Wednesday 22 May 2013

How much electricity in terms of Faraday is required to produce


Q.13:How much electricity in terms of Faraday is required to produce
(i)       20.0 g of Ca from molten CaCl2.
(ii)     40.0 g of Al from molten Al2O3.
Solution:
(i)  
The balance reaction will
The charge on Ca in CaCl2
Ca + 2Cl      = 0
Cl has –1 charge so that
Ca  + 2(–1) = 0
Ca               = 2
We have to get 2o g Ca from Ca 2+
Number of required moles  = mass / molar mass
Molar mass of Ca is 40 g/mol and required mass of Ca is 20 g
Hence number of moles           = 20/40      = 0.5 mol
Electricity required to produce 1 mol of calcium = 2 F
The electricity required to produce 0.5 mol of calcium
= 0.5 × 2 F
   = 1 F
(ii)
Charge on Al in Al2O3
2Al  + 3O = 0
Oxygen has –2
2Al +3(–2)   = 0
Al         = 3
Change transfer  n = 3
Charge required for 1 mol of Al    = 3F
Number of moles of Al  = 40 /27  = 1.48

The electricity required to produce 1.48 mol of Al = 1.48  × 3 F
                                                                                = 4.44 F

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