Answer:
None of the above could take place at the anode.
Explanation:
In a cell, a redox reaction takes place, so one substance must reduce (gain electrons), and others must oxidize (lose electrons). At the anode, the substance oxidizes, and at the cathode the substance reduces.
Thus, we need to find what transformation oxidation is happening. To this, let's calculate the oxidation number (nox) of the atoms. If it's increasing, and oxidation is happening, if it's decreasing, a reduction is happening.
O₂ to H₂O:
O2 is a simple substance, so it has nox = 0. In a compound, O has nox -2 and H +1, so the nox of O is decreasing.
Cr₂O₇⁻² → Cr⁺²:
O has nox -2, so let's call the nox of Cr as x:
2x +7*(-2) = -2
2x -14 = -2
x = +6
And Cr⁺² has nox +2, so it's reducing.
F₂ to F⁻:
F₂ is a simple substance, so F has nox 0, and F⁻ has nox -1, then it's reducing.
HAsO₂ to As:
H has nox +1, and O has nox -2, so calling x the nox of As:
+1 + x + 2*(-2) = 0
1 + x - 4 = 0
x = +3
And As is a simple substance, that has nox 0, the it's reducing.
Thus, none of them is oxidizing, and none of them could take place at the anode.
