Rules of balancing redox reaction :
(1) separate the redox reaction into half-reaction
(2) Balance the atom in each half reaction
a) Balance all atom except H and O
b) Balance the oxygen atom with H2O
c) Balance the hydrogen atom with H+
d) In a basic medium, add one OH+ to each side
for every H+
(3) Balance the change with e−
(4) Make e− gain equivalent to election loss in the half-reaction
(5) Add the half reaction together
(6) Simplify the equation
(7)finally check the element and changes are balanced
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To find reduction potential for half reaction (E∘cell)
cl2+2e−→2cl−
given eqnpt2+2cl−→pt+cl2 Ecell=−0.15v...(1)
pt2+2e−→pt E=1.20V...(2)
eqn(1)⇒2cl2+pt→pt2+2cl− Ecell=0.15V...(3)
Adding eqn (2) and eqn(3)
pt2+2e−→pt
cl2+pt→pt2+2cl−
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cl2+2e−→2cl−
∴E∘cell=E∘oxidation+E∘redaction
=0.15+1.20
∴E∘cell=1.35v