Consult the table of standard electrode potentials and suggest three substances that can oxidise ferrous ions under suitable conditions.
The three substances that can oxidise ferrous ions (Fe²⁺) are:
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Atmospheric Oxygen (O₂): The context explicitly states that "The ferrous ions are further oxidised by atmospheric oxygen to ferric ions." Oxygen has a high standard electrode potential (E° = 1.23 V for the reaction O₂ + 4H⁺ + 4e⁻ → 2H₂O).
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Bromine (Br₂): From question 2.17(v), Br₂ is mentioned with Fe²⁺, indicating that bromine can oxidise ferrous ions to ferric ions.
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Any substance with higher reduction potential than Fe³⁺/Fe²⁺ couple: According to the context, as we go from top to bottom in the table of standard electrode potentials, the oxidising power decreases. Therefore, substances placed above the Fe³⁺/Fe²⁺ couple in the electrochemical series (having higher E° values) can act as stronger oxidising agents and oxidise ferrous ions.
Explanation
The question requires understanding the relationship between standard electrode potential and oxidising power. The context clearly states that atmospheric oxygen oxidises ferrous ions to ferric ions. Additionally, the principle explained in the context—that oxidising power decreases as we move down the electrochemical series—helps identify other suitable oxidising agents. Substances with higher standard reduction potentials than the Fe³⁺/Fe²⁺ couple can oxidise Fe²⁺ to Fe³⁺. The mention of Br₂ with Fe²⁺ in question 2.17 further supports this.
Solution Steps
Step 1: Identify that a substance can oxidise ferrous ions if it has a higher standard reduction potential than the Fe³⁺/Fe²⁺ couple.
Step 2: From the context, note that atmospheric oxygen (O₂) explicitly oxidises ferrous ions.
Step 3: Identify bromine (Br₂) as another suitable substance from question 2.17(v).
Step 4: Apply the general principle that substances with higher E° values (stronger oxidising agents) can oxidise Fe²⁺.