Which of the following reagents would oxidize Cr to Cr2 , but not Fe to Fe3 ?

Question

Ca Co Ca2+ Br2 Co2+ Br-

Which of the following reagents would oxidize Cu to Cu2 , but not Cl– to Cl2?

Br2 Ca Co2+ Co Br- Ca2+

Answer 1

For #1, find Cr^2+ +2e ==> Cr in the tables. You want the reverse of this or Cr ==> Cr^2+ + 2e E = +0.91 so anything below it in your table will work. Find Fe ==> Fe^3+ + 3e and you want the reverse of that or
Fe ==> Fe^3+ + 3e = ? I couldn't find that in my table. Thenlocate those elements/ions above. Any thing above Fe will not oxidize it and anything below Cr will oxidize it so those between the two will satisfy the question. Since I couldn't locate Fe I can't put any other numbers on this. One caution, Since Cr ==> Cr^2+ + 2e is an oxidation the other reaction MUST BE A REDUCTION; I.E.,
metal ion + e ==> metal or
non-metal + e ==> non-metal ion which eliminates Co (but perhaps not Co^2+) and Ca(perhaps not Ca^2+) and Br^- (but perhaps not Br2)

Answer 2

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