Page 1 of 1

Voltaic Cell Tuffy!!

Posted: Fri Mar 21, 2014 4:42 pm
by Kwanteq
Hi, this is my first post and will appreciate any help and comments.

I have a copper and zinc pile with the standard wet bridge. What takes place between the dissimilar metals is galvanic corrosion which is a natural and spontaneous reaction causing an electrical current or flow between the metals. Because the metals are not in direct contact and because of the wet (electrolyte) bridge if galvanic corrosion occur it must be via the electrolyte and the electrons will then flow through the electrolyte. The wet bridge is cotton soaked in an electrolyte made of a 50g NaCl per 1000ml distilled water.

Sodium Chloride breaks up in the water to make positive sodium ions (Na+) and negative chloride ions (Cl-). These ions reduce the energy needed for water to split into hydroxide ions (OH-) and hydrogen ions H+ (the hydrogen ions quickly find another water molecule and create hydronium ions, H3O+)

1. at the zinc electrode (anode)
The zinc ion combines with four hydroxide ions to form one ion of zincate (Zn(OH)42-), leaving two electrons behind on the zinc anode.
The chlorine ions from the salt then combine with the hydronium ions left over when the hydroxide ions were taken away by the zinc, and form hydrochloric acid.
Zn + 4OH => Zn(OH)42- + 2e-
4Cl + 4H20 => 4HCl + 4OH
ZN(OH)42- => ZnO + H2O + 2OH-

2. at the copper electrode (cathode)
four electrons combine with oxygen dissolved in the water and two molecules of water to form four hydroxide ions.
The sodium ions from the salt combine with these hydroxide ions to make sodium hydroxide.
The hydrochloric acid and the sodium hydroxide combine back into salt and water.
O2 + 2H2O + 4e- => 4OH
2Na+ + 4OH- => 4 NaOH
NaOH + HCl => NaCl + H2O

My questions are this:
a. Do I have the reactions correctly noted?
b. If a NaCl molecule is the end result of the reaction in the electrolyte is it acting like a sort of catalyst? because the salt comes out of the process unchanged
c. Will the copper electrode still corrode?
d. Will there be a buildup of zinc oxide on the zinc electrode?

3. We can calculate from the metallurgy Anodic Index the overall standard state cell potencial (Volts) from the half cell reactions - one being reduction (0.762 V)
and the other being oxidation (0.339 V) to yield a total of 1.101 V

My questions are:
a. Does the surface area have an effect on the voltage produced?
b. If so, what formula can be used to calculate the voltage?
c. How would I calculate the amperage of the cell? or would it be load dependant?

Then finally what can the life expectancy of such a cell be? How long will it keep producing usable electrical current?
I know I will have to put the cell in an air tight container to prevent the water in the electrolyte from evaporating and drying out!

Thank you in advance for any information that would shed some light on the questions I have. I know it is a mouthful!

Ben van der Poel

Re: Voltaic Cell Tuffy!!

Posted: Tue Apr 01, 2014 9:48 pm
by raytrent
Is this a science fair project? If so, what are you testing and what's your experimental procedure? We don't offer assistance with homework or other assignments, I'm afraid (my apologies if I have misunderstood your intent).

Re: Voltaic Cell Tuffy!!

Posted: Wed Apr 02, 2014 3:47 am
by Kwanteq
Let me start by saying thank you that you took the time to look at my post.

This is not a science fair project neither is this a school project, it is an experimental voltaic cell I would like to build for my personal experimental use and development. Although I'm not a complete expert in the field of voltaic cell reactions I have done as much research as possible. I want to assess if what i have put together as theory is in fact theoretically correct by knocking on the door of more knowledgeable people like yourself and others at science buddies.

I have on order 10 copper plates cut to size (385mm x 160mm x 5mm thick) as well as 10 off 98% pure (may contain small amount of Aluminium and other elements) zinc plates being casted and machined to the same dimensions as that of the copper plates. I plan to build a voltaic pile with these by separating alternating copper and zinc plates with cotton sheets of the same dimension as the plates after immersion in the NaCl electrolyte.

Therefore all my many questions, when I build the cell I will get answers from the practical experience, however I thought it good to know the theory behind what will actually occur when I build the cell and have a deeper understanding and knowledge about the chemical reactions that will take place.

May I ask if you could recommend some people at science buddies who will also be able to evaluate the theory I have as to whether it is correct. Thank you again!

Re: Voltaic Cell Tuffy!!

Posted: Tue Apr 15, 2014 9:17 pm
by raytrent
Well, this really isn't the purpose of Science Buddies, but in the interests of being helpful, your equations look pretty good to me (though I'll warn you that it's been a lot of years since I had a chemistry class). The only one that looks a little "fishy" is "NaOH + HCl => NaCl + H2O". I don't know whether it actually goes through an NaCl state, or if it would be more accurate to say "NaOH + HCl => Na+ + Cl- + H2O".

The NaCl is basically a catalyst, yes. Both electrodes "corrode", though that terminology is a bit inexact. It would be better to say that one electrode oxidizes and the other reduces.

No, the voltage doesn't depend on the surface area, The actual amperage in a circuit with the battery will follow Ohm's Law, but the internal resistance, and therefore the maximum amperage of the battery will, as will the total capacity of the battery. I don't know how to calculate those theoretically, so they sound like good subjects for an experiment :-).

Re: Voltaic Cell Tuffy!!

Posted: Wed Apr 16, 2014 5:28 am
by Kwanteq
Thank you Ray Trent for you input!
I agree with your explanation about the chemical equation in respect to the NaOH + HCl => Na+ + Cl- + H2O
I finally have the plates and are waiting for a specially made container in which will assemble the lot. Been very busy with another project and will post my findings once I have completed this cell.

If there are any one else who would like to post their opinions I will appreciate it a lot, thank you in advance.