Question

Which equation represents an oxidation-reduction reaction?

Answer 1

In an oxidation-reduction reaction there is an exchange of electrons.


The exchange of electrons implies change in the oxidation states: at least one element increases its oxidation number while other reduces it.


By simple ispection you can predict that in the equation b. there is a change in oxidation states of Cl and Mn.


Now you can check it:


Equation             4H   Cl     +    Mn  O2     ->    Mn  Cl2   + 2H2 O     + Cl2    

oxidation sates   1+  1-             4+   2-              2+   1-         1+   2-         0


The oxidation state of Cl in HCl is 1-  and it changed to 0 in Cl2


The oxidation state of Mn in MnO2 is 4+ and it changed to 2+ in MnCl2 


Answer b.