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Answer please! Which of the following is a redox reaction?

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}

 
Answers (1)
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As we learnt in

Redox Reaction -

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

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 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.

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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.

\thereforeThe 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.

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