Where Is Carbon Used in Dry Cells?

[Quotes]

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)

 

[Borek]

Carbon electrode is inert - it doesn't react, it just transfers the charge.

 

[James Pelezo]

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)/MnO_2(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.

 

[Quotes]

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

 

[Quotes]

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)

 

[James Pelezo]

Are you asking about a Mercury/Zinc Button Battery, or the Castner-Kellner mercury electrolysis of Brine Water to generate chlorine gas and NaOH?

 

[Quotes]

It's the first one I am asking about -Mercury/Zinc Button Battery. Sorry for not mentioning it...

 

[James Pelezo]

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 Hg^o(s).

 

[Quotes]

https://www.bing.com/images/search?...F28A18C3AE6E03CA&first=26&count=1&FORM=IDFRIR

And

https://www.bing.com/images/search?...69D80DF38633F24CD&first=3&count=1&FORM=IDFRIR

These are the links to the pictures

 

[Quotes]

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?

 

[Borek]

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.

 

[Quotes]

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?

 

[Quotes]

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.

May I know what did you mean by artificial classification? Where did I go wrong in making that statement.

 

[Borek]

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.

 

[Quotes]

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.

That indeed was a good answer. I got my doubt cleared. Thanks for the help!