Calculating the EMF of batteries

[batteryguy]

Hi all,

I'm a total noob when it comes to chemistry, and would need your help to understand the calculation of EMF for batteries.

I have a couple of questions about the calculation for Zinc-Carbon batteries in Wikipedia:
http://en.wikipedia.org/wiki/Zinc–carbon_battery

I can't manage to get 1.5V as a result...

So, here's a couple questions:

1) Anode reaction: Zn(s) → Zn2+(aq) + 2 e- [e° = -1.04 volts]
Why -1.04 ? Why not +0.76 like in all the textbooks and other webpages?
Like on this page: http://www.ausetute.com.au/calcelemf.html

2) Cathode reaction:
2MnO2(s) + 2 e- + 2NH4Cl(aq) → Mn2O3(s) + 2NH3(aq) + H2O(aq) + 2 Cl- [e° ≈ +.5 v]
How to get the 0.5V?


Thanks already for your help!

 

[Borek]

-0.76 (not +0.76) is a standard potential - that means it will be observed when activity of Zn²⁺ is exactly 1. As a first approximation that means concentration of Zn²⁺ equal to 1M - if the concentration is different, observed potential will be different (it can be calculated from the http://en.wikipedia.org/wiki/Nernst_equation).

Same for the cathode - to calculate potential you need to take into account activities of all substances present. In concentrated solutions (and that's what you find in the battery, it is filled with NH₄Cl paste) these are in practice impossible to calculate and it is easier to measure.

 

[batteryguy]

Thanks a lot! Very good explanation!