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Standard reduction potentials are given as: Co³+ + e- -> Co²+; E° = +1.82 V O2 + 4H+ + 4e- -> 2H2O; E° = +1.23 V Which statement correctly explains why Co³+ is not stable in aqueous solution?

1. Co³+ is a stronger oxidant than O2/H2O, so it oxidises water to O2 and is itself reduced to Co²+
2. Co³+ reduces water to H2 gas while being oxidised to Co⁴+
3. Co³+ disproportionates into Co metal and Co²+ in water
4. Co³+ simply hydrolyses without any redox change in oxidation state

Correct answer: Co³+ is a stronger oxidant than O2/H2O, so it oxidises water to O2 and is itself reduced to Co²+

Solution

The Co³+/Co²+ couple has E° = +1.82 V, which is well above the O2/H2O couple at +1.23 V. A higher reduction potential means Co³+ is the stronger oxidising agent. It therefore drives the oxidation of water to oxygen, getting reduced to Co²+ in the process. The net reaction has a positive EMF, so it is spontaneous and Co³+ cannot persist in aqueous medium.

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