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Cell Reaction


We will have
  1. Zinc electrode of the left-hand half-cell (LHC) will act as the negative terminal or anode. The reaction occurring at this electrode will be oxidation. Hence, we write the half-cell reaction occurring at this electrode as
    LHS: Anode: Oxidation reaction: Zn Zn2+(aq) + 2e-
  2. Copper electrode of the right-hand half-cell (RHC) will act as the positive terminal or cathode. The reaction occurring at this electrode will be reduction. Hence, we write the half-cell reaction occurring at this electrode as
    RHS: Cathode: Reduction reaction: Cu2+ + 2e- Cu
  3. The net cell reaction is obtained by adding these two half-cell reactions, i.e.
    Cell reaction = Oxidation reaction at LHC + Reduction reaction at RHC
    Reaction at LHC: Zn Zn2+(aq) + 2e-
    Reaction at RHC: Cu2+(aq) + 2e- Cu
    Overall Cell reaction: Zn + cu2+(aq) Zn2+(aq) + Cu

Cell Potential


The expression of emf is

emf = Higher reduction potential - Lower reduction potential  

Now, the ion with a higher reduction potential is reduced and the ion with the lower reduction potential is formed by the oxidation of the corresponding atom. Keeping in view the convention for writing a cell, we find that (1) the reduction reaction occurs at the right-hand half-cell and, therefore, it will be represented by a higher reduction potential, and (2)
oxidation reaction occurs at the left-hand half-cell and will, therefore, be represented by a lower reduction potential


Emf = Reduction potential of RHC - Reduction potential of LHC or
Ecell = ERHC - ELHC 
where ERHC and ELHC are the reduction potentials (or simply electrode potentials) at right-hand and left-hand half-cells, respectively. In the present case of Daniel cell, we will have
Ecell = ECu2+|Cu - EZn 2+|Zn
It is worth mentioning here that if the above cell on the paper is written as
Cu|Cu2+(aq)||Zn2+(aq)|Zn  
Then it will imply the following facts:
  1. Copper electrode will be taken as the negative terminal as this constitutes the left-hand electrode. The reaction occurring in the left-hand half-cell will be taken as the oxidation reaction:
    Cu Cu2+(aq) + 2e-
  2. Zinc electrode will be taken as the positive terminal as this constitutes the right-hand electrode. The reaction occurring in the right-hand half-cell will be taken as the reduction reaction:
    Zn2+(aq) + 2e- Zn
  3. The net cell reaction as obtained by the addition of the above two reactions will be
    Cu + Zn2+(aq) Cu2+(aq) + Zn  
The cell potential in this case will be
Ecell = ERHC - ELHC
      = EZn2+|Zn - ECu2+|Cu   
If the cell is written in a reverse order, the expressions for cell potential are also reversed. (or in other words Ecell will be expressed with an opposite sign).




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