Which reaction is NOT a redox reaction?

Solution & Explanation

\textbf{Answer:} (c) $\mathrm{BaCl}{}_2+\mathrm{Na}{}_2\mathrm{SO}{}_4\to \mathrm{BaSO}{}_4+2\mathrm{NaCl}$. \textbf{Solution:} In $\mathrm{BaCl}{}_2+\mathrm{Na}{}_2\mathrm{SO}{}_4\to \mathrm{BaSO}{}_4+2\mathrm{NaCl}$ there is no change in the oxidation state of any element. It is a double-displacement (precipitation) reaction, hence not a redox reaction. The others all involve oxidation-state changes: in (a) $\mathrm{Cl}$ goes $+5\to 0$ and $\mathrm{I}$ goes $0\to +5$; in (b) $\mathrm{H}$ goes $0\to +1$ and $\mathrm{Cl}$ goes $0\to -1$; in (d) $\mathrm{Zn}$ goes $0\to +2$ and $\mathrm{Cu}$ goes $+2\to 0$.