Manganese dioxide (MnO₂) reacts with hydrochloric acid (HCl) to produce magnesium chloride (MgCl₂), water (H₂O), and chlorine (Cl₂) gas. The chemical equation involved in the reaction is given as:

MnO₂ + 4 HCl → MnCl₂ + 2 H₂O + Cl₂

Which of the following substances is reduced in the given reaction?

Understanding Redox Reactions

• Oxidation: A substance loses electrons, gains oxygen, or loses hydrogen.
• Reduction: A substance gains electrons, loses oxygen, or gains hydrogen.

Step-by-Step Analysis of the Reaction

1. Reactants and Products:
   • MnO₂: Manganese dioxide.
   • HCl: Hydrochloric acid.
   • Products: MnCl₂, H₂O, and Cl₂.
2. Role of MnO₂:
   • MnO₂ starts as manganese dioxide, which contains manganese in a +4 oxidation state.
   • In MnCl₂, manganese transitions to a +2 oxidation state.
   • The reduction of MnO₂ occurs because it loses oxygen during the reaction, thereby gaining electrons.

3. Reduction Process:

   The loss of oxygen from MnO₂ results in the formation of MnCl₂. This loss of oxygen qualifies MnO₂ as the reduced substance in this redox reaction.

4. Oxidation Process (for comparison):

   HCl is oxidized as chlorine in Cl₂ changes from -1 to 0 oxidation state.

Key Takeaway

In this reaction:

• Reduced Substance: MnO₂ (loses oxygen to form MnCl₂).
• Oxidized Substance: HCl (chlorine is oxidized to form Cl₂).

Hence, the correct answer is MnO₂.