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Thread: How does the chemical reaction in Alkaline Batteries come about?

  1. #1 How does the chemical reaction in Alkaline Batteries come about? 
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    I found online the formula (by the way, Alkaline batteries are composed of: zinc as the anode; manganese dioxide as the cathode and potassium dioxide as the electrolyte) but I can't make sense of it. I'd really appreciate some help



    The half-reactions are:

    Zn(s) + 2OH(aq) → ZnO(s) + H2O(l) + 2e [e = 1.28 V]2MnO2(s) + H2O(l) + 2e → Mn2O3(s) + 2OH(aq) [e = +0.15 V] Overall reaction:

    Zn(s) + 2MnO2(s) ZnO(s) + Mn2O3(s) [e = 1.43 V]

    I don't understand how those two electrons appear in the first half reaction. Zinc's lost electrons are gained by Oxygen; and hydrogen and oxygen are perfectly bonded as H2O. Where could those two electrons have come from?

    Another thing that baffles me is the quantity of electrons Manganese can lose in its reduction state. If I'm not mistaken, Mn is +4 when bonding with two oxygen atoms... how does Mn2 bond with O3 then? Does it only lose three electrons?


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    Bullshit Intolerant PhDemon's Avatar
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    So where does the charge on the OH- go? It can't just disappear, the two electrons are needed to balance the charges. It is probably easiest (although oversimplified) to think of the 2e as coming from the Zn when it becomes Zn2+ and treating the 2OH- going to O2- + H2O separately.

    Mn can have a variety of oxidation states depending on the conditions and what it is reacting with, this is quite common in transition elements, in Mn2O3, Mn does indeed have an oxidation state of +3 (in fact Mn can exhibit all oxidation states from +2 to +7 depending on what it is reacting with and the conditions).


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    Quote Originally Posted by PhDemon View Post
    So where does the charge on the OH- go? It can't just disappear, the two electrons are needed to balance the charges. It is probably easiest (although oversimplified) to think of the 2e as coming from the Zn when it becomes Zn2+ and treating the 2OH- going to O2- + H2O separately.

    Mn can have a variety of oxidation states depending on the conditions and what it is reacting with, this is quite common in transition elements, in Mn2O3, Mn does indeed have an oxidation state of +3 (in fact Mn can exhibit all oxidation states from +2 to +7 depending on what it is reacting with and the conditions).
    Oh, right, I totally missed that - sign over the formula! Thought it was just a typo or something. Forgive my ignorance, but does it represent a -1 charge then?

    Ok, now I understand the reaction of Manganese. Didn't come to my mind that it was a transition metal lol.

    Thanks for all your help
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    Bullshit Intolerant PhDemon's Avatar
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    No problem, and yes a superscript + or - represents a charge on an ion.
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