# Which of the following is a redox reaction? Option 1) $NaCI+KNO_{3}\rightarrow NaNO_{3}+KCI$ Option 2) $CaC_{2}O_{4}+2HCI\rightarrow CaCI_{2}+H_{2}C_{2}O_{4}$ Option 3) $Mg(OH)_{2}+2NH_{4}CI\rightarrow MgCI_{2}+2NH_{4}OH$ Option 4) $Zn+2AgCN\rightarrow 2Ag+Zn(CN)_{2}$

As we learnt in

Redox Reaction -

Redox reaction as a class of reactions in which oxidation and reduction reactions occur simultaneously.

-

and,

Oxidation Number -

Oxidation number denotes the oxidation state of an element in a compound ascertained according to a set of rules formulated on the basis that electron pair in a covalent bond belongs entirely to more electronegative elements.

-

In a Redox Reaction,  the oxidation states of the constituents should change.

Keeping that in mind, let us look at each reaction:

$\overset{+1}{Na}\overset{-1}{Cl} +\overset{H}{K}\overset{+5}{N}\overset{-2}{O}_{3}\rightarrow\overset{+1}{Na}\overset{+5}{N}\overset{-2}{O}+\overset{+1}{K}\overset{-1}{Cl}$

Clearly, there is no change in the oxidation states.

$\overset{2+}{Ca}\overset{+3}{C}_{2}\overset{-2}{O}_{4}\:+\:2\overset{+1}{H}\overset{-2}{Cl}\rightarrow\overset{2+}{Ca}\overset{-1}{Cl}+\overset{+1}{H}_{2}\overset{+3}{C}_{2}\overset{-2}{O}_{4}$

Clearly, there is no change in the oxidation states.

$\overset{+2}{Mg}(\overset{-2}{O}\overset{+1}{H})_{2}+2\overset{-3}{N}\overset{+4}{H}_{4}Cl\rightarrow\overset{+2}{Mg}\overset{-1}{Cl}_{2}+2\overset{-3}{N}\overset{+1}{H}_{4}\overset{-2}{O}\overset{+1}{H}$

Clearly, there is no change in the oxidation states.

Finally ,

$\overset{0}{Zn}+2\overset{+1}{Ag}\overset{+2}{C}\overset{-3}{N}_{2}\rightarrow2\overset{0}{Ag}+\overset{+2}{Zn}(\overset{+2}{C}\overset{-3}{N})_{2}$

Clearly, there is a change in the oxidation status of the constituents.

$\therefore$The last reaction is redox.

The oxidation states show a change only in reaction (d).

Option 1)

$NaCI+KNO_{3}\rightarrow NaNO_{3}+KCI$

This option is incorrect.

Option 2)

$CaC_{2}O_{4}+2HCI\rightarrow CaCI_{2}+H_{2}C_{2}O_{4}$

This option is incorrect.

Option 3)

$Mg(OH)_{2}+2NH_{4}CI\rightarrow MgCI_{2}+2NH_{4}OH$

This option is incorrect.

Option 4)

$Zn+2AgCN\rightarrow 2Ag+Zn(CN)_{2}$

This option is correct.

### Preparation Products

##### JEE Main Rank Booster 2021

This course will help student to be better prepared and study in the right direction for JEE Main..

₹ 13999/- ₹ 9999/-
##### Knockout JEE Main April 2021 (Subscription)

An exhaustive E-learning program for the complete preparation of JEE Main..

₹ 6999/- ₹ 4999/-
##### Knockout JEE Main April 2022 (Subscription)

Knockout JEE Main April 2022 Subscription.

₹ 8999/- ₹ 5999/-
##### Knockout JEE Main April 2021

An exhaustive E-learning program for the complete preparation of JEE Main..

₹ 22999/- ₹ 14999/-
##### Knockout JEE Main April 2022

An exhaustive E-learning program for the complete preparation of JEE Main..

₹ 34999/- ₹ 24999/-