: Check Mn3+.
The \( Mn^{3+}/Mn^{2+} \) reduction potential is about \( +1.51 \) V. Strongly positive, so \( Mn^{3+} \) is an oxidising agent and cannot reduce \( H^+ \).
Step 5: Check Co3+.
The \( Co^{3+}/Co^{2+} \) reduction potential is about \( +1.92 \) V. Strongly positive, so \( Co^{3+} \) is an oxidising agent and cannot liberate \( H_2 \).
Step 6: State the answer.
Only \( Ti^{2+} \) and \( Cr^{2+} \) can reduce dilute acid to give hydrogen gas. \[ \boxed{Ti^{2+}, \ Cr^{2+}} \]