Question

${{\rm{H}}_{\rm{2}}}{\rm{S}}$ acts only as reducing agent while ${\rm{S}}{{\rm{O}}_2}$ can act both as reducing agent and oxidising agent. Explain.

Answer 1

Determine various oxidation states of sulphur. If sulphur is in its lowest oxidation state, then further reduction is not possible. So a sulphur atom in its lowest oxidation state cannot act as an oxidizing agent.

Complete answer:
The atomic number of sulphur is 16 and its electronic configuration is $\left[ {{\rm{Ne}}} \right]{\rm{3}}{{\rm{s}}^{\rm{2}}}{\rm{3}}{{\rm{p}}^{\rm{4}}}$ . Thus one atom of sulphur contains six valence electrons. Sulphur atom shows oxidation states of -2,+2,+4 and +6. The oxidation state of +2 is somewhat unstable.
The lowest oxidation of sulphur is -2. It is obtained when a sulphur atom gains two electrons to form sulphide anion with two negative charges.
The highest oxidation of sulphur is +6. It is obtained when a sulphur atom gives all six valence electrons. An example is a sulphur trioxide molecule. Sulphuric acid molecule is another example of a compound of sulphur with +6 oxidation state.
In hydrogen sulphide molecules, the oxidation state of sulphur is -2. It is the lowest oxidation state of sulphur. Sulphur atom cannot have an oxidation state lower than -2 as it cannot gain more than 2 electrons. In the valence shell of sulphur atom, only 6 electrons are present and sulphur cannot have more than 8 valence electrons as it has only s, and p orbitals. Sulphur does not have d orbitals. Thus, sulphur in -2 oxidation state cannot undergo reduction.So it cannot oxidize other substances. Hence, hydrogen sulphide cannot act as an oxidizing agent. Hydrogen sulphide can only act as a reducing agent, because it is easy to oxidise sulfur in its lowest oxidation state.
The oxidation state of sulphur in sulphur dioxide molecules is +4. Sulphur in +4 oxidation state can be oxidized to a higher oxidation state of +6. Hence, sulphur dioxide can act as a reducing agent. At the same time, sulphur in +4 oxidation state can be reduced to lower oxidation number. Hence, sulphur dioxide can act as an oxidizing agent. Thus, sulphur dioxide can act both as an oxidizing agent and reducing agent.

Note: In a redox reaction, an oxidizing agent itself undergoes reduction. Similarly, a reducing agent itself undergoes oxidation. Increase in the oxidation number is oxidation and decrease in oxidation number is reduction.