Consider the reaction $$ ext{MnO}_2(s) + 4 ext{HCl}(aq) \rightarrow \text{Cl}_2(g) + 2 ext{H}_2 ext{O} + ext{MnCl}_2(aq) $$ The atoms whose oxidation numbers change during this reaction are A - VCE - SSCE Chemistry - Question 6 - 2004 - Paper 1

To determine which atoms have changing oxidation states, we first assign oxidation numbers based on common rules.

1. Identify oxidation states in reactants:

   • In ext{MnO}_2, the oxidation state of Mn is +4 (O is -2).
   • In ext{HCl}, Cl is -1 and H is +1.

2. Identify oxidation states in products:

   • In ext{Cl}_2, Cl is 0 (elemental state).
   • In ext{H}_2 ext{O}, H is +1 and O is -2.
   • In ext{MnCl}_2, Mn is +2 and Cl is -1.

3. Analyze the changes:

   • Mn changes from +4 in ext{MnO}_2 to +2 in ext{MnCl}_2.
   • Cl changes from -1 in ext{HCl} to 0 in ext{Cl}_2 (therefore it is oxidized).
   • The oxidation state of O does not change (remains -2 in both reactants/products).
   • H remains +1 in both the reactant ext{HCl} and product ext{H}_2 ext{O}.

Based on this analysis, the appropriate answer is: B. Mn, Cl and O. However, since the final correct option focuses only on changes in oxidation numbers for Mn and Cl, we conclude with B. Mn and Cl as correct; thus, re-examining the options leads to the most accurate identification of the Changing oxidation states are from B. Mn and Cl.