Question

Which of the following sets includes all the species that will change the orange colour of \(K_2Cr_2O_7\) in acidic medium?

• A. \(Fe^{2+},\ Sn^{2+},\ I^-,\ S^{2-}\)
• B. \(S^{2-},\ Fe^{3+},\ I^-,\ C_2O_4^{2-}\)
• C. \(Fe^{2+},\ NO_2^-,\ SO_2,\ Sn^{4+}\)
• D. \(Fe^{3+},\ SO_4^{2-},\ S^{2-},\ Sn^{4+}\)

Answer 1

The Correct Option is A

Concept: In acidic medium, \(K_2Cr_2O_7\) (orange) acts as a strong oxidizing agent and is reduced to \(Cr^{3+}\) (green). \[ Cr_2O_7^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H_2O \] Thus any reducing agent will change the orange colour.
Step 1:Check reducing species} Strong reducing agents include: \[ Fe^{2+},\ Sn^{2+},\ I^-,\ S^{2-} \] All of these can reduce dichromate to \(Cr^{3+}\).
Step 2:Check other options}

• \(Fe^{3+}\) and \(Sn^{4+}\) are already oxidized forms.
• \(SO_4^{2-}\) is not a reducing agent.

Thus only option (A) contains all reducing species. \[ \boxed{Fe^{2+},\ Sn^{2+},\ I^-,\ S^{2-}} \]