Question

Which of the following is not a redox reaction?
(1) ZnO + C ---------> Zn + CO
(2) MnO2 + 4HCl ---------> MnCl2 + 2H2O + Cl2
(3) CuO + H2 ---------> Cu + H2O
(4) Na2SO4 + BaCl2 ----------> BaSO4 + 2NaCl

Answer 1

The last one is not redox reaction and instead it is a double displacement reaction

Answer 2

answer : option (4)

explanation : redox reaction is the combination two half reactions ; oxidation half reaction and reduction half reaction.

option (1) ZnO + C ---> Zn + CO

we can write it,

$Zn^{+2}O^{-2}+C^0\rightarrow Zn^{0}+C^{2+}O^{-2}$

here it is clear that, oxidation number of Zn reduced to +2 to 0 whereas oxidation number of C increased 0 to +2.

hence, this reaction is a redox reaction.

Similarly, you can check all options !

option (2), $Mn^{+4}O^{-2}_2+4H^{+1}Cl^{-1}\rightarrow Mn^{+2}Cl^{-1}_2+2H_2O+Cl^0_2$

hence, this is also a redox reaction

option (3), $Cu^{+2}O^{-2}+H^0_2\rightarrow Cu^0+H^{+1}_2O^{-2}$

hence, it is clear that , this reaction is also a redox reaction.

option (4), $Na^{+1}_2S^{+6}O^{-2}_4+Ba^{+2}Cl^{-1}_2\rightarrow Ba^{+2}S^{+6}O^{-2}_4+2Na^{+1}Cl^{-1}$

here it is clear that, there doesn't change oxidation number of any element. so, it is not a redox reaction