Which of the following is a redox reaction?

Options

A. `NaCl + KNO_(3) to NaNO_(3) + KCl`
B. `CaC_(2)O_(4) + 2HCl to CaCl_(2) + H_(2)C_(2)O_(4)`
C. `Ca(OH)_(2) + 2NH_(4)Cl to CaCl_(2) + 2NH_(3) + 2H_(2)O`
D. `2K[Ag(CN)_(2)] + Zn to 2Ag + K_(2)[Zn(CN)_(4)]`

Answer

D

Explanation

(a) `overset(+1)(Na) overset(-1)(Cl) + overset(+1)K overset(-1)(NO_(3)) to overset(+1)(Na)overset(-1)(NO_(3)) + overset(+1)Koverset(-1)(Cl)`
Change in oxidation number.
(b) `overset(+2)CaC_(2)overset(-2)O_(4) + 2overset(+1)Hoverset(-1)(Cl) to overset(+2)(Ca) overset(-1)(Cl_(2)) + overset(+1)(H_(2))C_(2) O_(4)^(2-)`
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) `2K[Ag(CN)_(2)]+ Zn to 2Ag + K_(2)[Zn(CN)_(4)]`
`Ag^(+) to Ag` gain of `e^(-)`, reduction
`Zn to Zn^(2+)` loss of `e^(-)`, oxidation.