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E.M.F OF CELLS AND APPLICATION OF ELECTROCHEMICAL CELLS
The e.m.f of an electrochemical cell is the algebraic difference between the standard electrode potentials of the two metallic electrodes i.e e.m.f = Eocathode – Eo anode. A big e,m,f means a powerful cell. if we need a powerful cell the metals which serve as electrode should be far apart. The sign of e.m.f tells whether the reaction is spontaneous or not. A positive e.m.f shows that the reaction in the cell is spontaneous and so such cell can work otherwise it is not workable.
RELATIONSHIP BETWEEN E.M.F AND △G
△G = -nFE
Where n= number of moles of electron transfer, F = Faraday constant.
A negative value of △G shows the reaction is spontaneous
Solved examples
Find the e.m.f and △G of the cell represent by the half-cell below:
EoCu2+(aq) = +0.34v and EoAg+(aq)/Ag(s) = +0.80
Solution:
e.m.f = e.m.f = Eocathode – Eo anode.
= +0.80-0.34
= +0.46v
△G = -nFE
= 2X96500X0.46
= -88780J
APPLICATIONS OF ELECTROCHEMICAL CELLS
Examples of electrochemical cells are Daniell cell, Leclanche cell and lead acid accumulator (car battery). These cells are used in power generation.
Primary cells are not rechargeable. This because, the reactions in these cells are irreversible. Daniell and Leclanche cells are primary cells. Once they are discharged, they cannot be used again.
Secondary cells are rechargeable. The chemical reactions in these cells are reversible. They can be recharged after use. An example of this kind of cell is car battery. The batteries in our handset, laptops are secondary cells
EVALUATION:
- Distinguish between a primary cell and a secondary cell.
- Give four examples of a primary cell
- Describe the working principle of a Leclanche cell.
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