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Dry cell batteries

  1. Feb 8, 2010 #1
    i was wondering what are the chemical reactions involved in producing the electricity, that the batteries provide i.e. the reductant and the oxidant
     
  2. jcsd
  3. Feb 8, 2010 #2
    Zn and MnO2, I suppose
     
  4. Feb 9, 2010 #3
    what is the purpose of a Potassium Chloride salt bridge, and a ammonium chloride or zinc chloride electrolyte?
     
    Last edited: Feb 9, 2010
  5. Feb 9, 2010 #4

    mheslep

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    I'm not clear about your OP question: the text doesn't follow the title - dry cells. The purpose of the electrolyte in a dry cell is the same as a 'wet cell' to provide a substance with free ions which can migrate to and from the electrodes. The dry cell doesn't use a liquid electrolyte so it is typically more portable.
     
  6. Feb 9, 2010 #5
    OK thank you that was very helpful, i would also like to ask where in the battery does the manganese dioxide be reduced (i know the cathode, however i am unsure where the cathode physically is), also could you say the transfer of electrons through the anode to cathode via the electrolytes is called the redox reaction hence generating the electricity to power our devices i.e. torches.
     
  7. Feb 9, 2010 #6

    mheslep

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    As I understand it, electrons are not transferred through the electrolyte all the way from electrode to electrode. Instead, outside of the electrodes, electron motion is limited to the field adjacent the electrode. Only ions are free to move through the electrolyte. See here
    http://en.wikipedia.org/wiki/Electrolyte#Electrochemistry
     
  8. Feb 10, 2010 #7
    i'm sorry but i'm kind of confused. Can you tell me basically how the dry cell battery works in terms of chemical reactions such as "redox" reaction. (please include the reduction and oxidisation reactions, aswell as the purpose of the salt bridge and the electrolyte).

    "In dry-cell batteries, which are often used in flashlights, the electrons given up by zinc are taken up by ammonium ions (NH4+) present in the battery as ammonium chloride (NH4Cl)" [source Britannica], it doesn't make sense, why is the ammonium ions taking the electrons? isn't the ammonium chloride simply an electrolyte? shouldn't Manganese Chloride be taking the electrons to form the redox reaction? :S (sorry for the numerous questions but i am very confused please help)

    thank you :)
     
    Last edited: Feb 10, 2010
  9. Feb 10, 2010 #8

    Gokul43201

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    The basic concepts of a voltaic cell are covered nicely here: http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/electrochem.html#c2 (albeit using the example of a Daniel Cell, with Cu and Zn electrodes).

    The same general principle applies to all voltaic cells, but in particular, the reactions of the common NH4Cl/MnO2 dry cell are given here: http://library.kcc.hawaii.edu/external/chemistry/everyday_battery.html [Broken]
     
    Last edited by a moderator: May 4, 2017
  10. Feb 10, 2010 #9
    thank you very much these links are very helpful :) much appreciated.
     
    Last edited by a moderator: May 4, 2017
  11. Feb 11, 2010 #10
    "the reduction reaction occurs within the moist paste comprised of ammonium chloride (NH4Cl) and manganese dioxide (MnO2):

    2 NH4+ + 2 MnO2 + 2e- ------> Mn2O3 + 2 NH3 + H2O" [source: http://library.kcc.hawaii.edu/external/chemistry/everyday_battery.html [Broken] ]

    i don't understand where the chloride goes in this reaction. Is it simply an spectator ion?

    zinc is oxidized and manganese-dioxide is reduced forming the redox reaction generating an Enort of 1.5 volts, the reduction half reaction occurs in the "paste of MnO2, electrolyte and carbon" and the oxidation half reaction occurs in the anode "Zinc cylinder" (<--- is this statement a "summary" of the reactions occurring within a dry cell battery?)

    by the way... 2 NH4+ + 2 MnO2 + 2e- ------> Mn2O3 + 2 NH3 + H2O isnt this reaction the redox reaction or the reduction reaction?
     
    Last edited by a moderator: May 4, 2017
  12. Feb 11, 2010 #11

    Gokul43201

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    The Cl- liberated from the NH4Cl is taken up at the anode, by the oxidation of Zn to ZnCl2. But as such, it is not particularly important to the electrochemistry of the cell.

    More explicitly, you can combine the oxidation and reduction reactions to see what's happening to the chloride ions:

    2NH4Cl + Zn + 2MnO2 -----> 2NH3 + ZnCl2 + Mn2O3 + H2O

    Yes, it's a reasonable summary. Minor points: what you refer to as Enort is actually E0 ("E naught"), and the reduction reaction does not happen "in" the carbon (or graphite) electrode, but rather, at its surface.

    That's the reduction reaction (or a half-cell reaction). A red-ox reaction is what you get when you combine a reduction reaction with the corresponding oxidation reaction, just like the one I wrote out explicitly above.
     
    Last edited by a moderator: May 4, 2017
  13. Feb 13, 2010 #12
    thanks a lot very helpful
     
  14. Feb 15, 2010 #13
    can you say that the Silver oxide cell (button cell) works on the same principles that the dry cell battery does? (similarities - differences)

    sorry but in my text book the reduction reaction for manganese dioxide says: NH4+ + MnO2 + H2O + e- -----> Mn(OH)3 + NH3

    whilst my other source (encyclopedia Britannica) says that the reduction reaction is: 2 NH4+ + 2 MnO2 + 2e- ------> Mn2O3 + 2 NH3 + H2O

    are they the same equation?
     
    Last edited: Feb 15, 2010
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