Which compound can be used as a reducing agent in a reaction?

Analyze the question

A reducing agent is a substance that loses (donates) electrons and is itself oxidized. It does this most easily when an element in it is in a low or intermediate oxidation state and can move to a higher one.

Check each option

• A. Cl₂: Chlorine is a strong oxidizing agent. It readily gains electrons to form Cl⁻. Not a reducing agent.
• B. KMnO₄: Mn is at +7, its highest oxidation state, so it can only be reduced. It is a strong oxidizing agent.
• C. K₂Cr₂O₇ (written "KCr2O7" in the question, which is a typo): Cr is at +6, its highest state. It is an oxidizing agent.
• D. MnO₂: Mn is at +4. It usually acts as an oxidizing agent, for example when it oxidizes HCl to Cl₂.
• E. Na₂SO₃: S is at +4, an intermediate state. It can be oxidized to +6 (sulphate), so it acts as a reducing agent:

SO₃²⁻ + H₂O → SO₄²⁻ + 2H⁺ + 2e⁻

For example, sulphite decolourizes acidified KMnO₄ because it reduces Mn⁷⁺ to Mn²⁺.

Answer: Na₂SO₃ is the compound that can be used as a reducing agent.

Tip: Look at the oxidation state of the key element.

• Highest oxidation state (Mn⁺⁷, Cr⁺⁶) means oxidizing agent.
• Intermediate state with a higher one available (S⁺⁴, Fe²⁺) means reducing agent.
• Remember also that halogens like Cl₂ are oxidizing agents.