Electrochemistry

Electrochemistry

Question

(a) What type of a battery is lead storage battery? Write the anode and cathode reactions and the overall cell reaction occurring in the operation of a lead storage battery.

(b) Calculate the potential for half-cell containing 0.10 M K2Cr2O7 (aq), 0.20 M Cr3+(aq) and 1.0 x 10-4 M H+ (aq)

The half-cell reaction is Cr subscript 2 straight O subscript 7 superscript 2 minus end superscript left parenthesis aq right parenthesis straight space plus straight space 14 straight space straight H to the power of plus subscript left parenthesis aq right parenthesis end subscript straight space plus straight space 6 straight e to the power of minus straight space rightwards arrow straight space 2 Cr subscript left parenthesis aq right parenthesis end subscript superscript 3 plus end superscript straight space plus straight space 7 straight H subscript 2 straight O subscript left parenthesis straight l right parenthesis end subscript

And the standard electrode potential is given as E0 = 1.33 V.

 OR

(a) How many moles of mercury will be produced by electrolysing 1.0 M?

Hg (NO3)2 solution with a current of 2.00 A for 3 hours?

[Hg (NO3)2 = 200.6 g mol-1]

(b) A voltaic cell is set up at 25°C with the following half-cells Al3+ (0.001 M) and Ni2+ (0.50 M). Write an equation for the reaction that occurs when the cell generates an electric current and determine the cell potential.

 Error converting from MathML to accessible text.

Answer

(a) A lead storage battery is a secondary battery.

The following chemical equations take place in a lead storage battery.

At space anode colon space Pb subscript left parenthesis straight s right parenthesis end subscript space plus space SO subscript 4 superscript 2 minus end superscript subscript left parenthesis aq right parenthesis end subscript space rightwards arrow space PbSO subscript 4 left parenthesis straight s right parenthesis space plus 2 straight e to the power of minus

At space cathode colon space PbO subscript 2 space left parenthesis straight s right parenthesis end subscript space plus space SO subscript 4 superscript 2 minus end superscript subscript left parenthesis aq right parenthesis end subscript space plus space 4 straight H to the power of plus space plus 2 straight e to the power of minus space rightwards arrow space PbSO subscript 4 subscript space left parenthesis straight s right parenthesis end subscript space plus space 2 straight H subscript 2 straight O subscript left parenthesis straight l right parenthesis end subscript
The space overall space cell space reaction space is space given space by comma

Pb subscript left parenthesis straight s right parenthesis end subscript space plus space PbO subscript 2 space left parenthesis straight s right parenthesis end subscript space plus space 2 straight H subscript 2 SO subscript 4 space rightwards arrow space 2 PbSO subscript 4 subscript space left parenthesis straight s right parenthesis end subscript space plus space space 2 straight H subscript 2 straight O subscript left parenthesis straight l right parenthesis end subscript
When a battery is charged, the reverse of all these reactions takes place.

Hence, on charging, PbSO4(s) present at the anode and cathode is converted into Pb(s) and PbO2(s) respectively.
b) Error converting from MathML to accessible text.
Or

(a) Quantity of electricity passed = (2A) x (3 x 60 x 60s) = 21600 C

Thus, 2F i.e. 2 x 96500 C deposit Hg = 1 mole 21600 C will deposit Hg

 fraction numerator 1 over denominator 2 cross times 96500 end fraction cross times 21600

= 0.11 mole
Or

At anode:             Al (s)                  -->     Al3+ (aq) + 3e-] x2

At cathode:          Ni2+ + 2e-               -->   Ni(s)                ] x3

Cell reaction:     2Al(s) + 3Ni2+(aq) --->   2Al3+(aq)  + 3Ni (s)

Applying nernst equation to the above cell reaction 
Error converting from MathML to accessible text.

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