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Topic: Battery Chemistry  (Read 5246 times)

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Michael

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Battery Chemistry
« on: February 24, 2005, 08:33:42 PM »
Well to begin with i will say what i know then ask the questions afterwards.



NB: Nothing is connected to the battery as yet

A normal dry cell battery or Leclanché dry cell (arranged like the attached picture) contains 2 electordes a cathode and an anode. The cathode is carbon(graphite rod) and the anode is zinc.

The cathode is sourrounded by MnO2 (manganese dioxide) and carbon powder packed around the rod ( i'm assuming here that this is to help in electrical conduction)

The electrolyte is then found as a paste aroung the cathode containg NH4Cl and ZnCl2. Again i'm assuming that the MnO2 and/or carbon powder doesn't react with the paste.

The separator  is a sheet of porous material, such as paper or cardboard, soaked with electrolyte that prevents the electrode materials from mixing together and reacting when a battery is not being used. Without a separator the zinc anode could wear away prematurely and reduce the life of the battery.

So now the cathode and its materials around it is separated from the anode which is   zinc metal.
...
..
.
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Now you attach connecting wire from the negative terminal to the positive terminal.
What they say will happen is this:

The zinc (solid) metal will oxidize the zinc atoms at the surface of the anode oxidize (give up their oxygen electrons). The zinc atom then becomes an ion  (electrically charged atom) with a positive charge. The zinc ions move away from the anode, leaving their electrons behind on the anode's surface. The anode thus gains an excess of electrons and becomes more negatively charged than the cathode.

Zn(s) -> Zn2+(aq) + 2e-

Now the electrons will flow from a place of low potential to a place of higher potential hence the electrons will flow towards the positive terminal along the wire.

Reduction then takes place at the cathode involving the meterials around it.
As the electrons are added to them.

Now here is where it gets tricky and if i go wrong or miss out something please say.
First off, the NH4Cl gains 2 electrons and goes to NH3 gas and H2 gas and Cl2 gas.

2NH4Cl(aq) + 2e- -> 2NH3(g) + H2(g) + Cl2(g)

The MnO2(g) is then reduced when combining with the H2 gas produced by the NH4Cl and goes to Mn2O3(s) and H2O(l)

2MnO2(s) + H2(g) -> Mn2O3(s) + H2O(l)
Solid dimanganese trioxide plus water

Also, lets not forget the ZnCl2 the purpose of this they say is to dissolve the NH3(g) so as not to cause bubbles.

ZnCl2(aq) + 2NH3(g) -> Zn(NH3)2Cl2(s)
Solid zinc ammonium chloride

The Zinc metal keeps oxidizing and producing more and more electrons which is use up and hence the zn metal degrades. And i'm guessing here, the reason why you don't leave batteries in the appliance is because of the different products that would be continueously formed around the cathode and cause the battery to burst.?
Now that is what i analyse for my text books and other resources.

Now, what about the Cl2(g) form the NH4Cl? Where does it go or do.
And most importantly what causes the Zn metal to oxidize to Zn2+
When the battery is not attached to anything, is the Zn oxidizing all time
If not what causes to do so when it is attached to something, i'm guessing that before a battery is even used there are not electrons on the surface of the negative terminal form the Zn, so what causes it to oxidize?
   
« Last Edit: February 24, 2005, 08:37:15 PM by Michael »

Offline Borek

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Re:Battery Chemistry
« Reply #1 on: February 26, 2005, 04:58:28 AM »
Google "Leclanché dry cell"...

Complete set of reactions:

http://www.physchem.co.za/Redox/Cells.htm
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Michael

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Re:Battery Chemistry
« Reply #2 on: February 26, 2005, 01:26:48 PM »
Yes thats informative but it doesn't answer the questions....i have tried searching it but have not found it.

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Re:Battery Chemistry
« Reply #3 on: February 26, 2005, 06:34:53 PM »
It answers, even in not directly. Combining all reaction into one you will get

Zn + 2 MnO2 + 2 H+ -> Zn2+ + Mn2O3 + H2O

Note there is no Cl2 in this reaction, which suggests there was an error in your description of the whole process.

As for Zn oxidation - this is an redox process, electrons are going from Zn to Mn4+, that's why Zn is oxidized while Mn4+ is reduced.
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