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Redox Reactions

Question
CBSEENCH11006796

Write the anode reaction, the cathode reaction and the net cell reaction for the following cells. Which electrode will be the positive terminal in each cell?
1. space space Zn left parenthesis straight s right parenthesis space vertical line space space Zn to the power of 2 plus end exponent space vertical line vertical line space Br to the power of minus comma space space space Br subscript 2 space vertical line space Pt left parenthesis straight s right parenthesis
2. space space space Cr left parenthesis straight s right parenthesis space vertical line space Cr to the power of 3 plus end exponent space vertical line vertical line space straight I to the power of minus comma space space straight I subscript 2 space straight I space Pt left parenthesis straight s right parenthesis
3. space space space Pt left parenthesis straight s right parenthesis space vertical line space straight H subscript 2 left parenthesis straight g right parenthesis comma space straight H to the power of plus left parenthesis aq right parenthesis space vertical line vertical line space Cu to the power of 2 plus end exponent vertical line space Cu left parenthesis straight s right parenthesis.

Solution

1.   For the cell
    Zn left parenthesis straight s right parenthesis space plus space Zn to the power of 2 plus end exponent space vertical line vertical line space Br to the power of minus comma space space Br subscript 2 vertical line space Pt left parenthesis straight s right parenthesis
the space two space half space reactions space can space be space represented space as colon
left parenthesis straight i right parenthesis space Anode space reaction space left parenthesis oxidation space half space reaction right parenthesis colon
space space space space space space space space space space Zn left parenthesis straight s right parenthesis space space space rightwards arrow space space space Zn to the power of 2 plus end exponent space plus space 2 straight e to the power of minus
left parenthesis ii right parenthesis space Cathode space reaction space left parenthesis Reduction space half space reaction right parenthesis colon
space space space space space space space space space space space space Br subscript 2 plus space 2 straight e to the power of minus space rightwards arrow space 2 Br to the power of minus
Net space cell space reaction space left square bracket by space adding space left parenthesis straight i right parenthesis space and space left parenthesis ii right parenthesis right square bracket colon
space space space space space space space space Zn left parenthesis straight s right parenthesis space plus space Br subscript 2 space space rightwards arrow space space space Zn to the power of 2 plus end exponent space plus space 2 Br to the power of minus
Positive terminal:   The bromine electrode where reduction takes place.
2. For the cell Cr left parenthesis straight s right parenthesis space plus space Cr to the power of 3 plus end exponent space parallel to space space straight I to the power of minus comma space space straight I subscript 2 space vertical line space Pt left parenthesis straight s right parenthesis
The two half reactions can be represented as:
(i) Anode reaction (oxidation half reaction):
                Cr left parenthesis straight s right parenthesis space rightwards arrow space space space Cr to the power of 3 plus end exponent space left parenthesis aq right parenthesis space plus space 3 straight e to the power of minus
(ii) Cathode reaction (reduction half reaction):
               straight I subscript 2 space plus space 2 straight e to the power of minus space space space rightwards arrow space space 2 straight I to the power of minus
Net cell reaction: It is obtained by multiplying equation (i) by 2 and equation (ii) by 3 and then adding.
2 Cr left parenthesis straight s right parenthesis space space space rightwards arrow space space space space 2 Cr to the power of 3 plus end exponent space plus space 6 straight e to the power of minus
3 straight l subscript 2 left parenthesis straight s right parenthesis space plus space 6 straight e to the power of minus space space rightwards arrow space space space 6 straight I to the power of minus
2 Cr left parenthesis straight s right parenthesis space plus space 3 straight I subscript 2 left parenthesis straight s right parenthesis space rightwards arrow space space 2 Cr to the power of 3 plus end exponent space plus space 6 straight I to the power of minus

Positive terminal. The iodine electrode where reduction takes place.
3. For cell Pt left parenthesis straight s right parenthesis vertical line space straight H subscript 2 left parenthesis straight g right parenthesis comma space straight H to the power of plus left parenthesis aq right parenthesis space parallel to space Cu to the power of 2 plus end exponent space vertical line space Cu left parenthesis straight s right parenthesis
the two half reactions can be represented as:
(i) Anode reaction (oxidation half reaction):
                     straight H subscript 2 left parenthesis straight g right parenthesis space space rightwards arrow space space space 2 straight H to the power of plus left parenthesis aq right parenthesis space plus space 2 straight e to the power of minus
(ii) Cathode reaction (reduction half reaction):
                   Cu to the power of 2 plus end exponent left parenthesis aq right parenthesis space plus space 2 straight e to the power of minus space space space rightwards arrow space space space Cu left parenthesis straight s right parenthesis
Net cell reaction:  It is obtained by adding equations (i) and (ii).
straight H subscript 2 left parenthesis straight g right parenthesis space plus space Cu to the power of 2 plus end exponent left parenthesis aq right parenthesis space rightwards arrow space space 2 straight H to the power of plus left parenthesis aq right parenthesis space plus space Cu left parenthesis straight s right parenthesis
Positive terminal: The copper electrode where reduction takes place.