Question

Balance the below half-reaction, which takes place in acidic solution. cr2o72−(aq) → cr3 (aq) how many electrons are transferred in the balanced half-reaction

Answer 1

The equation is as follow,

                                          Cr₂O₇²⁻    →    Cr³⁺

Balance the Cr atoms,
                                          Cr₂O₇²⁻    →    2 Cr³⁺

Identify Oxidation Number of Elements on Both sides,

Left Hand Side;
                          Cr  =  +6
                           O  =  -2
Right Hand Side;
                          Cr  =  +3
                           O  =  -2

Means each Cr is gaining 3 electrons to change into +3 state, Hence, 6 electrons are gained by two Cr atoms, So,

                                       Cr₂O₇²⁻  +  6e⁻    →    2 Cr³⁺

Balance Oxygen atoms, as it is acidic medium, add H⁺ ion on the side containing large number of Oxygen atoms and H₂O on side containing less number of Oxygen atoms, So,

                                 H⁺  +  Cr₂O₇²⁻  +  6e⁻    →    2 Cr³⁺  +  H₂O

Now Balance H and O,

                          14 H⁺  +  Cr₂O₇²⁻  +  6e⁻    →    2 Cr³⁺  +  7 H₂O

6 electrons are transferred in this reaction.