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Which ion cannot catalyse the reaction between iodide (I extsuperscript{-}) and peroxodisulfate (S extsubscript{2}O extsubscript{8} extsuperscript{2-})? Use the data below to help you answer this question - AQA - A-Level Chemistry - Question 29 - 2018 - Paper 3
Question 29
Which ion cannot catalyse the reaction between iodide (I extsuperscript{-}) and peroxodisulfate (S extsubscript{2}O extsubscript{8} extsuperscript{2-})? Use the dat... show full transcript
Worked Solution & Example Answer:Which ion cannot catalyse the reaction between iodide (I extsuperscript{-}) and peroxodisulfate (S extsubscript{2}O extsubscript{8} extsuperscript{2-})? Use the data below to help you answer this question - AQA - A-Level Chemistry - Question 29 - 2018 - Paper 3
Step 1
Identify the relevant half-equations and standard reduction potentials
Answer
From the data provided, we have:
- S extsubscript{2}O extsubscript{8} extsuperscript{2-} + 2e extsuperscript{-} → 2SO extsubscript{4} extsuperscript{2-} | E extsuperscript{o} = +2.01 V
- Co extsuperscript{2+} + 2e extsuperscript{-} → Co | E extsuperscript{o} = +1.82 V
- Fe extsuperscript{2+} + 2e extsuperscript{-} → Fe | E extsuperscript{o} = +0.77 V
- I extsuperscript{-} + 2e extsuperscript{-} → 2I | E extsuperscript{o} = -0.54 V
- Cr extsuperscript{2+} + 2e extsuperscript{-} → Cr | E extsuperscript{o} = -0.41 V
Step 2
Evaluate which ion cannot act as a catalyst
Answer
For a species to act as a catalyst in the reaction, it must have a higher standard reduction potential than the species it is trying to reduce.
- Co extsuperscript{2+} and Fe extsuperscript{2+} can act efficiently because their E extsuperscript{o} values are positive, indicating a strong ability to gain electrons compared to I extsuperscript{-}.
- Cr extsuperscript{2+} has a negative E extsuperscript{o} value, making it less effective but still feasible as a catalyst.
However, Co extsuperscript{3+} (which could be considered from Co extsuperscript{2+}) does not appear listed but would also not catalyze since it does not have a favorable reduction potential compared to I extsuperscript{-}. Thus:
The answer is D Fe extsuperscript{3+} since it cannot catalyze the reaction.