Question

Which of the following is a redox reaction?

• A. $NaCl + KNO _{3} \longrightarrow NaNO _{3}+ KCl$
• B. $CaC _{2} O _{4}+2 HCl \longrightarrow CaCl _{2}+ H _{2} C _{2} O _{4}$
• C. $Ca ( OH )_{2}+2 NH _{4} Cl \longrightarrow CaCl _{2}+2 NH _{3}$
• D. $2 K \left[ Ag ( CN )_{2}\right]+ Zn \longrightarrow 2 Ag$ $+ K _{2}\left[ Zn ( CN )_{4}\right]$

Answer 1

Answer: (D)

(a) $\overset{+1}{Na}\overset{-1}{Cl} + \overset{+1}{K}\overset{-1}{NO_3} \to \overset{+1}{Na}\overset{-1}{NO_3}+\overset{+1}{K}+\overset{-1}{Cl}$

(b) $\overset{+2}{Ca}C_2\overset{-2}{O_4}+ 2\overset{+1}{H}\overset{-1}{Cl} \to \overset{+2}{Ca}\overset{-1}{Cl_2}+\overset{+1}{H_2}C_2\overset{-2}{O_4}$

(c)$\overset{+2}{Ca}\overset{-1}{(OH)_2} + \overset{-3}{2N}\overset{+1}{H_4}\overset{-1}{Cl} \to \overset{+2}{Ca}\overset{-1}{Cl_2}+\overset{-3}{2N}\overset{-1}{Cl_2}+\overset{-3}{2N}\overset{+1}{H_3}+2\overset{+1}{H_2}\overset{-2}{O}$

in all these cases during reaction, there is no change in oxidation state of ion or molecule or constituent atom, these are simply ionic reactions.

(d) $2 K \left[ Ag ( CN )_{2}\right]+ Zn \rightarrow 2 Ag + K _{2}\left[ Zn ( CN )_{4}\right]$

$Ag ^{+} \rightarrow$ Ag gaining of $e^{-},$ reduction $Zn \rightarrow Zn ^{2+}$ loss of $e ^{-}$, oxidation