In the following redox reaction: Cr2O7^{2-} + 3 HNO2 + 5 H^+ → 2 Cr^{3+} + 3 NO3^- + 4 H2O, which substance acts as the reducing agent?

• A. NO3^-
• B. Cr^{3+}
• C. Cr2O7^{2-}
• D. HNO2

Answer: (D)

Verified step by step guidance

1. Identify the oxidation states of the elements involved in the reaction to determine which species is oxidized and which is reduced. For example, find the oxidation state of chromium in Cr2O7^{2-} and Cr^{3+}, and nitrogen in HNO2 and NO3^-.
2. Determine which species undergoes oxidation (increase in oxidation state) and which undergoes reduction (decrease in oxidation state). The species that loses electrons is oxidized and acts as the reducing agent.
3. Check the changes in oxidation states: Chromium in Cr2O7^{2-} is reduced to Cr^{3+}, so Cr2O7^{2-} is the oxidizing agent.
4. Look at HNO2 and NO3^-: Nitrogen in HNO2 is oxidized to nitrogen in NO3^-, indicating HNO2 loses electrons and is therefore the reducing agent.
5. Conclude that the reducing agent is the species that donates electrons (is oxidized), which in this reaction is HNO2.