Question

A redox reaction among the following is
(i) $CdC{l}_2+2KOH\to Cd(OH{)}_2+2KCl$
(ii) $BaC{l}_2+K_{2}S{O}_4\to BaS{O}_4+2KCl$
(iii) $CaC{O}_2\to CaO+C{O}_2$
(iv) $2Ca+O_2\to 2CaO$.

• A. (i)
• B. (ii)
• C. (iii)
• D. (iv)

Answer 1

The correct option is D (iv)

Redox reaction has an oxidising and an reducing specie involved. Calculating the oxidation state of each elements will give a redox system.

$\left(i\right)\stackrel{+2}{Cd}\stackrel{-1}{{Cl}_2}+2\stackrel{+1}{K}\stackrel{-2}{O}\stackrel{+1}{H}\to \stackrel{+2}{Cd}(\stackrel{-1}{OH}{)}_2+2\stackrel{+1}{K}\stackrel{-1}{Cl}$

$\left(ii\right)\stackrel{+2}{Ba}\stackrel{-1}{{Cl}_2}+\stackrel{+1}{K_2}\stackrel{+6}{S}\stackrel{-2}{O_4}\to \stackrel{+2}{Ba}\stackrel{+6}{S}\stackrel{-2}{O_4}+2\stackrel{+1}{K}\stackrel{-1}{Cl}$

$\left(iii\right)\stackrel{0}{Ca}\stackrel{+4}{C}\stackrel{-2}{O_2}\to \stackrel{+2}{Ca}\stackrel{-2}{O}+\stackrel{+4}{C}\stackrel{-2}{O_2}$

$\left(iv\right)\stackrel{0}{2Ca}+\stackrel{0}{O_2}\to 2\stackrel{+2}{Ca}\stackrel{-2}{O}$

As in reaction (iii) only reduction of $Cafrom0to+2$ occurs and in reaction (iv) oxidation number of $Cafrom0to+2$ and $Ofrom0to-2$ both changes where oxidation and reduction both occur simultaneously. Therefore option D is the correct answer.