Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Carbon as cathode in dry cell?

  1. May 18, 2015 #1
    I was studying about dry cell and read that carbon(graphite) rod surrounded by MnO2 is used as cathode but when i read the chemical reaction equation at cathode given as :
    MnO2 + NH4+ + e - - - - > MnO(OH) +NH3
    I nowhere found carbon... So where is it actually used? (i found this for mercury cells too)
     
  2. jcsd
  3. May 18, 2015 #2

    Borek

    User Avatar

    Staff: Mentor

    Carbon electrode is inert - it doesn't react, it just transfers the charge.
     
  4. May 18, 2015 #3

    James Pelezo

    User Avatar
    Gold Member

    Borek is right in that the carbon electrode does not react, but if I may add (if interested), the ratio of manganese dioxide and carbon powder in the cathode paste affects the characteristics of the cell; more carbon powder lowers the internal resistance, but more manganese dioxide improves capacity. This implies that there is an optimum ratio of C(s)/MnO2(s) in the electrolyte gel.

    Also, and a little FYI for safety, when discarding dry-cell batteries, always put a piece of insulating tape (e.g., masking tape, electrical tape, duct tape, etc.) over the electrodes of dry-cell batteries. Discarding several dry-cell batteries (in, say a sack or waste bag without taping the electrodes) may bring the electrodes in contact and produce enough heat to ignite a fire. There have been reports of discarded batteries igniting and causing considerable damage to homes. Good question and nice reply Borek.
     
  5. May 19, 2015 #4
    Thank you so much..... Both of you cleared my doubt quite well.... And james thanks for that extra piece of information too. Thanks once again
     
  6. May 20, 2015 #5
    I have got one more question about cells. What is the funtion of the gasket in the mercury cell? (I saw it in the diagram just below the outer covering i. e. Below the cell can)
     
  7. May 20, 2015 #6

    James Pelezo

    User Avatar
    Gold Member

    Are you asking about a Mercury/Zinc Button Battery, or the Castner-Kellner mercury electrolysis of Brine Water to generate chlorine gas and NaOH?
     
  8. May 20, 2015 #7
    It's the first one I am asking about -Mercury/Zinc Button Battery. Sorry for not mentioning it...
     
  9. May 20, 2015 #8

    James Pelezo

    User Avatar
    Gold Member

    If you don't mind, could you send the link with the photo of interest? or, the photo ... Just so we're on the same page on this. The term 'gasket' in this issue may have different meanings between us; that is, insulator vs semi-permeable plate. If it's the semi-permeable plate (if I may guess), it is a porous material/plate that has been soaked in a strong electrolyte such as NaOH and functions as the 'Salt Bridge' separating the anode (Zn/ZnOxide oxidation side) from the cathode (Hg-Oxide/Hg reduction side). As the battery discharges, the Zinc reacts with the hydroxide forming a Zn-Hydroxide Complex which decomposes to Zn-Oxide. The sodium from the salt bridge migrates toward the HgO/Hg cathode to counter balance loss of cationic charge when mercury in mercuric oxide is reduced to Hgo(s).
     
    Last edited: May 20, 2015
  10. May 21, 2015 #9
  11. May 31, 2015 #10
    Another question again!... Can we describe a secondary cell as a cell which works initially as a electrochemical cell and when it comes to recharging it functions as electrolytic cell?
     
  12. May 31, 2015 #11

    Borek

    User Avatar

    Staff: Mentor

    Why not? But to be honest I fell like it is an artificial classification. Reversible electrochemical process doesn't care about which way it goes.
     
  13. May 31, 2015 #12
    But the reactants in the galvanic cell which get used up while supplying electrical energy require electric current from an outer source when it comes to recharging... and which means that the reaction is non spontaneous in the reverse direction. So how can we say that it is an reversible process (which are spontaneous by nature).
    Please clarify this its quite confusing, and are there any corrections I can make to the statement in my previous post?
     
  14. May 31, 2015 #13
    May I know what did you mean by artificial classification? Where did I go wrong in making that statement.
     
  15. May 31, 2015 #14

    Borek

    User Avatar

    Staff: Mentor

    Sure, charging requires putting energy into the cell, discharging means you draw the energy from the cell. But it is still the same setup, and the same reaction, just run in different direction. While technically there is nothing wrong with treating it as two separate models, they are for most purposes identical - the only difference is a flip of sign. Thus you can either use one universal model to describe the cell regardless of whether it is charged or discharged, or use two separate models. The latter approach is artificial for me, it complicates things for no apparent gain.
     
  16. May 31, 2015 #15
    That indeed was a good answer. I got my doubt cleared. Thanks for the help!!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Carbon as cathode in dry cell?
  1. Zinc Carbon Cell (Replies: 3)

  2. Is it Carbon (Replies: 15)

  3. Dry cell batteries (Replies: 12)

Loading...