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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Nitin_Naudiyal on May 17, 2013, 01:48:49 AM

Title: Leclanche Cell ( Dry Cell )
Post by: Nitin_Naudiyal on May 17, 2013, 01:48:49 AM
I was reading about Leclanche's Cell,

It is mentioned that " The Complexation of Zn2+ by NH3 molecules decreases the concentration of Free Zn2+ ions. This Increases the Voltage of the cell. "

I want to know, how decrease in the concentration of Zn2+ ions increases the voltage of the cell.

Thank you in Advance
Title: Re: Leclanche Cell ( Dry Cell )
Post by: Borek on May 17, 2013, 02:02:04 AM
Can you write Nernst equation for the cell?
Title: Re: Leclanche Cell ( Dry Cell )
Post by: Nitin_Naudiyal on May 17, 2013, 02:51:46 AM
Oxidation at Anode:
Zn  :rarrow: Zn2+ + 2e-

Reduction at Cathode:
2NH4+ + 2MnO2 + 2e- :rarrow: Mn2O3 + 2NH3 + H2O

Net Cell Reaction:
Zn + 2NH4+ + 2MnO2  :rarrow: Zn2+ + Mn2O3 + 2NH3 + H2O

Nernst Equation:

Ecell = E0 - 0.0592/2 Log([Zn2+][NH3]2[H2O]/[NH4+]2)





Title: Re: Leclanche Cell ( Dry Cell )
Post by: Borek on May 17, 2013, 04:19:37 AM
Quote from: Nitin_Naudiyal on May 17, 2013, 02:51:46 AM
Ecell = E0 - 0.0592/2 Log([Zn2+][NH3]2[H2O]/[NH4+]2)

Assuming everything else doesn't change and rearranging:

[tex]E' = -\frac{0.0592}{2} \log([Zn^{2+}]) = \frac{0.0592}{2} pZn[/tex]
Title: Re: Leclanche Cell ( Dry Cell )
Post by: Nitin_Naudiyal on May 17, 2013, 04:56:33 AM
So, as the Concentration of Zn ions decreases, the value of the non standard state electrochemical condition in the Nernst Equation decreases and hence the value of Ecell increases i.e. Potential of the cell increases.

Am i correct ?
Title: Re: Leclanche Cell ( Dry Cell )
Post by: Borek on May 17, 2013, 06:13:17 AM
Yes, we are dealing with a formal potential.
Title: Re: Leclanche Cell ( Dry Cell )
Post by: Nitin_Naudiyal on May 17, 2013, 06:25:43 AM
Thank You for your Help